Search

Context. Dense filaments/feathers are kpc-scale dusty features present in nearby main sequence galaxies. Distinct from the spiral arms, filaments constitute a major portion of dense gas concentration. They are expected to play an important role in star formation and are known to harbour star-forming regions and H II regions. Aims. We explore the origin of filaments/feathers in disc galaxies via global gravitational instability. Methods. We conduct a parameter study using three-dimensional hydrodynamical simulations of isolated disc galaxies that are isothermal, self-gravitating and initialised in equilibrium. Our galaxies are uniquely characterised by two dimensionless parameters, the Toomre $Q$ and the rotational Mach number, $\mathcal{M}_{\rm c} = v_{\rm c}/c_{\rm s}$ (ratio of circular velocity to sound speed). We carry out simulations covering a wide range in both. Results. We find that galaxies with $Q = 1$ form filaments within a single rotation, while galaxies with $Q \geq 2$ do not. These filaments are kpc long and are semi-regularly spaced along the azimuth. Their morphology, density contrast and formation timescale vary with $\mathcal{M}_{\rm c}$, with filament spacing and instability onset time both inversely proportional to $\mathcal{M}_{\rm c}$ and the density contrast increasing with $\mathcal{M}_{\rm c}$. However, their growth rates in all $Q = 1$ galaxies are $\sim 0.5~\Omega$, where $\Omega$ is the angular frequency. We compare the filament spacing in our simulations with the ones from JWST/MIRI and HST observations of nearby galaxies and find them in agreement. Conclusions. Our study suggests that self-gravity and rotation are sufficient to form filaments, even in the absence of spiral arms or magnetic fields. Their morphologies are primarily determined by $\mathcal{M}_{\rm c}$, which parametrises the importance of thermal versus rotational support., Comment: 15 pages, 17 figures. Accepted in A&A

We present results from a suite of 3D high-resolution hydrodynamic simulations of supernova-driven outflows from galactic disc regions with a range of gas surface density, metallicity, and supernova scale height. We use this suite to quantify how outflow properties -- particularly the loading factors for mass, metallicity, and energy -- vary with these parameters. We find that the winds fall into three broad categories: steady and hot, multiphase and moderately bursty, and cool and highly bursty. The first of these is characterised by efficient metal and energy loading but weak mass loading, the second by moderate loading of mass, metals, and energy, and the third by negligible metal and energy loading but substantial mass loading. The most important factor in determining the kind of wind a galaxy will produce is the ratio of supernova to gas gas scale heights, with the latter set by a combination of supernova rate, metallicity-dependent cooling rate, and the gravitational potential. These often combine in counterintuitive ways -- for example increased cooling causes cold clouds to sink into the galactic midplane more rapidly, lowering the volume-filling factor of dense gas and making the environment more favourable for strong winds. Our findings suggest that the nature of galactic winds is likely highly sensitive to phenomena such as runaway stars occuring at a large height and dense gas and are poorly captured in most simulations, and that metal loading factors for type Ia supernovae may be substantially larger than those for type II, with important implications for galactic chemical evolution., Comment: 18 pages, accepted for publication in MNRAS. Movies from QED simulations can be viewed at https://quokka-astro.github.io/quokka/gallery/

Since molecular clouds form stars, at least some parts of them must be in a state of collapse. However, there is a long-standing debate as to whether that collapse is local, involving only a small fraction of the cloud mass, or global, with most mass in a state of collapse up to the moment when it is dispersed by stellar feedback. In principle it is possible to distinguish these possibilities from clouds' virial ratios, which should be a factor of two larger for collapse than for equilibrium, but systematic uncertainties have thus far prevented such measurements. Here we propose a new analysis method to overcome this limitation: while the absolute value of a cloud's virial ratio is too uncertain to distinguish global from local collapse, the differential change in virial ratio as a function of surface density is also diagnostic of clouds' dynamical state, and can be measured with far fewer systematic uncertainties. We demonstrate the basic principles of the method using simple analytic models of supported and collapsing clouds, validate it from full 3D simulations, and discuss possible challenges in applying the method to real data. We then provide a preliminary application of the technique to recent observations of the molecular clouds in Andromeda, showing that most of them are inconsistent with being in a state of global collapse., Comment: 14 pages, 7 figures, submitted to The Open Journal of Astrophysics

We revisit the role of pressure in the structure, stability and confinement of Giant Molecular Clouds (GMCs) in light of recently published observations and analysis of the GMCs in the Andromeda galaxy (M31). That analysis showed, that in the absence of any external pressure, most GMCs (57\% by number) in M31 would be gravitationally unbound. Here, after a more detailed examination of the global measurements of surface densities and velocity dispersions, we find that GMCs in M31, when they can be traced to their outermost boundaries, require external pressures for confinement that are consistent with estimates for the mid-plane pressure of this galaxy. We introduce and apply a novel methodology to measure the radial profile of internal pressure within any GMC that is spatially resolved by the CO observations. We show that for the best resolved examples in M31 the internal pressures increase steeply with surface density in a power-law fashion with $p_{int} \sim \Sigma^2$. At high surface densities many of these extragalactic GMCs break from the single power-law and exhibit upward curvature. Both these characteristics of the variation of internal pressure with surface density are in agreement with theoretical expectations for hydrostatic equilibrium at each radial surface of a GMC, including the outermost boundary.

The origin of nebular HeII-emission in both local and high-redshift galaxies remains an unsolved problem. Various theories have been proposed to explain it, including HeII-ionization by high mass X-ray binaries, ultra-luminous X-ray sources, or "stripped" He stars, shock ionization, and hidden AGNs. All these theories have shortcomings, however, leaving the cause of nebular HeII emission unclear. We investigate the hypothesis that the photons responsible for driving nebular HeII emissions are produced by the evolution of single massive stars and/or WR stars. We combine models of stellar evolution with population synthesis and nebular models to identify the most favorable scenarios for producing nebular HeII via this channel. We find that, if WR winds are clumpy enough to become close to optically thin, stellar populations with a wide range of metallicities and rotation rates can produce HeII ionizing photons at rates sufficient to explain the observed nebular $I(HeII)/I(\mathrm{H}\beta)$ ratio $\sim 0.004-0.07$ found in HeII-emitting galaxies. Metal-poor, rapidly rotating stellar populations ($[\mathrm{Fe}/\mathrm{H}]=-2.0$, $v/v_\mathrm{crit}=0.4$) also reach these levels of HeII production even for partially clumpy winds. These scenarios also yield HeII, H$\beta$, and "Blue-Bump" line equivalent widths comparable to those observed in HeII emitters. Only for laminar, non-clumpy winds, do we fail to find combinations of metallicity and stellar rotation rate that yield $I(HeII)/I(\mathrm{H}\beta)$ values as high as those observed in HeII-emitters. Contrary to previous findings, we conclude that single WR stars can be a strong source for nebular HeII emission if their winds are sufficiently clumpy allowing significant escape of hard ionizing photons., Comment: submitted to A&A, 16 pages, 16 figures

Recommendation systems predominantly utilize two-tower architectures, which evaluate user-item rankings through the inner product of their respective embeddings. However, one key limitation of two-tower models is that they learn a pair-agnostic representation of users and items. In contrast, pair-wise representations either scale poorly due to their quadratic complexity or are too restrictive on the candidate pairs to rank. To address these issues, we introduce Context-based Graph Neural Networks (ContextGNNs), a novel deep learning architecture for link prediction in recommendation systems. The method employs a pair-wise representation technique for familiar items situated within a user's local subgraph, while leveraging two-tower representations to facilitate the recommendation of exploratory items. A final network then predicts how to fuse both pair-wise and two-tower recommendations into a single ranking of items. We demonstrate that ContextGNN is able to adapt to different data characteristics and outperforms existing methods, both traditional and GNN-based, on a diverse set of practical recommendation tasks, improving performance by 20% on average., Comment: 14 pages, 1 figure, 5 tables

The evolution and distribution of metals within galaxies are critical for understanding galactic evolution and star formation processes, but the mechanisms responsible for shaping this distribution remain uncertain. In this study, we carry out high-resolution simulations of an isolated Milky Way-like galaxy, including a star-by-star treatment of both feedback and element injection. We include seven key isotopes of observational and physical interest, and which are distributed across different nucleosynthetic channels--primarily AGB stars (N, Ba, Ce), supernovae (O, Mg, S), and Wolf-Rayet stars (C) show measurably different correlation statistics in space and time and their fluctuations. This difference arises from the distinct ejection mechanisms associated with each nucleosynthetic process. The large-scale properties ensure that different elements, despite having different nucleosynthetic origins, are highly correlated with one another (>0.85 for all, >0.99 for same orgins), and their spatial correlations vary together in time. However small-scale variations naturally break elements into distinct nucleosynthetic familiars, with elements originating from the same channels correlating better with each other than with elements from different origins. Our findings suggest both challenges and opportunities for ongoing efforts to use chemical measurements of gas and stars to unravel the history and physics of galaxy assembly.

Supernova-driven galactic outflows play a vital but still poorly-understood role in galactic chemical evolution, and one of the largest uncertainties about them is the extent to which they consist of supernova ejecta that are unmixed, or only poorly-mixed, with the remainder of the interstellar medium (ISM). Simulations of wind launching make a range of predictions about the extent of mixing between the wind and the ISM, but thus far these have proven challenging to test observationally. In this study, we post-process high-resolution simulations of outflows from the QED simulation suite to generate synthetic X-ray spectra from galactic winds, which we then analyse using standard observational procedures, in order to search for detectable markers of wind mixing. Our synthetic observations reveal that partially-mixed winds show significant and detectable metallicity gradients when viewed edge-on, with metallicity decreasing away from the central galactic disc. We explore how this signature results from imperfect mixing and the extent to which measurements of it can be used to diagnose the level of mixing in winds. We determine the signal-to-noise ratio (SNR) requirements for such measurements to be reliable, and provide a simple quantitative model that can be used to connect metallicity gradients to mixing between the hot ($T>10^{6}$ K) and cold ($T<10^{4}$ K) phases in observations that reach the required SNR, providing a framework to interpret current and future observations., Comment: The paper was submitted to MNRAS. 11 pages, 9 figures

We present a novel method for automatically detecting and characterising semi-resolved star clusters: clusters where the observational point-spread function (PSF) is smaller than the cluster's radius, but larger than the separations between individual stars. We apply our method to a 1.77 deg$^2$ field located in the Large Magellanic Cloud (LMC) using the VISTA survey of the Magellanic Clouds (VMC), which surveyed the LMC in the $YJK_\text{s}$ bands. Our approach first models the position-dependent PSF to detect and remove point sources from deep $K_\text{s}$ images; this leaves behind extended objects such as star clusters and background galaxies. We then analyse the isophotes of these extended objects to characterise their properties, perform integrated photometry, and finally remove any spurious objects this procedure identifies. We demonstrate our approach in practice on a deep VMC $K_\text{s}$ tile that contains the most active star-forming regions in the LMC: 30 Doradus, N158, N159, and N160. We select this tile because it is the most challenging for automated techniques due both to crowding and nebular emission. We detect 682 candidate star clusters, with an estimated contamination rate of 13% from background galaxies and chance blends of physically unrelated stars. We compare our candidates to publicly available James Webb Space Telescope data and find that at least 80% of our detections appear to be star clusters., Comment: 26 pages, 17 figures, submitted to MNRAS

Molecular clouds (MCs) are the birthplaces of new stars in galaxies. A key component of MCs are photodissociation regions (PDRs), where far-ultraviolet radiation plays a crucial role in determining the gas's physical and chemical state. Traditional PDR models assume chemical steady state (CSS), where the rates of H$_2$ formation and photodissociation are balanced. However, real MCs are dynamic and can be out of CSS. In this study, we demonstrate that combining H$_2$ emission lines observed in the far-ultraviolet or infrared with column density observations can be used to derive the rates of H$_2$ formation and photodissociation. We derive analytical formulae that relate these rates to observable quantities, which we validate using synthetic H$_2$ line emission maps derived from the SILCC-Zoom hydrodynamical simulation. Our method estimates integrated H$_2$ formation and dissociation rates with an accuracy $\approx 30$ % (on top of uncertainties in observed H$_2$ emission maps and column densities). Our simulations, valid for column densities $N \leq 2 \times 10^{22}$ cm$^{-2}$, cover a wide dynamic range in H$_2$ formation and photodissociation rates, showing significant deviations from CSS, with 74 % of the MC's mass deviating from CSS by a factor greater than 2. Our analytical formulae can effectively distinguish between regions in and out of CSS. When applied to actual H$_2$ line observations, our method can assess the chemical state of MCs, providing insights into their evolutionary stages and lifetimes. A NASA Small Explorer mission concept, Eos, will be proposed in 2025 and is specifically designed to conduct the types of observations outlined in this study., Comment: Accepted for publication in ApJ. **Comments Welcome!** This paper is the first ("Paper-I") in 2-paper series. Paper II, entitled "The Molecular Cloud Lifecycle II: Formation and Destruction of Molecular Clouds Diagnosed via H 2 Fluorescent Emission Emission" may be found here: https://ui.adsabs.harvard.edu/abs/2024ApJ...975..269B/abstract

Mixed-frame formulations of radiation-hydrodynamics (RHD), where the radiation quantities are computed in an inertial frame but matter quantities are in a comoving frame, are advantageous because they admit algorithms that conserve energy and momentum to machine precision and combine more naturally with adaptive mesh techniques, since unlike pure comoving-frame methods they do not face the problem that radiation quantities must change frame every time a cell is refined or coarsened. However, implementing multigroup RHD in a mixed-frame formulation presents challenges due to the complexity of handling frequency-dependent interactions and the Doppler shift of radiation boundaries. In this paper, we introduce a novel method for multigroup RHD that integrates a mixed-frame formulation with a piecewise powerlaw approximation for frequency dependence within groups. This approach ensures the exact conservation of total energy and momentum while effectively managing the Lorentz transformation of group boundaries and evaluation of group-averaged opacities. Our method takes advantage of the locality of matter-radiation coupling, allowing the source term for $N_g$ frequency groups to be handled with simple equations with a sparse Jacobian matrix of size $N_g + 1$, which can be inverted with $O(N_g)$ complexity. This results in a computational complexity that scales linearly with $N_g$ and requires no more communication than a pure hydrodynamics update, making it highly efficient for massively parallel and GPU-based systems. We implement our method in the GPU-accelerated RHD code QUOKKA and demonstrate that it passes a wide range of numerical tests, including preserving the asymptotic diffusion limit. We demonstrate that the piecewise powerlaw method shows significant advantages over traditional opacity averaging methods for handling rapidly variable opacities with modest frequency resolution., Comment: 18 pages, 8 figures, MNRAS in press. The code described in this article is available at: https://github.com/quokka-astro/quokka

Cosmic rays (CRs) are the primary driver of ionization in star forming molecular clouds (MCs). Despite their potential impacts on gas dynamics and chemistry, no simulations of star cluster formation following the creation of individual stars have included explicit cosmic ray transport (CRT) to date. We conduct the first numerical simulations following the collapse of a $2000 M_{\odot}$ MC and the subsequent star formation including CRT using the STARFORGE framework implemented in the GIZMO code. We show that when CR-transport is streaming-dominated, the CR energy in the cloud is strongly attenuated due to energy losses from the streaming instability. Consequently, in a Milky Way like environment the median CR ionization rate (CRIR) in the cloud is low ($ \zeta \lesssim 2 \times 10^{-19} \rm s^{-1}$) during the main star forming epoch of the calculation and the impact of CRs on the star formation in the cloud is limited. However, in high-CR environments, the CR distribution in the cloud is elevated ($\zeta \lesssim 6 \times 10^{-18}$), and the relatively higher CR pressure outside the cloud causes slightly earlier cloud collapse and increases the star formation efficiency (SFE) by $50 \%$ to $\sim 13 \%$. The initial mass function (IMF) is similar in all cases except with possible variations in a high-CR environment. Further studies are needed to explain the range of ionization rates observed in MCs and explore star formation in extreme CR environments., Comment: Accepted to The Astrophysical Journal

The gas-phase metallicity distribution within galaxies records critical information about galactic evolution. In this work we investigate how active galactic nuclei (AGN) influence this distribution by measuring the two-point correlation functions of gas-phase metallicity in 95 non-AGN and 37 AGN-host galaxies from the Calar Alto Legacy Integral Field spectroscopy Area integral field spectrographic survey. We measure metallicity using a novel Bayesian method that properly includes both stellar and AGN contributions to emission line fluxes and allows us to measure metallicities in both AGN-host and non-AGN galaxies in a single, consistent framework. We find that the two-point correlation functions of both AGN-host and non-AGN galaxies are well-fit by a simple injection-diffusion model, and that the correlation lengths $l_\mathrm{corr}$ we derive for the non-AGN galaxies are reasonably consistent with those obtained in earlier work. The AGN-host galaxies generally have smaller $l_\mathrm{corr}$ than non-AGN galaxies at fixed stellar mass, but similar $l_\mathrm{corr}$ at fixed star formation rate (SFR), suggesting that the primary effect of hosting an AGN in this sample is a reduction in SFR at fixed stellar mass, and that this in turn suppresses the correlation length. Our findings further indicate that, while both SFR and stellar mass are positively correlated with metallicity correlation length $l_\mathrm{corr}$, the former is more fundamental, implying that fluctuations in the metallicity distribution within galaxies are driven more by short-term responses to physical processes such as star formation that can change much faster than a Hubble time., Comment: 16 pages, 15 figures, accepted by MNRAS

Cosmic rays travelling through interstellar space have their propagation directions repeatedly scattered by fluctuating interstellar magnetic fields. The nature of this scattering is a major unsolved problem in astrophysics, one that has resisted solution largely due to a lack of direct observational constraints on the scattering rate. Here we show that very high-energy $\gamma$-ray emission from the globular cluster Terzan 5, which has unexpectedly been found to be displaced from the cluster, presents a direct probe of this process. We show that this displacement is naturally explained by cosmic rays accelerated in the bow shock around the cluster propagating a finite distance before scattering processes re-orient enough of them towards Earth to produce a detectable $\gamma$-ray signal. The angular distance between the cluster and the signal places tight constraints on the scattering rate, which we show are consistent with a model whereby scattering is primarily due to excitation of magnetic waves by the cosmic rays themselves. The analysis method we develop here will make it possible to use sources with similarly displaced non-thermal X-ray and TeV $\gamma$-ray signals as direct probes of cosmic ray scattering across a range of Galactic environments., Comment: Accepted to Nature Astronomy; 16 pages, 7 figures; this submission contains the main article, supplementary information, and extended data in a single document

This study examines the limitations of H$\alpha$ luminosity as a tracer of star formation rates (SFR) in spatially resolved observations. We carry out high-resolution simulations of a Milky Way-like galaxy including both supernova and photoionization feedback, and from these we generate synthetic H$\alpha$ emission maps that we compare to maps of the true distribution of young stellar objects (YSOs) on scales from whole-galaxy to individual molecular clouds ($\lesssim 100$ pc). Our results reveal significant spatial mismatches between H$\alpha$ and true YSO maps on sub-100 pc scales, primarily due to ionizing photon leakage, with a secondary contribution from young stars drifting away from their parent molecular clouds. On small scales these effect contribute significantly to the observed anti-correlation between gas and star formation, such that there is noticeably less anti-correlation if we replace an H$\alpha$-based star formation map with a YSO-based one; this in turn implies that previous studies have underestimated the time it takes for young stars to disperse their parent molecular clouds. However, these effects are limited in dense regions with hydrogen columns $N_\mathrm{H} > 3 \times 10^{21}$ cm$^{-2}$, where the H$\alpha$- and YSO-based SFR maps show better agreement. Based on this finding we propose a calibration model that can precisely measure the SFR of large molecular clouds (mean radius > 100 pc) with a combination of H$\alpha$ and CO observations, which provides a foundation for future study of star formation processes in extragalactic molecular clouds., Comment: 12 Pages, 9 Figures, accepted by MNRAS

We address the formation of giant clumps in violently unstable gas-rich disc galaxies at cosmic noon. While these are commonly thought to originate from gravitational Toomre instability, cosmological simulations have indicated that clumps form even in regions where the Toomre $Q$ parameter is well above unity, which should be stable according to linear Toomre theory (Inoue et al., 2016). Examining one of these cosmological simulations, we find that it exhibits an excess in compressive modes of turbulence with converging motions. The energy in converging motions within proto-clump regions is $\sim 70\%$ of the total turbulent energy, compared to $\sim 17\%$ expected in equipartition. When averaged over the whole disc, $\sim 32\%$ of the turbulent energy is in converging motions, with a further $\sim 8\%$ in diverging motions. Thus, a total of $\sim 40\%$ of the turbulent energy is in compressive modes, with the rest in solenoidal modes, compared to the $(1/3)-(2/3)$ division expected in equipartition. By contrast, we find that in an isolated-disc simulation with similar properties, resembling high-$z$ star-forming galaxies, the energy in the different turbulence modes are in equipartition, both in proto-clump regions and over the whole disc. We conclude that the origin of the excessive converging motions in proto-clump regions is external to the disc, and propose several mechanisms that can induce them. This is an additional mechanism for clump formation, complementary to and possibly preceding gravitational instability., Comment: 6 pages, 4 figures, Submitted to MNRAS Letters Comments welcome!

New JWST near-infrared imaging of the nearby galaxy NGC 628 from the Cycle 1 program JWST-FEAST is combined with archival JWST mid-infrared imaging to calibrate the 21 $\mu$m emission as a star formation rate indicator (SFR) at $\sim$120 pc scales. The Pa$\alpha$ ($\lambda$1.8756 $\mu$m) hydrogen recombination emission line targeted by FEAST provides a reference SFR indicator that is relatively insensitive to dust attenuation, as demonstrated by combining this tracer with the HST H$\alpha$ imaging. Our analysis is restricted to regions that appear compact in nebular line emission and are sufficiently bright to mitigate effects of both age and stochastic sampling of the stellar initial mass function. We find that the 21 $\mu$m emission closely correlates with the nebular line emission, with a power-law with exponent=1.07$\pm$0.01, in agreement with past results. We calibrate a hybrid SFR indicator using a combination of H$\alpha$ and 24 $\mu$m (extrapolated from 21 $\mu$m) tracers and derive the proportionality constant between the two tracers $b=0.095\pm0.007$, which is $\sim$ 3-5 times larger than previous derivations using large regions/entire galaxies. We model these discrepancies as an increasing contribution to the dust heating by progressively older stellar populations for increasing spatial scales, in agreement with earlier findings that star formation is hierarchically distributed in galaxies. Thus, use of hybrid SFR indicators requires prior knowledge of the mean age of the stellar populations dominating the dust heating, which makes their application uncertain. Conversely, non-linear calibrations of SFRs from L(24) alone are more robust, with a factor $\lesssim$2.5 variation across the entire range of L(24) luminosities from HII regions to galaxies., Comment: 28 pages, 9 figures. Accepted for publication on the Astrophysical Journal

We present maps of ionized gas (traced by Pa$\alpha$ and Br$\alpha$) and 3.3 $\mu$m Polycyclic Aromatic Hydrocarbon (PAH) emission in the nearby spiral galaxy NGC 628, derived from new JWST/NIRCam data from the FEAST survey. With this data, we investigate and calibrate the relation between 3.3 $\mu$m PAH emission and star formation rate (SFR) in and around emerging young star clusters (eYSCs) on a scale of ${\sim}40$ pc. We find a tight (correlation coefficient $\rho$${\sim}$0.9) sub-linear (power-law exponent $\alpha$${\sim}$0.75) relation between the 3.3 $\mu$m PAH luminosity surface density and SFR traced by Br$\alpha$ for compact, cospatial (within 0.16$''$ or ${\sim}$7 pc) peaks in Pa$\alpha$, Br$\alpha$, and 3.3 $\mu$m (eYSC-I). The scatter in the relationship does not correlate well with variations in local interstellar medium (ISM) metallicity due to a radial metallicity gradient, but rather is likely due to stochastic sampling of the stellar initial mass function (IMF) and variations in the PAH heating and age of our sources. The deviation from a linear relation may be explained by PAH destruction in more intense ionizing environments, variations in age, and IMF stochasticity at intermediate to low luminosities. We test our results with various continuum subtraction techniques using combinations of NIRCam bands and find that they remain robust with only minor differences in the derived slope and intercept. An unexpected discrepancy is identified between the relations of hydrogen recombination lines (Pa$\alpha$ versus Br$\alpha$; H$\alpha$ versus Br$\alpha$)., Comment: 28 pages, 10 figures, 3 tables

The observed rest-UV luminosity function at cosmic dawn ($z \sim 8-14$) measured by JWST revealed an excess of UV-luminous galaxies relative to many pre-launch theoretical predictions. A high star-formation efficiency (SFE) and a top-heavy initial mass function (IMF) are among the mechanisms proposed for explaining this excess. Although a top-heavy IMF has been proposed for its ability to increase the light-to-mass ratio (\(\Psi_{\mathrm{UV}}\)), the resulting enhanced radiative pressure from young stars could decrease the star formation efficiency (SFE), potentially driving galaxy luminosities back down. In this Letter, we use idealized radiation hydrodynamic simulations of star cluster formation to explore the effects of a top-heavy IMF on the SFE of clouds typical of the high pressure conditions found at these redshifts. We find that the SFE in star clusters with solar neighbourhood-like dust abundance decreases with increasingly top-heavy IMF's -- by $\sim 20 \%$ for an increase of factor 4 in $\Psi_{\mathrm{UV}}$, and by $50 \%$ for a factor $ \sim 10$ in $\Psi_{\mathrm{UV}}$. However, we find that an expected decrease in the dust-to-gas ratio ($\sim 0.01 \times \mathrm{Solar}$) at these redshifts can completely compensate for the enhanced light output. This leads to a (cloud-scale; $\sim 10 \, \mathrm{pc}$) SFE that is $\gtrsim 70\%$ even for a factor 10 increase in $\Psi_{\mathrm{UV}}$, implying that highly efficient star formation is unavoidable for high surface density and low metallicity conditions. Our results suggest that a top-heavy IMF, if present, likely coexists with efficient star formation in these galaxies., Comment: 12 pages, 4 figures. Accepted to the Astrophysical Journal Letters

The molecular phase of supernova-driven outflows originates from the cold, molecular gas in the disc of a star-forming galaxy, and may carry a substantial fraction of the wind mass flux in some galaxies, but it remains poorly understood. Observations of this phase come mostly from very nearby galaxies due its low surface brightness and covering fraction, and simulations often lack the spatial resolution necessary to resolve it. Here we analytically estimate the survivability of this phase in order to understand under what conditions an galactic wind can contain a significant molecular phase. We show that the molecular content of outflows is primarily determined by two dimensionless numbers: a generalised Eddington ratio describing the strength of the outflow and an ionisation parameter-like quantity describing the strength of the radiation field per baryon. We apply this model to a sample of galaxies and show that, while any molecules entrained in the winds of normal star-forming galaxies should be destroyed close to the galactic disc, the outflows of strong starburst should become increasingly dominated by molecules., Comment: 8 pages, 5 figures, submitted to MNRAS. Comments welcome

Numerical radiation-hydrodynamics (RHD) for non-relativistic flows is a challenging problem because it encompasses processes acting over a very broad range of timescales, and where the relative importance of these processes often varies by orders of magnitude across the computational domain. Here we present a new implicit-explicit (IMEX) method for numerical RHD that has a number of desirable properties that have not previously been combined in a single method. Our scheme is based on moments and allows machine-precision conservation of energy and momentum, making it highly suitable for adaptive mesh refinement applications; it requires no more communication than hydrodynamics and includes no non-local iterative steps, making it highly suitable for massively parallel and GPU-based systems where communication is a bottleneck; and we show that it is asymptotically-accurate in the streaming, static diffusion, and dynamic diffusion limits, including in the so-called asymptotic diffusion regime where the computational grid does not resolve the photon mean free path. We implement our method in the GPU-accelerated RHD code QUOKKA and show that it passes a wide range of numerical tests., Comment: 14 pages, 6 figures, submitted to MNRAS; Code available for download at https://github.com/quokka-astro/quokka

Galaxies' stellar masses, gas-phase oxygen abundances (metallicity), and star formation rates (SFRs) obey a series of empirical correlations, most notably the mass-metallicity relation (MZR) and fundamental metallicity relation (FZR), which relates oxygen abundance to a combination of stellar mass and SFR. However, due to the difficulty of measuring oxygen abundances and SFRs in galaxies that host powerful active galactic nuclei (AGN), to date it is unknown to what extent AGN-host galaxies also follow these correlations. In this work, we apply Bayesian methods to the MaNGA integral field spectrographic (IFS) survey that allow us to measure oxygen abundances and SFRs in AGN hosts, and use these measurements to explore how the MZR and FZR differ between galaxies that do and do not host AGN. We find similar MZRs at stellar masses above $10^{10.5} \mathrm{M}_\odot$, but that at lower stellar masses AGN hosts show up to $\sim 0.2$ dex higher oxygen abundances. The offset in the FZR is significantly smaller, suggesting that the larger deviation in the MZR is a result of AGN-host galaxies having systematically lower SFRs at fixed stellar mass. However, within the AGN-host sample there is little correlation between SFR and oxygen abundance. These findings support a scenario in which an AGN can halt efficient gas accretion, which drives non-AGN host galaxies to both higher SFR and lower oxygen abundance, resulting in the galaxy evolving off the star-forming main sequence (SFMS). As a consequence, as the SFR declines for an individual system its metallicity remains mostly unchanged., Comment: 18 pages, 14 figures, accepted for publication in MNRAS

Stellar feedback in dwarf galaxies remains, to date, poorly explored, yet is crucial to understanding galaxy evolution in the early Universe. In particular, pre-supernova feedback has recently been found to play a significant role in regulating and disrupting star formation in larger spiral galaxies, but it remains uncertain if it also plays this role in dwarfs. We study the ionised gas properties and stellar content of individual star-forming regions across three nearby, low-metallicity, dwarf starburst galaxies (J0921, KKH046, and Leo P) to investigate how massive stars influence their surroundings and how this influence changes as a function of environment. To achieve this, we extracted integrated spectra of 30 HII regions from archival VLT/MUSE integral field spectroscopic observations of these three dwarf starburst galaxies. We fitted the HII regions' main emission lines with Gaussian profiles to derive their oxygen abundances, electron densities, and luminosities, and we used the Stochastically Lighting Up Galaxies (SLUG) code to derive the stellar mass, age, and bolometric luminosity of the stellar populations driving the HII regions. We then quantified two pre-supernova stellar feedback mechanisms, namely the direct radiation pressure and photoionisation feedback, and explored how feedback strength varies with HII region properties. Our findings suggest that stellar feedback has less of an impact on evolved regions, with both the pressure of the ionised gas and the direct radiation pressure decreasing as a function of HII region size. We also find that these stellar feedback mechanisms are dependent on the metallicity of the HII regions. These findings extend results from stellar feedback studies of more massive star-forming galaxies to the low-mass, low-metallicity regime., Comment: The paper has been published in A&A

We study the cosmological evolution of the two-point correlation functions of galactic gas-phase metal distributions using the 28 simulated galaxies from the Auriga Project. Using mock observations of the $z = 0$ snapshots to mimic our past work, we show that the correlation functions of the simulated mock observations are well matched to the correlation functions measured from local galaxy surveys. This comparison suggests that the simulations capture the processes important for determining metal correlation lengths, the key parameter in metallicity correlation functions. We investigate the evolution of metallicity correlations over cosmic time using the true simulation data, showing that individual galaxies undergo no significant systematic evolution in their metal correlation functions from $z\sim 3$ to today. In addition, the fluctuations in metal correlation length are correlated with but lag ahead fluctuations in star formation rate. This suggests that re-arrangement of metals within galaxies occurs at a higher cadence than star formation activity, and is more sensitive to the changes of environment, such as galaxy mergers, gas inflows / outflows, and fly-bys., Comment: 12 pages, 10 figures, 1 table, accepted for publication in MNRAS

Molecular hydrogen (H$_2$) formation and dissociation are key processes that drive the gas lifecycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds (SILCC) zoom-in simulation suite, we explore the utility of future observations of H$_2$ dissociation and formation for tracking the lifecycle of molecular clouds. The simulations used in this work include non-equilibrium H$_2$ formation, stellar radiation, sink particles, and turbulence. We find that, at early times in the cloud evolution, H$_2$ formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H$_2$ formation is also associated with a higher early star formation rate. For the clouds studied here, H$_2$ is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady-state about 2 Myrs after the first stars form. At the latest stage of cloud evolution, dissociation outweighs formation and the clouds enter a dispersal phase. We discuss how theories for the molecular cloud lifecycle and the star formation efficiency may be distinguished with observational measurements of H$_2$ fluorescence with a space-based high-resolution FUV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H$_2$ dissociation and formation rates, which we demonstrate can be connected to different phases in a molecular cloud's star-forming life, including cloud building, rapidly star-forming, H$_2$ chemical equilibrium, and cloud destruction., Comment: Submitted to ApJ, comments welcome

We compare the molecular cloud properties in sub-galactic regions of two galaxies, barred spiral NGC 1313, which is forming many massive clusters, and flocculent spiral NGC 7793, which is forming significantly fewer massive clusters despite having a similar star formation rate to NGC 1313. We find that there are larger variations in cloud properties between different regions within each galaxy than there are between the galaxies on a global scale, especially for NGC 1313. There are higher masses, linewidths, pressures, and virial parameters in the arms of NGC 1313 and center of NGC 7793 than in the interarm and outer regions of the galaxies. The massive cluster formation of NGC 1313 may be driven by its greater variation in environments, allowing more clouds with the necessary conditions to arise, although no one parameter seems primarily responsible for the difference in star formation. Meanwhile NGC 7793 has clouds that are as massive and have as much kinetic energy as clouds in the arms of NGC 1313, but have densities and pressures more similar to the interarm regions and so are less inclined to collapse and form stars. The cloud properties in NGC 1313 and NGC 7793 suggest that spiral arms, bars, interarm regions, and flocculent spirals each represent distinct environments with regard to molecular cloud populations. We see surprisingly little difference in surface densities between the regions, suggesting that the differences in surface densities frequently seen between arm and interarm regions of lower-resolution studies are indicative of the sparsity of molecular clouds, rather than differences in their true surface density., Comment: 24 pages, 12 figures, accepted for publication in ApJ

We present a comparative study of the molecular gas in two galaxies from the LEGUS sample: barred spiral NGC 1313 and flocculent spiral NGC 7793. These two galaxies have similar masses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clusters than NGC 7793, especially young massive clusters (<10 Myr, >10^4 Msol). Using ALMA CO(2-1) observations of the two galaxies with the same sensitivities and resolutions of 13 pc, we directly compare the molecular gas in these two similar galaxies to determine the physical conditions responsible for their large disparity in cluster formation. By fitting size-linewidth relations for the clouds in each galaxy, we find that NGC 1313 has a higher intercept than NGC 7793, implying that its clouds have higher kinetic energies at a given size scale. NGC 1313 also has more clouds near virial equilibrium than NGC 7793, which may be connected to its higher rate of massive cluster formation. However, these virially bound clouds do not show a stronger correlation with young clusters than that of the general cloud population. We find surprisingly small differences between the distributions of molecular cloud populations in the two galaxies, though the largest of those differences are that NGC 1313 has higher surface densities and lower free-fall times., Comment: 25 pages, 10 figures, accepted for publication in ApJ

We present the extended ALMA MaNGA QUEnching and STar formation survey, a combination of the original 46 ALMaQUEST galaxies plus new ALMA observations for a further 20 interacting galaxies. Three well-studied scaling relations are fit to the 19,999 star-forming spaxels in the extended sample, namely the resolved Schmidt-Kennicutt (rSK) relation, the resolved star forming main sequence (rSFMS) and the resolved molecular gas main sequence (rMGMS). We additionally investigate the relationship between the dynamical equilibrium pressure (PDE) and star formation rate surface density (Sigma_SFR), which we refer to as the resolved PDE (rPDE) relation. Contrary to previous studies that have focussed on normal star-forming galaxies and found an approximately linear rPDE relation, the presence of more vigourously star-forming galaxies in the extended ALMaQUEST sample reveals a marked turnover in the relation at high pressures. Although the scatter around the linear fit to the rPDE relation is similar to the other three relations, a random forest analysis, which can extract non-linear dependences, finds that PDE is unambiguously more important than either Sigma_H2 or Sigma_star for predicting Sigma_SFR. We compare the observed rPDE relation to the prediction of the pressure-regulated feedback-modulated (PRFM) model of star formation, finding that galaxies residing on the global SFMS do indeed closely follow the rPDE relation predicted by the PRFM theory. However, galaxies above and below the global SFMS show significant deviations from the model. Galaxies with high SFR are instead consistent with models that include other contributions to turbulence in addition to the local star formation feedback., Comment: Accepted for publication in MNRAS

Many astrophysical small-scale dynamos (SSDs) amplify weak magnetic fields via highly compressible, supersonic turbulence, but established SSD theories have overlooked these compressible effects. To address this, we perform visco-resistive SSD simulations across a range of sonic Mach numbers ($\mathcal{M}$), hydrodynamic Reynolds numbers ($\mathrm{Re}$), and magnetic Prandtl numbers ($\mathrm{Pm}$). We develop robust methods to measure kinetic and magnetic energy dissipation scales ($\ell_\nu$ and $\ell_\eta$) and the scale of strongest magnetic fields ($\ell_\mathrm{p}$) during the kinematic phase. We demonstrate that $\ell_\nu/\ell_\eta \sim \mathrm{Pm}^{1/2}$ is a universal feature for $\mathrm{Pm} \geq 1$ SSDs, regardless of $\mathcal{M}$ or $\mathrm{Re}$. Incompressible SSDs (either $\mathcal{M} \leq 1$ or $\mathrm{Re} < \mathrm{Re}\mathrm{crit} \approx 100$) concentrate magnetic energy at $\ell_\mathrm{p} \sim \ell_\eta$ with inversely correlated field strength and curvature. However, for compressible SSDs ($\mathcal{M} > 1$ and $\mathrm{Re} > \mathrm{Re}\mathrm{crit}$), shocks concentrate magnetic energy in large structures with $\ell_\mathrm{p} \sim (\ell_\mathrm{turb} / \ell_\mathrm{shock})^{1/3} \ell_\eta \gg \ell_\eta$, where $\ell_\mathrm{shock}$ is the characteristic shock width, and $\ell_\mathrm{turb}$ is the outer scale of the turbulent field. In this regime, magnetic field-line curvature becomes nearly independent of field strength. These results have implications for galaxy mergers and cosmic ray transport models in the interstellar medium., Comment: 30 pages, 15 figures, dataset available from MNRAS

Galaxies obey a number of empirical correlations between their radio, {\gamma}-ray, and infrared emission, but the physical origins of these correlations remain uncertain. Here we use the CONGRuENTS model for broadband non-thermal emission from star-forming galaxies, which self-consistently calculates energy-dependent transport and non-thermal emission from cosmic ray hadrons and leptons, to predict radio and {\gamma}-ray emission for a synthetic galaxy population with properties drawn from a large deep-field survey. We show that our synthetic galaxies reproduce observed relations such as the FIR-radio correlation, the FIR-{\gamma} correlation, and the distribution of radio spectral indices, and we use the model to explain the physical origins of these relations. Our results show that the FIR-radio correlation arises because the amount of cosmic ray electron power ultimately radiated as synchrotron emission varies only weakly with galaxy star formation rate as a result of the constraints imposed on gas properties by hydrostatic balance and turbulent dynamo action; the same physics dictates the extent of proton calorimetry in different galaxies, and thus sets the FIR-{\gamma}-ray correlation. We further show that galactic radio spectral indices result primarily from competition between thermal free-free emission and energy-dependent loss of cosmic ray electrons to bremsstrahlung and escape into galactic halos, with shaping of the spectrum by inverse Compton, synchrotron, and ionisation processes typically playing a sub-dominant role. In addition to explaining existing observations, we use our analysis to predict a heretofore unseen correlation between the curvature of galaxies' radio spectra and their pion-driven {\gamma}-ray emission, a prediction that will be testable with upcoming facilities., Comment: 17 pages, 14 figures

Multiphase galactic outflows, generated by supernova feedback, are likely to be more metal-rich than the interstellar media from which they are driven due to incomplete mixing between supernova ejecta and the ambient ISM. This enrichment is important for shaping galactic metallicities and metallicity gradients, but measuring it quantitatively from simulations requires resolution high enough to resolve mass, momentum and energy exchanges between the different phases of the outflows. In this context, we present simulations of outflows, driven by SN feedback, conducted using \textsc{Quokka}, a new GPU-optimised AMR radiation-hydrodynamics code. This code allows us to reach combinations of resolution, simulation volume, and simulation duration larger than those that have previously been possible, and to resolve all gas phases from cold neutral medium, $T \sim 100$ K, to hot ionised gas, $T \gtrsim 10^7$ K. In this, a first of a series of papers exploring generation and evolution of multiphase outflows from a wide range of galactic environments and star formation rates, we quantify the extent of selective metal loading in Solar neighbourhood-like environments. We explain the selective metal loading we find as a result of the transport of metals within and between phases, a phenomenon we can study owing to the parsec-scale resolution that our simulations achieve. We also quantify the sensitivity of metal loading studies to numerical resolution, and present convergence criteria for future studies., Comment: Submitted to MNRAS. Full title="Quokka-based Understanding of Outflows Derived from Extensive, Repeated, Accurate, Thorough, Demanding, Expensive, Memory-consuming, Ongoing Numerical Simulations of Transport, Removal, Accretion, Nucleosynthesis, Deposition, and Uplifting of Metals (QUOD ERAT DEMONSTRANDUM, i.e, QED)"

We investigate the variation in the upper end of stellar initial mass function (uIMF) in 375 young and compact star clusters in five nearby galaxies within $\sim 5$ Mpc. All the young stellar clusters (YSCs) in the sample have ages $\lesssim 4$ Myr and masses above 500 $M_{\odot}$, according to standard stellar models. The YSC catalogs were produced from Hubble Space Telescope images obtained as part of the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. They are used here to test whether the uIMF is universal or changes as a function of the cluster's stellar mass. We perform this test by measuring the H$\alpha$ luminosity of the star clusters as a proxy for their ionizing photon rate, and charting its trend as a function of cluster mass. Large cluster numbers allow us to mitigate the stochastic sampling of the uIMF. The advantage of our approach relative to previous similar attempts is the use of cluster catalogs that have been selected independently of the presence of H$\alpha$ emission, thus removing a potential sample bias. We find that the uIMF, as traced by the H$\alpha$ emission, shows no dependence on cluster mass, suggesting that the maximum stellar mass that can be produced in star clusters is universal, in agreement with previous findings.

The ionising radiation of young and massive stars is a crucial form of stellar feedback. Most ionising (Lyman-continuum; LyC, $\lambda < 912A$) photons are absorbed close to the stars that produce them, forming compact HII regions, but some escape into the wider galaxy. Quantifying the fraction of LyC photons that escape is an open problem. In this work, we present a semi-novel method to estimate the escape fraction by combining broadband photometry of star clusters from the Legacy ExtraGalactic UV Survey (LEGUS) with HII regions observed by the Star formation, Ionized gas, and Nebular Abundances Legacy Survey (SIGNALS) in the nearby spiral galaxy NGC 628. We first assess the completeness of the combined catalogue, and find that 49\% of HII regions lack corresponding star clusters as a result of a difference in the sensitivities of the LEGUS and SIGNALS surveys. For HII regions that do have matching clusters, we infer the escape fraction from the difference between the ionising power required to produce the observed HII luminosity and the predicted ionising photon output of their host star clusters; the latter is computed using a combination of LEGUS photometric observations and a stochastic stellar population synthesis code SLUG (Stochastically Lighting Up Galaxies). Overall, we find an escape fraction of $f_{esc} = 0.09^{+0.06}_{-0.06}$ across our sample of 42 HII regions; in particular, we find HII regions with high $f_{esc}$ are predominantly regions with low H$\alpha$-luminosity. We also report possible correlation between $f_{esc}$ and the emission lines [O ii]/[N ii] and [O ii]/H$\beta$., Comment: Accepted for publication at MNRAS

Dust is important for star formation because it is the crucial component that couples gas to stellar radiation fields, allowing radiation feedback to influence gas fragmentation and thus the stellar initial mass function (IMF). Variations in dust abundance therefore provide a potential avenue by which variation in galaxy metallicity might affect the IMF. In this paper we present a series of radiation-magnetohydrodynamic simulations in which we vary the metallicity and thus the dust abundance from 1% of Solar to 3$\times$ Solar, spanning the range from the lowest metallicity dwarfs to the most metal-rich early-type galaxies found in the local Universe. We design the simulations to keep all dimensionless parameters constant so that the interaction between feedback and star-forming environments of varying surface density and metallicity is the only factor capable of breaking the symmetry between the simulations and modifying the IMF, allowing us to cleanly isolate and understand the effects of each environmental parameter. We find that at a fixed surface density more metal-rich clouds tend to form a slightly more bottom-heavy IMF than metal-poor ones, primarily because in metal-poor gas radiation feedback is able to propagate further, heating somewhat larger volumes of gas. However, shifts in IMF with metallicity at a fixed surface density are much smaller than shifts with surface density at fixed metallicity; metallicity-induced IMF variations are too small to explain the variations in mass-to-light ratio reported in galaxies of different mass and metallicity. We, therefore, conclude that metallicity variations are much less important than variations in surface density in driving changes in the IMF and that the latter rather than the former are most likely responsible for the IMF variations found in early-type galaxies., Comment: 10 pages, 12 figures, submitted in MNRAS

Hundreds of high-velocity atomic gas clouds exist above and below the Galactic Centre, with some containing a molecular component. However, the origin of these clouds in the Milky Way's wind is unclear. This paper presents new high-resolution MeerKAT observations of three atomic gas clouds and studies the relationship between the atomic and molecular phases at $\sim 1$ pc scales. The clouds' atomic hydrogen column densities, $N_{\mathrm{HI}}$, are less than a $\mbox{few}\times 10^{20}$ cm$^{-2}$, but the two clouds closest to the Galactic Centre nonetheless have detectable CO emission. This implies the presence of H$_{2}$ at levels of $N_{\mathrm{HI}}$ at least a factor of ten lower than in the typical Galactic interstellar medium. For the cloud closest to the Galactic Centre, there is little correlation between the $N_{\mathrm{HI}}$ and the probability that it will harbour detectable CO emissions. In contrast, for the intermediate cloud, detectable CO is heavily biased toward the highest values of $N_{\mathrm{HI}}$. The cloud most distant from the Galactic Centre has no detectable CO at similar $N_{\mathrm{HI}}$ values. Moreover, we find that the two clouds with detectable CO are too molecule-rich to be in chemical equilibrium, given the depths of their atomic shielding layers, which suggests a scenario whereby these clouds consist of pre-existing molecular gas from the disc that the Galactic wind has swept up, and that is dissociating into atomic hydrogen as it flows away from the Galaxy. We estimate that entrained molecular material of this type has a $\sim \mathrm{few}-10$ Myr lifetime before photodissociating., Comment: 11 pages, 6 figures, 2 tables. Accepted for publication in MNRAS