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We report LOFAR and Very Large Array (VLA) detections of extended radio emission in the spiral galaxy HCG 97b, hosted by an X-ray bright galaxy group. The extended radio emission detected at 144 MHz, 1.4 GHz, and 4.86 GHz is elongated along the optical disc and has a tail that extends 27 kpc in projection towards the centre of the group at GHz frequencies or 60 kpc at 144 MHz. Chandra X-ray data show two off-nuclear ultra-luminous X-ray sources (ULXs), with the farther one being a plausible candidate for an accreting intermediate-mass black hole (IMBH). The asymmetry observed in both CO emission morphology and kinematics indicates that HCG 97b is undergoing ram-pressure stripping, with the leading side at the south-eastern edge of the disc. Moreover, the VLA 4.86 GHz image reveals two bright radio blobs near one ULX, aligning with the disc and tail, respectively. The spectral indices in the disc and tail are comparable and flat (α > −1), suggesting the presence of recent outflows potentially linked to ULX feedback. This hypothesis gains support from estimates showing that the bulk velocity of the relativistic electrons needed for transport from the disc to the tail is approximately |${\sim}1300\, \rm km~s^{-1}$|⁠. This velocity is much higher than those observed in ram-pressure stripped galaxies (⁠|$100{-}600\, \rm km~s^{-1}$|⁠), implying an alternative mechanism aiding the stripping process. Therefore, we conclude that HCG 97b is subject to ram pressure, with the formation of its stripped radio tail likely influenced by the putative IMBH activities. [ABSTRACT FROM AUTHOR]

Using over 3000 compact groups (CGs) of galaxies extracted from mock catalogues built from semi-analytical models (SAMs) of galaxy formation, we study whether the CG assembly channel affects the z  = 0 properties of galaxies and their evolution. The evolution of CG galaxy properties with time is a clear function of their stellar masses. For instance, high-stellar-mass CG galaxies have lived their last 8 Gyr with little cold gas content while maintaining their reservoir of hot gas, while low-mass CG galaxies still preserve some of their cold gas content at the present but they have completely drained their hot gas reservoir. Beyond that, we find that the evolution of CG galaxies is also a function of the assembly history of the CGs: with more extreme losses of gas content, faster mass gain rates for black holes and more marked suppression of star formation as a function of cosmic time as we go from recent to early CG assembly. Thus, CGs constitute another laboratory for galaxy assembly bias, as the later assembling groups have later star formation. Our results show that classifying CGs according to their assembly channel is a way of distinguishing different paths by which galaxies transform their properties throughout their history. [ABSTRACT FROM AUTHOR]

We present results of a Hubble Space Telescope (HST) UBVI -band study of star clusters in tidal tails, using new WFC3 and ACS imaging to complement existing WFPC2 data. We survey 12 tidal tails across seven merging systems, deriving ages and masses for 425 star cluster candidates (SCCs). The stacked mass distribution across all systems follows a power law of the form d N /d M  ∝  M β, with β = −2.02 ± 0.15, consistent with what is seen in other star-forming environments. GALEX and Swift UV imaging provide star formation rates (SFRs) for our tidal tails, which when compared with ages and masses of our SCCs, allows for a determination of the cluster formation efficiency (CFE). We find the CFE increases with increasing SFR surface density, matching the theoretical model. We confirm this fit down at SFR densities lower than previously measured (log ΣSFR (M⊙ yr−1 kpc−2) ≈ −4.2), as related to the CFE. We determine the half-light radii for a refined sample of 57 SCCs with our HST WFC3 and ACS imaging, and calculate their dynamical age, finding the majority of them to be gravitationally bound. We also provide evidence of only low-mass (<104 M⊙) cluster formation in our nearest galaxy, NGC 1487, consistent with the theory that this system is a dwarf merger. [ABSTRACT FROM AUTHOR]

We obtain the kinematic distributions of stars (synthetic model line absorption) and ionized gas (H α line emission) for star-forming regions residing in Calar Alto Legacy Integral Field Area survey tidally perturbed (perturbed) and non-tidally perturbed (control) galaxies. We set the uncertainties of the velocity dispersion by measuring the statistical variability of the data sets themselves. Using these adopted uncertainties and considering the sensitivity of the grating device, we establish thresholds of reliability that allow us to select reliable velocity dispersions. From this selection, we pair the star-forming spaxels between control and perturbed galaxies at the closest shifts in velocity (de-redshifting). We compare their respective distributions of velocity dispersion. In perturbed galaxies, median velocity dispersions for the stellar and gaseous components are minimally higher and equal, respectively, than those in control galaxies. The spread in velocity dispersion and the velocity shift–velocity dispersion space agree with this result. Unlike the well-known trend in strongly interacting systems, the stellar and ionized-gas motions are not disturbed by the influence of close companions. For the gaseous component, this result is due to the poor statistical variability of its data, a consequence of the tightness in velocity dispersion derived from high spectral line intensities. This analysis concludes the series, which previously showed star-forming regions in galaxies with close companions undergoing more prominent gas inflows, resulting in differences in their star formation and consequent metal content. [ABSTRACT FROM AUTHOR]

Tidal dwarf galaxies (TDGs) are low-mass objects that form within tidal and/or collisional debris ejected from more massive interacting galaxies. We use CO(1–0) observations from Atacama Large Millimeter/submillimeter Array and integral-field spectroscopy from Multi-Unit Spectroscopic Explorer to study molecular and ionized gas in three TDGs: two around the collisional galaxy NGC 5291 and one in the late-stage merger NGC 7252. The CO and H α emission is more compact than the H  i emission and displaced from the H  i dynamical centre, so these gas phases cannot be used to study the internal dynamics of TDGs. We use CO, H  i , and H α data to measure the surface densities of molecular gas (Σmol), atomic gas (Σatom), and star formation rate (ΣSFR), respectively. We confirm that TDGs follow the same spatially integrated ΣSFR–Σgas relation of regular galaxies, where Σgas = Σmol + Σatom, even though they are H  i dominated. We find a more complex behaviour in terms of the spatially resolved ΣSFR–Σmol relation on subkpc scales. The majority (⁠|$\sim 60~{{\ \rm per\ cent}}$|⁠) of star-forming regions in TDGs lie on the same ΣSFR–Σmol relation of normal spiral galaxies but show a higher dispersion around the mean. The remaining fraction of star-forming regions (⁠|$\sim 40~{{\ \rm per\ cent}}$|⁠) lie in the starburst region and are associated with the formation of massive super star clusters, as shown by Hubble Space Telescope images. We conclude that the local star formation activity in TDGs proceeds in a hybrid fashion, with some regions comparable to normal spiral galaxies and others to extreme starbursts. [ABSTRACT FROM AUTHOR]

The role of magnetic fields in galaxy evolution is still an unsolved question in astrophysics. We have previously shown that magnetic fields play a crucial role in major mergers between disc galaxies; in hydrodynamic simulations of such mergers, the Auriga model produces compact remnants with a distinctive bar and ring morphology. In contrast, in magnetohydrodynamic (MHD) simulations, remnants form radially extended discs with prominent spiral arm structure. In this paper, we analyse a series of cosmological 'zoom-in' simulations of major mergers and identify exactly how magnetic fields are able to alter the outcome of the merger. We find that magnetic fields modify the transport of angular momentum, systematically hastening the merger progress. The impact of this altered transport depends on the orientation of the field, with a predominantly non-azimuthal (azimuthal) orientation increasing the central baryonic concentration (providing support against collapse). Both effects act to suppress an otherwise existent bar-instability, which in turn leads to a fundamentally different morphology and manifestation of feedback. We note, in particular, that stellar feedback is substantially less influential in MHD simulations, which allows for the later accretion of higher angular momentum gas and the subsequent rapid radial growth of the remnant disc. A corollary of the increased baryonic concentration in MHD simulations is that black holes are able to grow twice as large, although this turns out to have little impact on the remnant's development. Our results show that galaxy evolution cannot be modelled correctly without including magnetic fields. [ABSTRACT FROM AUTHOR]

We present a study of the morphology of star formation and the associated nuclear activity in a sample of eight closely interacting southern galaxies, which are in different stages of interaction, starting with nearly merged nuclei that have one prominent bulge to more widely spaced interacting galaxies. We have used far-ultraviolet (FUV) observations from the Ultraviolet Imaging Telescope (UVIT), near-Infrared observations from the Infrared Survey Facility (IRSF) telescope, and archival optical data from the Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) integral field spectrograph. Analysing resolved stellar populations across the disc of the interacting galaxies can provide unique insights into how interactions affect galaxy properties, such as morphology, star formation rates, and chemical composition. We take advantage of the unprecedented capabilities of MUSE and UVIT to carry out a highly detailed spatially and spectrally resolved study of star formation rate, star formation histories, metallicity, and active galactic nucleus activity in the sample of eight interacting galaxies that are in different stages of interaction. Most of our sample galaxies are gas-rich and show evidence of recent, massive star formation in tidal tails, rings, and spiral arms. This is evident from their FUV and Hα emissions, which trace young, massive star-forming regions. We compared the star formation rate in the barred and unbarred galaxies in our sample and found that the barred galaxies do not show significant enhancement in star formation rate or large-scale difference in star formation morphology compared to unbarred galaxies. IC5250 and NGC7733N show extended nuclear outflows of sizes ∼5 and 8 kpc, respectively. [ABSTRACT FROM AUTHOR]

Large-scale asymmetries (i.e. lopsidedness) are a common feature in the stellar density distribution of nearby disc galaxies both in low- and high-density environments. In this work, we characterize the present-day lopsidedness in a sample of 1435 disc-like galaxies selected from the TNG50 simulation. We find that the percentage of lopsided galaxies (10  per cent–30  per cent) is in good agreement with observations if we use similar radial ranges to the observations. However, the percentage (58  per cent) significantly increases if we extend our measurement to larger radii. We find a mild or lack of correlation between lopsidedness amplitude and environment at z  = 0 and a strong correlation between lopsidedness and galaxy morphology regardless of the environment. Present-day galaxies with more extended discs, flatter inner galactic regions, and lower central stellar mass density (i.e. late-type disc galaxies) are typically more lopsided than galaxies with smaller discs, rounder inner galactic regions, and higher central stellar mass density (i.e. early-type disc galaxies). Interestingly, we find that lopsided galaxies have, on average, a very distinct star formation history within the last |$10\, \mathrm{Gyr}$|⁠ , with respect to their symmetric counterparts. Symmetric galaxies have typically assembled at early times (∼8– |$6\, \mathrm{Gyr}$| ago) with relatively short and intense bursts of central star formation, while lopsided galaxies have assembled on longer time-scales and with milder initial bursts of star formation, continuing building up their mass until z  = 0. Overall, these results indicate that lopsidedness in present-day disc galaxies is connected to the specific evolutionary histories of the galaxies that shaped their distinct internal properties. [ABSTRACT FROM AUTHOR]

We study a compact nucleus embedded in an early-type dwarf galaxy, MATLAS-167, which is in the process of disruption by the tidal force of the neighbouring giant S0 galaxy, NGC 936, in a group environment. Using the imaging data of the MATLAS survey, we analyse the stellar tidal tail of MATLAS-167 and its central compact nucleus, designated as NGC 936_UCD. We find that NGC 936_UCD has a luminosity of Mg = −11.43 ± 0.01 mag and a size of 66.5 ± 17 pc, sharing the global properties of ultra-compact dwarf galaxies (UCDs) but significantly larger and brighter compared to the typical UCD populations observed in the Virgo cluster. By integrating the total luminosity of both the tidal stream and MATLAS-167, we estimate that the disrupted dwarf progenitor possesses a luminosity of Mg = −15.92 ± 0.06 mag, a typical bright early-type dwarf galaxy luminosity. With the help of the optical spectrum observed by the SDSS survey, we derive the simple stellar population properties of NGC 936_UCD: a light-weighted age of 5.6 ± 0.7 Gyr and metallicity of [Z/H] = −0.83 ± 0.3 dex. Our findings suggest that tidal threshing is a possible formation mechanism of bright UCD populations in close proximity to giant galaxies. [ABSTRACT FROM AUTHOR]

We present the first results from GALAXY CRUISE, a community (or citizen) science project based on data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The current paradigm of galaxy evolution suggests that galaxies grow hierarchically via mergers, but our observational understanding of the role of mergers is still limited. The data from HSC-SSP are ideally suited to improve our understanding with improved identifications of interacting galaxies thanks to the superb depth and image quality of HSC-SSP. We launched a community science project, GALAXY CRUISE, in 2019 and have collected over two million independent classifications of 20686 galaxies at z  < 0.2. We first characterize the accuracy of the participants' classifications and demonstrate that it surpasses previous studies based on shallower imaging data. We then investigate various aspects of interacting galaxies in detail. We show that there is a clear sign of enhanced activities of super-massive black holes and star formation in interacting galaxies compared to those in isolated galaxies. The enhancement seems particularly strong for galaxies undergoing violent mergers. We also show that the mass growth rate inferred from our results is roughly consistent with the observed evolution of the stellar mass function. The second season of GALAXY CRUISE is currently underway and we conclude with future prospects. We make the morphological classification catalog used in this paper publicly available at the GALAXY CRUISE website, which will be particularly useful for machine-learning applications. [ABSTRACT FROM AUTHOR]

The excellent sensitivity and optimum resolution of LoTSS DR2 at 144 MHz has enabled us to discover a giant radio galaxy (J1225+4011) with three distinct episodes of jet activity, making it a member of a class of objects called triple-double radio galaxies (TDRGs). The source extends overall up to 1.35 Mpc in projected size, with the second episode extending to 572 kpc, and the inner episode to 118 kpc. J1225+4011 is only the fourth radio source showing a triple-double radio structure. All four sources have overall sizes greater than 700 kpc, making them giants. We also present the LoTSS 144 MHz map of the TDRG J0929+4146 and report its updated size. Lastly, we have summarized and discussed the radio properties of all TDRGs for the first time to understand their growth and evolution. Our observations suggest that the power of their jets may decrease with time. [ABSTRACT FROM AUTHOR]

Recent observational studies have uncovered a small number of very metal-poor (VMP) stars with cold kinematics in the Galactic disc and bulge. However, their origins remain enigmatic. We select a total of 138 Milky Way (MW) analogues from the TNG50 cosmological simulation based on their z = 0 properties: discy morphology, stellar mass, and local environment. In order to make more predictive statements for the MW, we further limit the spatial volume coverage of stellar populations in galaxies to that targeted by the upcoming 4MOST high-resolution survey of the Galactic disc and bulge. We find that across all galaxies, ∼20 per cent of VMP ([Fe/H] < −2) stars belong to the disc, with some analogues reaching 30 per cent. About 50 ± 10 per cent of the VMP disc stars are, on average, older than 12.5 Gyr and ∼70 ± 10 per cent come from accreted satellites. A large fraction of the VMP stars belong to the halo (∼70) and have a median age of 12 Gyr. Our results with the TNG50 cosmological simulation confirm earlier findings with simulations of fewer individual galaxies, and suggest that the stellar disc of the MW is very likely to host significant amounts of very- and extremely-metal-poor stars that, although mostly of ex situ origin, can also form in situ , reinforcing the idea of the existence of a primordial Galactic disc. [ABSTRACT FROM AUTHOR]

Jellyfish galaxies are prototypical examples of satellite galaxies undergoing strong ram pressure stripping (RPS). We analyse the evolution of 512 unique, first-infalling jellyfish galaxies from the TNG50 cosmological simulation. These have been visually inspected to be undergoing RPS sometime in the past 5 Byr (since z  = 0.5), have satellite stellar masses |$M_\star ^{\rm sat}\sim 10^{8\!-\!10.5}\, {\rm M}_\odot$|⁠ , and live in hosts with |$M_{\rm 200c}\sim 10^{12\!-\!14.3}\, {\rm M}_\odot$| at z  = 0. We quantify the cold gas (T ≤ 104.5 K) removal using the tracer particles, confirming that for these jellyfish, RPS is the dominant driver of cold gas loss after infall. Half of these jellyfish are completely gas-less by z  = 0, and these galaxies have earlier infall times and smaller satellite-to-host mass ratios than their gaseous counterparts. RPS can act on jellyfish galaxies over long time-scales of ≈1.5–8 Gyr. Jellyfish in more massive hosts are impacted by RPS for a shorter time span and, at a fixed host mass, jellyfish with less cold gas at infall and lower stellar masses at z  = 0 have shorter RPS time spans. While RPS may act for long periods of time, the peak RPS period – where at least 50 per cent of the total RPS occurs – begins within ≈1 Gyr of infall and lasts ≲2 Gyr. During this period, the jellyfish are at host-centric distances ∼0.2–2 R 200c, illustrating that much of RPS occurs at large distances from the host galaxy. Interestingly, jellyfish continue forming stars until they have lost ≈98 per cent of their cold gas. For groups and clusters in TNG50 |$(M_{\rm 200c}^{\rm host}\sim 10^{13\!-\!14.3}\, {\rm M}_\odot)$|⁠ , jellyfish galaxies deposit more cold gas (⁠|$\sim 10^{11\!-\!12}\, {\rm M}_\odot$|⁠) into haloes than what exists in them at z  = 0, demonstrating that jellyfish, and in general satellite galaxies, are a significant source of cold gas accretion. [ABSTRACT FROM AUTHOR]

We present a novel algorithm to detect double nuclei galaxies (DNG) called Gothic (G raph-b O os T ed iterated HI ll C limbing) – that detects whether a given image of a galaxy has two or more closely separated nuclei. Our aim is to test for the presence of dual/multiple active galactic nuclei (AGN) in galaxies that visually represent a DNG. Although galaxy mergers are common, the detection of dual AGN is rare. Their detection is very important as they help us understand the formation of supermassive black hole (SMBH) binaries, SMBH growth and AGN feedback effects in multiple nuclei systems. There is thus a need for an algorithm to do a systematic survey of existing imaging data for the discovery of DNGs and dual AGNs. We have tested Gothic on an established sample of DNGs with a 100 per cent detection rate and subsequently conducted a blind search of 1 million SDSS DR16 galaxies (with spectroscopic data available) lying in the redshift range of z  = 0 to 0.75. From the list of candidate DNGs found, we have detected 159 dual AGNs, of which 2 are triple AGN systems. Our results show that dual AGNs are not common, and triple AGN even rarer. The colour (u–r) magnitude plots of the DNGs indicate that star formation is quenched as the nuclei come closer and as the AGN fraction increases. The quenching is especially prominent for dual/triple AGN galaxies that lie at the extreme end of the red sequence. [ABSTRACT FROM AUTHOR]

We present the results of our new H  i observations for the nearby galaxy NGC 4449 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). More diffuse H  i gas was detected by FAST compared with previous studies. The measured extent of H  i gas is more than ∼19 arcmin (20 kpc) larger than that in the Very Large Array map, and the H  i gas disc is ∼22 times the optical diameter of NGC 4449. The total H  i mass detected for NGC 4449 is 3.07 |$\times 10^9\, \rm M_{\odot }$|⁠ , which is in agreement with that detected by the Effelsberg telescope. We found one H  i cloud from the channel map that is in superposition with the optical dwarf galaxy NGC 4449B. This cloud is slightly offset from the NGC 4449B disc, suggesting that it could be gas stripped from NGC 4449B's optical body when it was disrupted and pulled into the nuclear region of NGC 4449. We also find a region in the outer part of the H  i envelope with a peculiar velocity field and double-peaked spectral profiles, whose velocity distribution is consistent with the nearby companion dwarf galaxy DDO 125. This implies that NGC 4449 may have stripped gas out of DDO 125. We suggest that the irregularity of NGC 4449's huge H  i gas halo might originate from interactions with NGC 4449B and DDO 125. [ABSTRACT FROM AUTHOR]

Post-starburst galaxies are believed to be in a rapid transition between major merger starbursts and quiescent ellipticals. Their optical spectrum is dominated by A-type stars, suggesting a starburst that was quenched recently. While optical observations suggest little ongoing star formation, some have been shown to host significant molecular gas reservoirs. This led to the suggestion that gas depletion is not required to end the starburst, and that star formation is suppressed by other processes. We present NOEMA CO(1−0) observations of 15 post-starburst galaxies with emission lines consistent with active galactic nucleus (AGN) photoionization. We collect post-starburst candidates with molecular gas measurements from the literature, with some classified as classical E + A, while others with line ratios consistent with AGN and/or shock ionization. Using far-infrared observations, we show that systems that were reported to host exceptionally large molecular gas reservoirs host in fact obscured star formation, with some systems showing star formation rates comparable to ULIRGs. Among E + A galaxies with molecular gas measurements, 7 out of 26 (26 per cent) host obscured starbursts. Using far-infrared observations, post-starburst candidates show similar SFR– |$M_{\mathrm{H_2}}$| and Kennicutt–Schmidt relations to those observed in star-forming and starburst galaxies. In particular, there is no need to hypothesize star formation quenching by processes other than the consumption of molecular gas by star formation. The combination of optical, far-infrared, and CO observations indicates that some regions within these galaxies have been recently quenched, while others are still forming stars in highly obscured regions. All this calls into question the traditional interpretation of such galaxies. [ABSTRACT FROM AUTHOR]

Flattened and kinematically correlated planes of dwarf satellite galaxies have been observed in the Local Volume. The slinging out of satellites during host galaxy mergers has been suggested as a formation mechanism for these peculiar structures. We statistically examined the impact of major mergers on present-time satellite systems for the first time in a full cosmological context using the IllustrisTNG suite of hydrodynamic simulations. Mergers with mass ratios above 1/3 generally have a negligible or adverse impact on the phase-space correlation of observationally motivated satellites. Even high angular momentum mergers are inefficient at slinging satellites outward due to the extended nature of simulated satellite distributions. Furthermore, any potential merger imprint is partially washed out by post-merger accretion of satellites, while satellites bound to the merging haloes since the merger's beginning are disrupted and stripped of mass – minimizing the merger's influence on the present-time distribution of the most massive satellites after |$2{\!-\!}5\, \mathrm{Gyr}$|⁠. Constraining our sample to satellites bound to their host throughout the full duration of their system's last merger, we recover no particular improvement in their phase-space correlation. Instead, such participant satellites experience a contraction of their radial distribution during and after the merger, resulting in smaller absolute plane heights (but comparable axial ratios). Overall, major mergers do not appear to form correlated planes in a statistical sample. Mergers that efficiently transfer their angular momentum to satellite distributions can marginally enhance their phase-space correlation, but cannot form highly flattened and orbitally coherent configurations as observed in our local Universe. [ABSTRACT FROM AUTHOR]

Simulations of structure formation in the standard cold dark matter cosmological model quantify the dark matter halos of galaxies. Taking into account dynamical friction between dark matter halos, we investigate the past orbital dynamical evolution of the Magellanic Clouds in the presence of the Galaxy. Our calculations are based on a three-body model of rigid Navarro–Frenk–White profiles for dark matter halos but were verified in a previous publication by comparison to high-resolution N-body simulations of live self-consistent systems. Under the requirement that the LMC and SMC had an encounter within 20 kpc between 1 and 4 Gyr ago in order to allow the development of the Magellanic Stream, using the latest astrometric data, the dynamical evolution of the MW/LMC/SMC system is calculated backwards in time. With the employment of the genetic algorithm and a Markov-Chain Monte-Carlo method, the present state of this system is unlikely, with a probability of <10−9 (6σ complement), because the solutions found do not fit into the error bars for the observed plane-of-sky velocity components of the Magellanic Clouds. This implies that orbital solutions that assume dark matter halos, according to cosmological structure formation theory, to exist around the Magellanic Clouds and the Milky Way are not possible with a confidence of more than 6 sigma.

The gravitational wave (GW) antenna LISA will detect the signal from coalescing massive black hole binaries (MBHBs) of 104 − 107 M⊙, providing clues as to their formation and growth throughout cosmic history. Some of these events will be localised with a precision of several to less than a deg2, enabling the possible identification of their host galaxy. This work explores the properties of the host galaxies of LISA MBHBs below z ≲ 3. We generate a simulated lightcone using the semi-analytical model L-Galaxies applied to the merger trees of the high-resolution N-body cosmological simulation Millennium-II. The model shows that LISA MBHBs are expected to be found in optically dim (r > 20), star-forming (sSFR > 10−10 yr−1), gas-rich (fgas > 0.6), and disc-dominated (B/T < 0.7) low-mass galaxies of stellar masses 108 − 109 M⊙. However, these properties are indistinguishable from those of galaxies harbouring single massive black holes of comparable mass, making it difficult to select LISA hosts among the whole population of low-mass galaxies. Motivated by this, we explore the possibility of using merger signatures to select LISA hosts. We find that 40%−80% of the galaxies housing LISA MBHBs display merger features related to the interaction that brought the secondary MBH to the galaxy. Despite this, around 60% of dwarf galaxies placed in the surroundings of the LISA hosts will show these kinds of features as well, challenging the unequivocal detection of LISA hosts through the search for merger signatures. Consequently, the detection of an electromagnetic transient associated with the MBHB merger will be vital in order to pinpoint the star-forming dwarf galaxy where these binary systems evolve and coalesce. [ABSTRACT FROM AUTHOR]

Context. The physical mechanisms driving starbursts in dwarf galaxies are unclear, and the effects of mergers on star formation in these galaxies are still uncertain. Aims. We explore how the merger process affects star formation in metal-poor dwarf galaxies by analyzing high spatial resolution (∼70 pc) integral field spectrograph observations of ionized gas. Methods. We used archival data from the Very Large Telescope/Multi Unit Spectroscopic Explorer to map the spatial distribution of strong emission lines (e.g., Hβ, Hα, [O III] λ5007, and [N II] λ6583) in the nearby merging star-forming dwarf galaxy system NGC 4809/4810. Results. We identify 112 star-forming knots scattered among the two galaxies, where the gas-phase metallicity distribution is inhomogeneous and mixes with metal-poor and metal-rich ionized gas. Star-forming knots at the interacting region show lower metallicity, the highest star formation rates (SFRs), and the highest ratio of the SFR to the resolved main-sequence relation (rMSR). Ionized gas exhibits an obvious northeast–southwest velocity gradient in NGC 4809, but appears to be mixed in NGC 4810. The high virial parameters and the stellar mass-size relation of the H II regions indicate that these regions are dominated by direct radiation pressure from massive stars (or clusters) and expand persistently. We find two different relations of the stellar mass surface density to stellar age in NGC 4809 and NGC 4810, and the stellar ages of NGC 4810 are systematically younger than those in NGC 4809. Conclusions. Our study suggests that the merging stage of two dwarf galaxies can induce starburst activities in the interaction areas despite the metal-deficient environment. Considering the high specific SFRs and different stellar ages, we propose that the interaction initially triggered star formation in NGC 4809 and then drove star formation in NGC 4810. [ABSTRACT FROM AUTHOR]

Context. In the standard cosmological model of galaxy evolution, mergers and interactions play a fundamental role in shaping galaxies. Galaxies that are currently isolated are thus interesting because they allow us to distinguish between internal and external processes that affect the galactic structure. However, current observational limits may obscure crucial information in the low-mass or low-brightness regime. Aims. We use optical imaging of a subsample of the AMIGA catalogue of isolated galaxies to explore the impact of different factors on the structure of these galaxies. In particular, we study the type of disc break as a function of the degree of isolation and the presence of interaction indicators such as tidal streams or plumes, which are only detectable in the ultra-low surface brightness regime. Methods. We present ultra-deep optical imaging in the r band of a sample of 25 low-redshift (z < 0.035) isolated galaxies. Through careful data processing and analysis techniques, the nominal surface brightness limits achieved are comparable to those to be obtained on the ten-year LSST coadds (μr,lim ≳ 29.5 mag arcsec−2 [3σ; 10″ × 10″]). We place special emphasis on preserving the low surface brightness features throughout the processing. Results. The extreme depth of our imaging allows us to study the interaction signatures of 20 galaxies since Galactic cirrus is a strong limiting factor in the characterisation of interactions for the remaining 5 of them. We detect previously unreported interaction features in 8 (40% ± 14%) galaxies in our sample. We identify 9 galaxies (36% ± 10%) with an exponential disc (Type I), 14 galaxies (56% ± 10%) with a down-bending (Type II) profile, and only 2 galaxies (8% ± 5%) with up-bending (Type III) profiles. Isolated galaxies have considerably more purely exponential discs and fewer up-bending surface brightness profiles than field or cluster galaxies. We find clear minor merger activity in some of the galaxies with single exponential or down-bending profiles, and both of the galaxies with up-bending profiles show signatures of a past interaction. Conclusions. We show the importance of ultra-deep optical imaging in revealing faint external features in galaxies that indicate a probable history of interaction. We confirm that up-bending profiles are likely produced by major mergers, while down-bending profiles are probably formed by a threshold in star formation. Unperturbed galaxies that slowly evolve with a low star formation rate could induce the high rate of Type I discs in isolated galaxies. [ABSTRACT FROM AUTHOR]

The chemical evolution history of the Small Magellanic Cloud (SMC) has been a matter of debate for decades. The challenges in understanding the SMC chemical evolution are related to a very slow star formation rate (SFR) combined with bursts triggered by the multiple interactions between the SMC and the Large Magellanic Cloud, a significant (∼0.5 dex) metallicity dispersion for the SMC cluster population younger than about 7.5 Gyr, and multiple chemical evolution models tracing very different paths through the observed age–metallicity relation of the SMC. There is no doubt that these processes were complex. Therefore, a step-by-step strategy is required in order to better understand the SMC chemical evolution. We adopted an existing framework to split the SMC into regions on the sky, and we focus on the west halo in this work, which contains the oldest and most metal-poor stellar populations and is moving away from the SMC, that is, in an opposite motion with respect to the Magellanic Bridge. We present a sample containing ∼60% of all west halo clusters to represent the region well, and we identify a clear age–metallicity relation with a tight dispersion that exhibits a 0.5 dex metallicity dip about 6 Gyr ago. We ran chemical evolution models and discuss possible scenarios to explain this metallicity dip, the most likely being a major merger accelerating the SFR after the event. This merger should be combined with inefficient internal gas mixing within the SMC and different SFRs in different SMC regions because the same metallicity dip is not seen in the AMR of the SMC combining clusters from all regions. We try to explain the scenario to better understand the SMC chemo-dynamical history. [ABSTRACT FROM AUTHOR]

The relation between the resolved star formation rate (SFR) per unit area and the nonthermal radio continuum emission is studied in 21 Virgo cluster galaxies and the two nearby spiral galaxies, NGC 6946 and M 51. For the interpretation and understanding of our results, we used a 3D model where star formation, 2D cosmic-ray (CR) propagation, and the physics of synchrotron emission are included. Based on the linear correlation between the SFR per unit area and the synchrotron emission and its scatter, radio-bright and radio-dim regions can be robustly defined for our sample of spiral galaxies. We identified CR diffusion or streaming as the physical causes of radio-bright regions of unperturbed symmetric spiral galaxies as NGC 6946. The enhanced magnetic field in the region of interstellar medium (ISM) compression via ram pressure is responsible for the southwestern radio-bright region in NGC 4501. We identified the probable causes of radio-bright regions in several galaxies as CR transport, via either gravitational tides (M 51) or galactic winds (NGC 4532) or ram pressure stripping (NGC 4330 and NGC 4522). Three galaxies are overall radio dim: NGC 4298, NGC 4535, and NGC 4567. Based on our model of synchrotron-emitting disks, we suggest that the overall radio-dim galaxies have a significantly lower magnetic field than expected by equipartition between the magnetic and turbulent energy densities. We suggest that this is linked to the difference between the timescales of the variation in the SFR and the small-scale dynamo. In NGC 4535, shear motions increase the total magnetic field strength via the induction equation, which leads to enhanced synchrotron emission with respect to the SFR in an otherwise radio-dim galactic disk. Radio-bright regions frequently coincide with asymmetric ridges of polarized radio continuum emission, and we found a clear albeit moderate correlation between the polarized radio continuum emission and the radio/SFR ratio. When compression or shear motions of the ISM are present in the galactic disk, the radio-bright regions are linked to the commonly observed asymmetric ridges of polarized radio continuum emission and represent a useful tool for the interaction diagnostics. The magnetic field is enhanced (as observed in NGC 4535 and NGC 4501) and ordered by these ISM compression and shear motions. Whereas the enhancement of the magnetic field is rather modest and does not significantly influence the radio-SFR correlation, the main effect of ISM compression and shear motions is the ordering of the magnetic field, which significantly affects the CR transport. Cosmic-ray energy losses and transport also affect the spectral index, which we measured between 4.85 and 1.4 GHz. The influence of CR losses and transport on the spectral index distribution with respect to the synchrotron/SFR ratio is discussed with the help of model calculations. Based on our results, we propose a scenario for the interplay between star formation, CR electrons, and magnetic fields in spiral galaxies. [ABSTRACT FROM AUTHOR]

We present CEERS JWST/NIRCam imaging of a massive galaxy group at z = 1.85, to explore the early JWST view on massive group formation in the distant Universe. The group contains ≳16 members (including six spectroscopic confirmations) down to log10(M⋆/M⊙) = 8.5, including the brightest group galaxy (BGG) in the process of actively assembling at this redshift. The BGG is comprised of multiple merging components extending ∼3.6″ (30 kpc) across the sky. The BGG contributes 69% of the group's total galactic stellar mass, with one of the merging components containing 76% of the total mass of the BGG and a star formation rate > 1810 M⊙ yr−1. Most importantly, we detected intra-halo light (IHL) in several HST and JWST/NIRCam bands, allowing us to construct a state-of-the-art rest-frame UV-NIR spectral energy distribution of the IHL for the first time at this high redshift. This allows stellar population characterisation of both the IHL and member galaxies, as well as the morphology distribution of group galaxies versus their star formation activity when coupled with Herschel data. We created a stacked image of the IHL, giving us a sensitivity to extended emission of 28.5 mag arcsec−2 at rest-frame 1 μm. We find that the IHL is extremely dust poor (Av ∼ 0), containing an evolved stellar population of log10(t50/yr) = 8.8, corresponding to a formation epoch for 50% of the stellar material 0.63 Gyr before z = 1.85. There is no evidence of ongoing star formation in the IHL. The IHL in this group at z = 1.85 contributes ∼10% of the total stellar mass, comparable with what is observed in local clusters. This suggests that the evolution of the IHL fraction is more self-similar with redshift than predicted by some models, challenging our understanding of IHL formation during the assembly of high-redshift clusters. JWST is unveiling a new side of group formation at this redshift, which will evolve into Virgo-like structures in the local Universe. [ABSTRACT FROM AUTHOR]

We report the 3D kinematics of 27 Mira-like stars in the northern, eastern, and southern periphery of the Large Magellanic Cloud (LMC), based on Gaia proper motions and a dedicated spectroscopic follow-up. Low-resolution spectra were obtained for more than 40 Mira-like candidates, selected to trace known substructures in the LMC periphery. Radial velocities and stellar parameters were derived for all stars. Gaia data release 3 astrometry and photometry were used to discard outliers, derive periods for those stars with available light curves, and determine their photometric chemical types. The 3D motion of the stars in the reference frame of the LMC revealed that most of the stars, in all directions, have velocities consistent with being part of the LMC disc population, out of equilibrium in the radial and vertical directions. A suite of numerical simulations was used to constrain the most likely past interaction history between the Clouds given the phase-space distribution of our targets. Model realizations in which the Small Magellanic Cloud (SMC) had three pericentric passages around the LMC best resemble the observations. The interaction history of those model realizations has a recent SMC pericentric passage (∼320 Myr ago), preceded by an SMC crossing of the LMC disc at ∼0.97 Gyr ago, having a radial crossing distance of only ∼4.5 kpc. The previous disc crossing of the SMC was found to occur at ∼1.78 Gyr ago, with a similar radial crossing distance of ∼5.6 kpc. [ABSTRACT FROM AUTHOR]

We present a spatially resolved stellar population analysis of 61 jellyfish galaxies and 47 control galaxies observed with ESO/MUSE attempting to understand the general trends of the stellar populations as a function of the stripping intensity and mass. This is the public sample from the GASP programme, with 0.01 < z < 0.15 and 8.9 < log (M ⋆/M⊙) < 12.0. We apply the spectral population synthesis code fado to fit self-consistently both the stellar and nebular contributions to the spectra of the sources. We present 2D morphological maps for mean stellar ages, metallicities, gas-phase oxygen abundances, and star formation rates for the galaxies with Integrated Nested Laplace Approximation (inla), which is efficient in reconstructing spatial data of extended sources. We find that 'extreme stripping' and 'stripping' galaxies are typically younger than the other types. Regarding stellar and nebular metallicities, the 'stripping' and 'control passive' galaxies are the most metal-poor. Based on the phase space for jellyfish cluster members we find trends in ages, metallicities, and abundances with different regions of the diagram. We also compute radial profiles for the same quantities. We find that both the stripping and the stellar masses seem to influence the profiles, and we see differences between various groups and distinct mass bins. The radial profiles for different mass bins present relations already shown in the literature for undisturbed galaxies, i.e. profiles of ages and metallicities tend to increase with mass. However, beyond ∼0.75 effective radius, the ages of the most massive galaxies become similar to or lower than the ages of the lower mass ones. [ABSTRACT FROM AUTHOR]

We conduct a comparison of the merging galaxy populations detected by a sample of visual identification of tidal features around galaxies as well as spectroscopically detected close pairs of galaxies to determine whether our method of selecting merging galaxies biases our understanding of galaxy interactions. Our volume-limited parent sample consists of 852 galaxies from the Galaxy And Mass Assembly (GAMA) survey in the redshift range 0.04 ≤ z ≤ 0.20 and stellar mass range 9.50  ≤  log  |$_{10}(M_{\star }/\rm {M}_{\odot })\le ~11.0$|⁠. We conduct our comparison using images from the Ultradeep layer of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) to visually classify galaxies with tidal features and compare these to the galaxies in the GAMA spectroscopic close-pair sample. We identify 198 galaxies possessing tidal features, resulting in a tidal feature fraction f tidal = 0.23 ± 0.02. We also identify 80 galaxies involved in close pairs, resulting in a close pair fraction f pair = 0.09 ± 0.01. Upon comparison of our tidal feature and close pair samples we identify 42 galaxies that are present in both samples, yielding a fraction f both = 0.05 ± 0.01. We find evidence to suggest that the sample of close pairs of galaxies is more likely to detect early stage mergers, where two separate galaxies are still visible, and the tidal feature sample detects later-stage mergers, where only one galaxy nucleus remains visible. The overlap of the close pair and tidal feature samples likely detect intermediate-stage mergers. Our results are in good agreement with the predictions of cosmological hydrodynamical simulations regarding the populations of merging galaxies detected by close pair and tidal feature samples. [ABSTRACT FROM AUTHOR]

Thin galactic discs and nuclear stellar discs (NSDs) are fragile structures that can be easily disturbed by merger events. By studying the age of the stellar populations in present-day discs, we can learn about the assembly history of galaxies and place constraints on their past merger events. Following on the steps of our initial work, we explore the fragility of such disc structures in intermediate-mass-ratio dry encounters using the previously constructed N -body model of the Fornax galaxy NGC 1381 (FCC 170), which hosts both a thin galactic disc and an NSD. We dismiss major and minor encounters, as the former were previously shown to easily destroy thin-disc structures, whereas the latter take several Hubble times to complete in the specific case of FCC 170. The kinematics and structure of the thin galactic disc are dramatically altered by the mergers, whereas the NSD shows a remarkable resilience, exhibiting only a smooth increase of its size when compared to the model evolved in isolation. Our results suggest that thin galactic discs are better tracers for intermediate-mass-ratio mergers, while NSDs may be more useful for major encounters. Based on our simulations and previous analysis of the stellar populations, we concluded that FCC 170 has not experienced any intermediate-mass-ratio dry encounters for at least ∼10 Gyr, as indicated by the age of its thin-disc stellar populations. [ABSTRACT FROM AUTHOR]

We present a study on the incidence of major mergers and their impact on the triggering of nuclear activity in 47 type I and 236 type II optically selected AGN from the MaNGA DR15 sample. From an estimate of non-parametric image predictors (Gini , M20, concentration (C), asymmetry (A), clumpiness (S), Sérsic index (n), and shape asymmetry (AS)) using the SDSS images, in combination with a Linear Discriminant Analysis Method, we identified major mergers and merger stages. We reinforced our results by looking for bright tidal features in our post-processed SDSS and DESI legacy images. We find a statistically significant higher incidence of major mergers of 29 per cent ± 3 per cent in our type I+II AGN sample compared to 22 per cent ± 0.8 per cent for a non-AGN sample matched in redshift, stellar mass, colour, and morphological type, finding also a prevalence of post-coalescence (51 per cent ± 5 per cent) over pre-coalescence (23 per cent ± 6 per cent) merger stages. The levels of AGN activity among our massive major mergers are similar to those reported in other works using [O  iii] tracers. However, similar levels are produced by our AGN-galaxies hosting stellar bars, suggesting that major mergers are important promoters of nuclear activity but are not the main nor the only mechanism behind the AGN triggering. The tidal strength parameter Q was considered at various scales looking for environmental differences that could affect our results on the merger incidence, finding non-significant differences. Finally, the H-H β diagram could be used as an empirical predictor for the flux coming from an AGN source, useful to correct photometric quantities in large AGN samples emerging from surveys. [ABSTRACT FROM AUTHOR]

We report the discovery of a 100 kpc H  i tail in the merging galaxy pair NGC 4490/85 detected by the Five-Hundred-meter Aperture Spherical radio Telescope (FAST). The tidal tails extended in both the south and north directions, and they are much longer than that reported previously based on the VLA interferometric maps. The NGC 4490/85 is surrounded by a large gas envelope, and a starburst low-metallicity dwarf galaxy MAPS 1231+42 is found to be connected with the gas envelope, indicating that galaxy interaction trigged the intense star formation in it. Based on the fact that the metallicity in MAPS 1231+42 is one order of magnitude lower than that in the two discs of NGC 4490 and NGC 4485, we speculate that the gas near this galaxy should be primordial and could be due to gas inflow from the circumgalactic medium. We also found a collimated gas component pointing at a nearby dwarf galaxy KK 149, suggesting that this galaxy might also be interacting with the NGC 4490 pair. We discuss the possible origin of the long tidal tails and the extended gas envelope in this merging system based on the new data from FAST. [ABSTRACT FROM AUTHOR]

Post-starburst galaxies (PSBs) are transition galaxies showing evidence of recent rapid star formation quenching. To understand the role of galaxy mergers in triggering quenching, we investigate the incidence of PSBs and resolved PSB properties in post-merger galaxies using both SDSS single-fibre spectra and MaNGA resolved IFU spectra. We find post-mergers have a PSB excess of 10–20 times that relative to their control galaxies using single-fibre PSB diagnostics. A similar excess of ∼ 19 times is also found in the fraction of central (C)PSBs and ring-like (R)PSBs in post-mergers using the resolved PSB diagnostic. However, 60 per cent of the CPSBs + RPSBs in both post-mergers and control galaxies are missed by the single-fibre data. By visually inspecting the resolved PSB distribution, we find that the fraction of outside-in quenching is seven times higher than inside-out quenching in PSBs in post-mergers while PSBs in control galaxies do not show large differences in these quenching directions. In addition, we find a marginal deficit of H  i gas in PSBs relative to non-PSBs in post-mergers using the MaNGA-H  i data. The excesses of PSBs in post-mergers suggest that mergers play an important role in triggering quenching. Resolved IFU spectra are important to recover the PSBs missed by single-fibre spectra. The excess of outside-in quenching relative to inside-out quenching in post-mergers suggests that AGNs are not the dominant quenching mechanism in these galaxies, but that processes from the disc (gas inflows/consumption and stellar feedback) play a more important role. [ABSTRACT FROM AUTHOR]

We present a kinematic analysis of the Small Magellanic Cloud using 3700 spectra extracted from the European Southern Observatory archive. We used data from Gaia and near-infrared photometry to select stellar populations and discard Galactic foreground stars. The sample includes main-sequence red giant branch and red clump stars, observed with Fibre Large Array Multi Wavelength Spectrograph. The spectra have a resolving power λ/Δλ from 6500 to 38 000. We derive radial velocities by employing a full spectrum fitting method using a penalized pixel fitting routine. We obtain a mean radial velocity for the galaxy of 159 ± 2 km s−1, with a velocity dispersion of 33 ± 2 km s−1. Our velocities agree with literature estimates for similar (young or old) stellar populations. The radial velocity of stars in the Wing and bar-like structures differ as a consequence of the dynamical interaction with the Large Magellanic Cloud. The higher radial velocity of young main-sequence stars in the bar compared to that of supergiants can be attributed to star formation around 40 Myr ago from gas already influenced by tidal stripping. Similarly, young main-sequence stars in the northern part of the bar, resulting from a prominent star forming episode 25 Myr ago, have a higher radial velocity than stars in the southern part. Radial velocity differences between the northern and southern bar overdensities are also traced by giant stars. They are corroborated by studies of the cold gas and proper motion indicating stretching/tidal stripping of the galaxy. [ABSTRACT FROM AUTHOR]

We investigate the role of the close environment on the nuclear activity of a sample of 436 nearby (z < 0.3) QSO 2's – selected from SDSS-III spectra, via comparison of their environment and interaction parameters with those of a control sample of 1308 galaxies. We have used the corresponding SDSS images to obtain the number of neighbour galaxies N , tidal strength parameter Q and asymmetry parameters. We find a small excess of N in the QSOs compared to its three controls, and no difference in Q. The main difference is an excess of asymmetry in the QSOs hosts, which is almost twice that of the control galaxies. This difference is not due to the hosts' morphology, since there is no difference in their Galaxy Zoo classifications. HST images of two highly asymmetric QSO 2 hosts of our sample show that both sources have a close companion (at projected separations ∼ 5 kpc), which we thus conclude is the cause of the observed asymmetry in the lower resolution SDSS images. The mean projected radius of the controls is 〈 r 〉 = 8.53 ± 0.06 kpc, while that of the QSO hosts is 〈 r 〉 = 9.39 ± 0.12 kpc, supporting the presence of interaction signatures in the outer regions of the QSO hosts. Our results favour a scenario in which nuclear activity in QSO 2's is triggered by close galaxy interactions – when the distance between the host and companion is of the order of the galaxy radius, implying that they are already in the process of merger. [ABSTRACT FROM AUTHOR]

We study interacting galaxy pairs in the TNG100-1 and TNG300-1 cosmological simulations using previously generated closest companion samples. We study the specific star-formation rates (sSFRs) of massive (⁠|$10^{10} \, {\rm M}_{\odot } \lt M_* \lt 10^{12} \, {\rm M}_{\odot }$|) galaxies at z ≤ 0.2 as a function of separation from the closest companion galaxy. We split our sample based on whether the companion galaxy is star-forming or passive. We find that galaxies with close star-forming companions have sSFRs that are enhanced (on average) by a factor of 2.9 ± 0.3 in TNG100-1 and 2.27 ± 0.06 in TNG300-1 compared to controls, with enhancements present out to separations of ∼300 kpc. Galaxies with passive companions in TNG300-1 exhibit mild sSFR suppression (∼12 per cent) at 100–300 kpc and small sSFR enhancements at separations below 50 kpc. sSFR suppression is strongest in pairs where the galaxy's stellar mass is more than 2 times that of its passive companion. By generating a stellar mass-matched ('twinned') sample in TNG300-1, we show that differences in sSFR trends between companion types are not a result of intrinsic stellar mass differences in star-forming versus passive galaxies. We compare with an analogous sample of galaxy pairs from SDSS, finding consistent results between observations and simulations. Overall, we find that star-forming galaxies show enhanced sSFRs regardless of companion type, but that galaxies with close passive companions are more likely to be passive themselves. [ABSTRACT FROM AUTHOR]

We utilize high-resolution cosmological simulations to reveal that high-redshift galaxies tend to undergo a robust 'wet compaction' event when near a 'golden' stellar mass of |$\sim \!\!10^{10}\, \rm M_\odot$|. This is a gaseous shrinkage to a compact star-forming phase, a 'blue nugget' (BN), followed by central quenching of star formation to a compact passive stellar bulge, a 'red nugget' (RN), and a buildup of an extended gaseous disc and ring. Such nuggets are observed at cosmic noon and seed today's early-type galaxies. The compaction is triggered by a drastic loss of angular momentum due to, e.g. wet mergers, counter-rotating cold streams, or violent disc instability. The BN phase marks drastic transitions in the galaxy structural, compositional, and kinematic properties. The transitions are from star forming to quenched inside-out, from diffuse to compact with an extended disc or ring and a stellar envelope, from dark matter to baryon central dominance, from prolate to oblate stellar shape, from pressure to rotation support, from low to high metallicity, and from supernova to AGN feedback. The central black hole growth, first suppressed by supernova feedback when below the golden mass, is boosted by the compaction, and the black hole keeps growing once the halo is massive enough to lock in the supernova ejecta. [ABSTRACT FROM AUTHOR]

One of the most protracted problems in astronomy has been understanding the evolution of galaxy morphology. Much discussion has surrounded how lenticular galaxies may form a bridging population between elliptical and spiral galaxies. However, with recourse to a galaxy's central black hole mass, accretion-built spiral galaxies have emerged as the bridging population between low-mass lenticular galaxies and the dusty merger-built lenticular galaxies contiguous with elliptical galaxies and 'brightest cluster galaxies' in the black hole/galaxy mass diagram. Spiral galaxies, including the Milky Way, appear built from gas accretion and minor mergers onto what were initially lenticular galaxies. These connections are expressed as a new morphology sequence, dubbed the 'Triangal', which subsumes elements of the Hubble sequence and the van den Bergh trident and reveals the bridging nature of the often overlooked ellicular galaxies. Furthermore, a quadratic black hole/galaxy mass relation is found to describe ordinary elliptical galaxies. The relation is roughly parallel to the quadratic-like relations observed for the central spheroidal component of spiral galaxies, dust-rich lenticular galaxies, and old dust-poor lenticular galaxies. The brightest cluster galaxies are offset according to expectations from an additional major merger. The findings have implications for feedback from active galactic nuclei, mapping morphology into simulations, and predicting gravitational wave signals from colliding supermassive black holes. A new galaxy speciation model is presented. It disfavours the 'monolithic collapse' scenario for spiral, dusty lenticular, and elliptical galaxies. It reveals substantial orbital angular momentum in the Universe's first galaxies and unites dwarf and ordinary 'early-type' galaxies. [ABSTRACT FROM AUTHOR]

Using CANUCS imaging we found an apparent major merger of two |$z$| ∼ 5 ultra-low-mass galaxies (M ⋆∼107M⊙ each) that are doubly imaged and magnified ∼12–15× by the lensing cluster MACS 0417. Both galaxies are experiencing young (∼100 Myr), synchronised bursts of star formation with log (sSFR/Gyr−1) ∼1.3–1.4, yet SFRs of just ∼0.2M⊙ yr−1. They have sub-solar (Z ∼ 0.2Z⊙) gas-phase metallicities and are connected by an even more metal-poor star-forming bridge. The galaxy that forms from the merger will have a mass of at least M ⋆∼2 × 107 M⊙, at least half of it formed during the interaction-induced starburst. More than half of the ionizing photons produced by the system (before and during the merger) will have been produced during the burst. This system provides the first detailed look at a merger involving two high- |$z$| ultra-low-mass galaxies of the type believed to be responsible for reionizing the Universe. It suggests that such galaxies can grow via a combination of mass obtained through major mergers, merger-triggered starbursts, and long-term in-situ star formation. If such high- |$z$| mergers are common, then merger-triggered starbursts could be significant contributors to the ionizing photon budget of the Universe. [ABSTRACT FROM AUTHOR]

The triggering mechanism for the most luminous, quasar-like active galactic nuclei (AGN) remains a source of debate, with some studies favouring triggering via galaxy mergers, but others finding little evidence to support this mechanism. Here, we present deep Isaac Newton Telescope/Wide Field Camera imaging observations of a complete sample of 48 optically selected type 2 quasars – the QSOFEED sample (⁠|$L_{\rm [O\, \small {III}]}\gt 10^{8.5}\, \mathrm{L}_{\odot }$|⁠ ; z < 0.14). Based on visual inspection by eight classifiers, we find clear evidence that galaxy interactions are the dominant triggering mechanism for quasar activity in the local universe, with 65 |$^{+6}_{-7}$|  per cent of the type 2 quasar hosts showing morphological features consistent with galaxy mergers or encounters, compared with only 22 |$^{+5}_{-4}$|  per cent of a stellar-mass- and redshift-matched comparison sample of non-AGN galaxies – a 5σ difference. The type 2 quasar hosts are a factor of 3.0 |$^{+0.5}_{-0.8}$| more likely to be morphologically disturbed than their matched non-AGN counterparts, similar to our previous results for powerful 3CR radio AGN of comparable [O  iii ] emission-line luminosity and redshift. In contrast to the idea that quasars are triggered at the peaks of galaxy mergers as the two nuclei coalesce, and only become visible post-coalescence, the majority of morphologically disturbed type 2 quasar sources in our sample are observed in the pre-coalescence phase (61 |$^{+8}_{-9}$|  per cent). We argue that much of the apparent ambiguity that surrounds observational results in this field is a result of differences in the surface brightness depths of the observations, combined with the effects of cosmological surface brightness dimming. [ABSTRACT FROM AUTHOR]

We present, for the first time, spatially resolved spectroscopy for the entire Hickson Compact Group 31 obtained with the MUSE instrument at the VLT and an in-depth analysis of this compact group. To obtain a complete understanding of the system, we derived radial velocity and dispersion velocity maps, maps of the ionization mechanism of the system, chemical abundances and their distribution over the whole system, star formation rates and ages of the different star-forming regions, and the spatial distribution of the Wolf–Rayet stellar population. We also reconstructed the star formation history of the galaxies HCG 31 A, C, B, and F, measured the emission-line fluxes, and performed a stellar population synthesis. Our main findings are: (i) that there is clearly disturbed kinematics due to the merger event that the system is experiencing; (ii) that the ionization is produced exclusively via star formation except for the nucleus of the galaxy HCG 31 A, where there is a small contribution of shocks; (iii) that there is low oxygen abundance distributed homogeneously through the system; (iv) that there is a prominent population of carbon Wolf–Rayet stars in the central zone of the group; and (v) that there are clear evidences of the tidal origin of the galaxies HCG 31 E, HCG 31 H, and HCG 31 F because they show quite high oxygen abundances for their stellar mass. All these findings are clear evidence that HCG 31 is currently in an early merging phase and manifesting a starburst in its central region. [ABSTRACT FROM AUTHOR]

Nearby metal-poor starburst dwarf galaxies present a unique opportunity to probe the physics of high-density star formation with a detail and sensitivity unmatched by any observation of the high- z Universe. Here, we present the first results from a chemodynamical study of the nearby, gas-rich starburst dwarf CGCG 007-025. We use VLT/MUSE integral field spectroscopy to characterize the properties of the star-forming (SF) gas, from its metal content to its kinematics. The star formation rate (SFR) surface density presents a clumpy distribution, with the brightest knot hosting a 5 Myr young, Wolf–Rayet (WR) population (revealed by the presence of the characteristic 5808 Å WR bump). The ionized gas kinematics are dominated by disordered motions. A superposition of a narrow (σ ≈ 30km s−1), intermediate (150 km s−1), and broad (1000 km s−1) kinematic components are needed to model the emission-line profiles in the brightest SF region, suggesting the presence of energetic outflows from massive stars. The gas-phase metallicity of the galaxy spans 0.6 dex and displays a strong anticorrelation with SFR surface density, dropping to 12 + log(O/H) = 7.7 in the central SF knot. The spatially resolved BPTs indicate the gas is being ionized purely by SF processes. Finally, the anticorrelation between the SFR and the gas metallicity points out to accretion of metal-poor gas as the origin of the recent off-centre starburst, in which the infalling material ignites the SF episode. [ABSTRACT FROM AUTHOR]

NGC 5291, an early-type galaxy surrounded by a giant H  i ring, is believed to be formed from collision with another galaxy. Several star forming complexes and tidal dwarf galaxies are distributed along the collisional ring which are sites of star formation in environments where extreme dynamical effects are involved. Dynamical effects can affect the star formation properties and the spatial distribution of star forming complexes along the tidal features. To study and quantify the star formation activity in the main body and in the ring structure of the NGC 5291 system, we use high spatial resolution FUV and NUV imaging observations from the Ultraviolet Imaging Telescope onboard AstroSat. A total of 57 star-forming knots are identified to be part of this interacting system out of which 12 are new detections (star forming complexes that lie inside the H  i contour) compared to the previous measurements from lower resolution UV imaging. We estimate the attenuation in UV for each of the resolved star-forming knots using the UV spectral slope β, derived from the FUV − NUV colour. Using the extinction corrected UV fluxes, we derive the star formation rate of the resolved star forming complexes. The extinction corrected total star formation rate of this system is estimated as 1.75 ± 0.04 |$\mathrm{M_\odot \, yr}^{-1}$|⁠. The comparison with dwarf galaxy populations (BCD, Sm, and dIm galaxies) in the nearby Universe shows that many of the knots in the NGC 5291 system have SFR values comparable to the SFR of BCD galaxies. [ABSTRACT FROM AUTHOR]

It is difficult to accurately identify galaxy mergers and it is an even larger challenge to classify them by their mass ratio or merger stage. In previous work we used a suite of simulated mergers to create a classification technique that uses linear discriminant analysis to identify major and minor mergers. Here, we apply this technique to 1.3 million galaxies from the SDSS DR16 photometric catalogue and present the probability that each galaxy is a major or minor merger, splitting the classifications by merger stages (early, late, post-coalescence). We present publicly available imaging predictor values and all of the above classifications for one of the largest-yet samples of galaxies. We measure the major and minor merger fraction (f merg) and build a mass-complete sample of galaxies, which we bin as a function of stellar mass and redshift. For the major mergers, we find a positive slope of f merg with stellar mass and negative slope of f merg with redshift between stellar masses of 10.5 < M *(log M⊙) < 11.6 and redshifts of 0.03 < z < 0.19. We are able to reproduce an artificial positive slope of the major merger fraction with redshift when we do not bin for mass or craft a complete sample, demonstrating the importance of mass completeness and mass binning. We determine that the positive trend of the major merger fraction with stellar mass is consistent with a hierarchical assembly scenario. The negative trend with redshift requires that an additional assembly mechanism, such as baryonic feedback, dominates in the local Universe. [ABSTRACT FROM AUTHOR]

Using the data set of The Three Hundred project, i.e. 324 hydrodynamical resimulations of cluster-sized haloes and the regions of radius 15 |${{h^{-1}\, {\rm Mpc}}}$| around them, we study galaxy pairs in high-density environments. By projecting the galaxies' 3D coordinates onto a 2D plane, we apply observational techniques to find galaxy pairs. Based on a previous theoretical study on galaxy groups in the same simulations, we are able to classify the observed pairs into 'true' or 'false', depending on whether they are gravitationally bound or not. We find that the fraction of true pairs (purity) crucially depends on the specific thresholds used to find the pairs, ranging from around 30 to more than 80 per cent in the most restrictive case. Nevertheless, in these very restrictive cases, we see that the completeness of the sample is low, failing to find a significant number of true pairs. Therefore, we train a machine learning algorithm to help us identify these true pairs based on the properties of the galaxies that constitute them. With the aid of the machine learning model trained with a set of properties of all the objects, we show that purity and completeness can be boosted significantly using the default observational thresholds. Furthermore, this machine learning model also reveals the properties that are most important when distinguishing true pairs, mainly the size and mass of the galaxies, their spin parameter, gas content, and shape of their stellar components. [ABSTRACT FROM AUTHOR]

Supermassive black holes (SMBHs) are believed to reside at the centre of massive galaxies such as brightest cluster galaxies (BCGs). However, as BCGs experienced numerous galaxy mergers throughout their history, the central BH can be significantly kicked from the central region by these dynamical encounters. By combining the TNG300 simulations and orbital integrations, we demonstrate that mergers with satellite galaxies on radial orbits are a main driver for such BH displacements in BCGs. BHs can get ejected to distances varying between a few parsecs to hundreds of kiloparsecs. Our results clearly establish that SMBH offsets are common in BCGs and more precisely a third of our BHs are off-centred at z  = 0. This orbital offset can be sustained for up to at least 6 Gyr between z  = 2 and z  = 0 in half of our BCGs. Since the dense gas reservoirs are located in the central region of galaxies, we argue that the consequences of off-centre SMBHs in BCGs are to quench any BH growth and BH feedback. [ABSTRACT FROM AUTHOR]

We present a study of the metal-enriched halo gas, traced using Mg  ii and [O  ii ] emission lines, in two large, blind galaxy surveys – the MUSE (Multi Unit Spectroscopic Explorer) Analysis of Gas around Galaxies (MAGG) and the MUSE Ultra Deep Field (MUDF). By stacking a sample of ≈600 galaxies (stellar masses |$M_*$|  ≈ 106–12  |$\rm M_\odot$|⁠), we characterize for the first time the average metal line emission from a general population of galaxy haloes at 0.7 ≤ z ≤ 1.5. The Mg  ii and [O  ii ] line emission extends farther out than the stellar continuum emission, on average out to ≈25 and ≈45 kpc, respectively, at a surface brightness (SB) level of 10−20  |$\rm erg\, s^{-1}\, cm^{-2}\, arcsec^{-2}$|⁠. The radial profile of the Mg  ii SB is shallower than that of the [O  ii ], suggesting that the resonant Mg  ii emission is affected by dust and radiative transfer effects. The [O  ii ] to Mg  ii SB ratio is ≈3 over ≈20–40 kpc, also indicating a significant in situ origin of the extended metal emission. The average SB profiles are intrinsically brighter by a factor of ≈2–3 and more radially extended by a factor of ≈1.3 at 1.0 < z ≤ 1.5 than at 0.7 ≤ z ≤ 1.0. The average extent of the metal emission also increases independently with increasing stellar mass and in overdense group environments. When considering individual detections, we find extended [O  ii ] emission up to ≈50 kpc around ≈30–40 per cent of the group galaxies, and extended (≈30–40 kpc) Mg  ii emission around two z ≈ 1 quasars in groups, which could arise from outflows or environmental processes. [ABSTRACT FROM AUTHOR]

We draw the K -band luminosity functions (CLFs) of young massive clusters (YMCs) hosted by 34 SUNBIRD targets to evaluate the impact of the host galaxy environment on their YMC properties. The depth and high resolution of the NIR images (PSF ∼ 0.1") allow us to test whether CLF power-law slopes (α) of high star-forming galaxies are similar to those of gas-poor low star formation rate (SFR) galaxies. We found that α ranges between 1.53 and 2.41 with a median value of 1.87 ± 0.23. We also performed correlation searches between α and the host global properties and noticed that α decreases with an increasing SFR and SFR density. On sub-galactic scales, CLF slopes of cluster-rich galaxies differ by ∼0.5. Our NIR CLF analyses suggest that the extreme environment of high SFR galaxies such as the SUNBIRD sample is likely to affect the formation mechanisms of YMCs and hence to govern the ongoing small-scale SF processes of the host galaxy. [ABSTRACT FROM AUTHOR]

We employ the Feedback In Realistic Environments (FIRE-2) physics model to study how the properties of giant molecular clouds (GMCs) evolve during galaxy mergers. Due to the rarity of mergers in the local Universe, samples of nearby merging galaxies suitable for studies of individual GMCs are limited. Idealized simulations provide us with a new window to study GMC evolution during a merger, and assist in interpreting observations. We conduct a pixel-by-pixel analysis of the simulated molecular gas properties in both undisturbed control galaxies and galaxy mergers. The simulated GMC-pixels follow a similar trend in a diagram of velocity dispersion (σ v ) versus gas surface density (Σmol) as observed in normal spiral galaxies in the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For simulated mergers, we see a significant increase in both the Σmol and σ v for GMC-pixels by a factor of 5 – 10, which put these pixels to be above the trend of PHANGS galaxies in the σ v vs Σmol diagram. This deviation indicates that GMCs in the simulated merger are more gravitationally unbound and have higher virial parameter (α vir) of 10 – 100, which is much larger than that of simulated control galaxies. Furthermore, we find that the increase in α vir generally happens at the same time as the increase in global star formation rate (SFR), which suggests feedback is playing a role in dispersing the gas. The correspondence between high α vir and SFR also suggests some other physical mechanisms besides self-gravity are helping the GMCs in starburst mergers to collapse and form stars. [ABSTRACT FROM AUTHOR]

The transformation and evolution of a galaxy is strongly influenced by interactions with its environment. Neutral hydrogen (H  i) is an excellent way to trace these interactions. Here, we present H  i observations of the spiral galaxy NGC 895, which was previously thought to be isolated. High-sensitivity H  i observations from the MeerKAT large survey project MIGHTEE reveal possible interaction features, such as extended spiral arms and the two newly discovered H  i companions, that drive us to change the narrative that it is an isolated galaxy. We combine these observations with deep optical images from the Hyper Suprime Camera to show an absence of tidal debris between NGC 895 and its companions. We do find an excess of light in the outer parts of the companion galaxy MGTH |$\_$| J022138.1-052631, which could be an indication of external perturbation and thus possible sign of interactions. Our analysis shows that NGC 895 is an actively star-forming galaxy with a SFR of 1.75 ± 0.09[M⊙/yr], a value typical for high-stellar mass galaxies on the star-forming main sequence. It is reasonable to state that different mechanisms may have contributed to the observed features in NGC 895, and this emphasizes the need to revisit the target with more detailed observations. Our work shows the high potential and synergy of using state-of-the-art data in both H  i and optical to reveal a more complete picture of galaxy environments. [ABSTRACT FROM AUTHOR]

Galaxy mergers are common processes in the Universe. As a large fraction of galaxies hosts at their centres a central supermassive black hole (SMBH), mergers can lead to the formation of a supermassive black hole binary (SMBHB). The formation of such a binary is more efficient when the SMBHs are embedded in a nuclear star cluster (NSC). NSCs are dense and massive stellar clusters present in the majority of the observed galaxies. Their central densities can reach up to |$10^7\, {\rm M_{\odot }}\,{\rm pc^{-3}}$| and their masses can be as large as a few |$10^7\, {\rm M_{\odot }}$|⁠. The direct detection of an SMBHB is observationally challenging. In this work, we illustrate how the large-scale structural and dynamical properties of an NSC can help to identify nucleated galaxies that recently went through a merger that possibly led to the formation of a central SMBHB. Our models show that the merger can imprint signatures on the shape, density profile, rotation, and velocity structure of the NSC. The strength of the signatures depends on the mass ratio between the SMBHs and on the orbital initial conditions of the merger. In addition, the number of hypervelocity stars produced in the mergers is linked to the SMBHB properties. The merger can also contribute to the formation of the nuclear stellar disc of the galaxy. [ABSTRACT FROM AUTHOR]

How does the host galaxy morphology influence a central quasar or vice versa? We address this question by measuring the asymmetries of 2424 SDSS quasar hosts at 0.2 < z < 0.8 using broad-band (grizy) images from the Hyper Suprime-Cam Subaru Strategic Program. Control galaxies (without quasars) are selected by matching the redshifts and stellar masses of the quasar hosts. A two-step pipeline is run to decompose the PSF and Sérsic components and then measure asymmetry indices (A CAS, A outer, and A shape) of each quasar host and control galaxy. We find a mild correlation between host asymmetry and AGN bolometric luminosity (L bol) for the full sample (spearman correlation of 0.37) while a stronger trend is evident at the highest luminosities (L bol > 45). This then manifests itself into quasar hosts being more asymmetric, on average, when they harbour a more massive and highly accreting black hole. The merger fraction also positively correlates with L bol and reaches up to 35 per cent for the most luminous. Compared to control galaxies, quasar hosts are marginally more asymmetric (excess of 0.017 in median at 9.4σ level) and the merger fractions are similar (⁠|$\sim 16.5~{{\ \rm per\ cent}}$|⁠). We quantify the dependence of asymmetry on optical band that demonstrates that mergers are more likely to be identified with the bluer bands and the correlation between L bol and asymmetry is also stronger in such bands. We stress that the band dependence, indicative of a changing stellar population, is an important factor in considering the influence of mergers on AGN activity. [ABSTRACT FROM AUTHOR]