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Major mergers between galaxies are predicted to fuel their central supermassive black holes (SMBHs), particularly after coalescence. However, determining the prevalence of active galactic nuclei (AGNs) in mergers remains a challenge, because AGN diagnostics are sensitive to details of the central structure (e.g. nuclear gas clouds, geometry, and orientation of a dusty torus) that are partly decoupled from SMBH accretion. X-rays, expected to be ubiquitous among accreting systems, are detectable through non-Compton-thick screens of obscuring material, and thus offer the potential for a more complete assessment of AGNs in mergers. But extant statistical X-ray studies of AGNs in mergers have been limited by either sparse, heterogeneous, or shallow on-sky coverage. We use new X-ray observations from the first SRG/eROSITA all-sky data release to characterize the incidence, luminosity, and observability of AGNs in mergers. Combining machine learning and visual classification, we identify 923 post-mergers in Dark Energy Camera Legacy Survey (DECaLS) imaging and select 4565 interacting galaxy pairs (with separations <120 kpc and mass ratios within 1:10) from the Sloan Digital Sky Survey. We find that galaxies with X-ray AGNs are 2.0 |$\pm$| 0.24 times as likely to be identified as post-mergers compared to non-AGN controls, and that post-mergers are 1.8 |$\pm$| 0.1 times as likely to host an X-ray AGN as non-interacting controls. A multiwavelength census of X-ray, optical, and mid-IR-selected AGNs suggests a picture wherein the underlying AGN fraction increases during pair-phase interactions, that galaxy pairs within ~20 kpc become heavily obscured, and that the obscuration often clears post-coalescence. [ABSTRACT FROM AUTHOR]

Merging and interactions can radically transform galaxies. However, identifying these events based solely on structure is challenging as the status of observed mergers is not easily accessible. Fortunately, cosmological simulations are now able to produce more realistic galaxy morphologies, allowing us to directly trace galaxy transformation throughout the merger sequence. To advance the potential of observational analysis closer to what is possible in simulations, we introduce a supervised deep learning convolutional neural network and vision transformer hybrid framework, Mummi (MU lti M odel M erger I dentifier). Mummi is trained on realism-added synthetic data from IllustrisTNG100-1, and is comprised of a multistep ensemble of models to identify mergers and non-mergers, and to subsequently classify the mergers as interacting pairs or post-mergers. To train this ensemble of models, we generate a large imaging data set of 6.4 million images targeting UNIONS with RealSimCFIS. We show that Mummi offers a significant improvement over many previous machine learning classifiers, achieving 95 per cent pure classifications even at Gyr long time-scales when using a jury-based decision-making process, mitigating class imbalance issues that arise when identifying real galaxy mergers from |$z=0$| to 0.3. Additionally, we can divide the identified mergers into pairs and post-mergers at 96 per cent success rate. We drastically decrease the false positive rate in galaxy merger samples by 75 per cent. By applying Mummi to the UNIONS DR5-SDSS DR7 overlap, we report a catalogue of 13 448 high-confidence galaxy merger candidates. Finally, we demonstrate that Mummi produces powerful representations solely using supervised learning, which can be used to bridge galaxy morphologies in simulations and observations. [ABSTRACT FROM AUTHOR]

Cosmological simulations have been used to study interacting galaxies as a function of galaxy pair separation, enabling comparisons with observational studies of galaxy pairs. The study of interacting galaxies as a function of time (i.e. merger stage) has mostly been limited to high-resolution merger simulations, due to the poor time sampling available in cosmological simulations. Building on an earlier study of galaxy pairs in the IllustrisTNG cosmological simulations, we reconstruct the orbits of galaxy pairs involving massive galaxies (M * > 1010 M⊙) at redshifts of 0 ≤ z < 1, using a novel kinematic interpolation scheme to model the orbits in between the IllustrisTNG snapshots (which are separated by 162 Myr, on average). We assess the accuracy of these interpolations using a pre-existing suite of merger simulations, and find that kinematic interpolations provide a remarkable improvement in accuracy compared with interpolations that use only radial separations or 3D positions. We find that nearly 90 per cent of the closest pairs (r < 25 kpc) have had a pericentre encounter within the past Gyr. Many of these close pairs are found on rapidly shrinking orbits, and roughly 85 per cent of these pairs will merge within 1 Gyr. However, approximately 3 per cent of these close pairs appear to be flyby systems that will never merge. These reconstructed orbits will be used in future studies to investigate how and when galaxy properties change during close encounters and mergers between galaxies in IllustrisTNG. [ABSTRACT FROM AUTHOR]

We investigate the connection between the most luminous active galactic nuclei (AGN), galaxy pairs, and post-mergers in the IllustrisTNG simulation. We select galaxy pairs and post-mergers with a mass ratio between 1:10 < μ < 1:1 and a redshift between 0 < z < 1. We compare the incidence of luminous AGN in pairs with matched non-pair controls, finding that AGN with luminosity L bol > 1044 erg s−1 occur in ∼26 per cent of paired galaxies with a companion within 20 kpc, compared with ∼14 per cent in controls (a fractional excess of ∼2). The enhancement of AGN in galaxy pairs is luminosity dependent, with the highest excess (a factor of ∼6 ± 2 at the closest separations) for AGN with L bol > 1045 erg s−1. Additionally, pairs exhibit a modest yet statistically significant excess of luminous AGN up to ∼150 kpc in separation. For pairs which merge between 0 < z < 1, AGN fractions are elevated between 1.5 Gyr before and after coalescence, with the highest excess closest to coalescence. Our results indicate that pre-coalescence interactions drive excesses of luminous AGN, but that luminous AGN in galaxy pairs are not ubiquitous. Finally, we investigate what fraction of AGN can be associated with an interaction (either having a companion within 100 kpc or a merger within the last 500 Myr). For AGN with L bol > 1045 erg s−1, ∼55 per cent are interacting, compared with a 30 per cent interaction fraction in AGN with 1044 < L bol < 1044.5 erg s−1. Our results support a picture in which interactions play a dominant role in (but are not the sole cause of) triggering the most luminous AGN. [ABSTRACT FROM AUTHOR]

Non-parametric morphology statistics have been used for decades to classify galaxies into morphological types and identify mergers in an automated way. In this work, we assess how reliably we can identify galaxy post-mergers with non-parametric morphology statistics. Low-redshift (z ≲ 0.2), recent (t post-merger ≲ 200 Myr), and isolated (r > 100 kpc) post-merger galaxies are drawn from the IllustrisTNG100-1 cosmological simulation. Synthetic r -band images of the mergers are generated with SKIRT9 and degraded to various image qualities, adding observational effects such as sky noise and atmospheric blurring. We find that even in perfect quality imaging, the individual non-parametric morphology statistics fail to recover more than 55 per cent of the post-mergers, and that this number decreases precipitously with worsening image qualities. The realistic distributions of galaxy properties in IllustrisTNG allow us to show that merger samples assembled using individual morphology statistics are biased towards low-mass, high gas fraction, and high mass ratio. However, combining all of the morphology statistics together using either a linear discriminant analysis or random forest algorithm increases the completeness and purity of the identified merger samples and mitigates bias with various galaxy properties. For example, we show that in imaging similar to that of the 10-yr depth of the Legacy Survey of Space and Time, a random forest can identify 89 per cent of mergers with a false positive rate of 17 per cent. Finally, we conduct a detailed study of the effect of viewing angle on merger observability and find that there may be an upper limit to merger recovery due to the orientation of merger features with respect to the observer. [ABSTRACT FROM AUTHOR]

Background National Cancer Institute-Children's Oncology Group Pediatric Molecular Analysis for Therapy Choice assigns patients aged 1-21 years with refractory solid tumors, brain tumors, lymphomas, and histiocytic disorders to phase II trials of molecularly targeted therapies based on detection of predefined genetic alterations. Patients whose tumors harbored EZH2 mutations or loss of SMARCB1 or SMARCA4 by immunohistochemistry were treated with EZH2 inhibitor tazemetostat. Methods Patients received tazemetostat for 28-day cycles until disease progression or intolerable toxicity (max 26 cycles). The primary endpoint was objective response rate; secondary endpoints included progression-free survival and tolerability of tazemetostat. Results Twenty patients (median age = 5 years) enrolled, all evaluable for response and toxicities. The most frequent diagnoses were atypical teratoid rhabdoid tumor (n = 8) and malignant rhabdoid tumor (n = 4). Actionable alterations consisted of SMARCB1 loss (n = 16), EZH2 mutation (n = 3), and SMARCA4 loss (n = 1). One objective response was observed in a patient with non-Langerhans cell histiocytosis with SMARCA4 loss (26 cycles, 1200 mg/m2/dose twice daily). Four patients with SMARCB1 loss had a best response of stable disease: epithelioid sarcoma (n = 2), atypical teratoid rhabdoid tumor (n = 1), and renal medullary carcinoma (n = 1). Six-month progression-free survival was 35% (95% confidence interval [CI] = 15.7% to 55.2%) and 6-month overall survival was 45% (95% CI = 23.1% to 64.7%). Treatment-related adverse events were consistent with prior tazemetostat reports. Conclusions Although tazemetostat did not meet its primary efficacy endpoint in this population of refractory pediatric tumors (objective response rate = 5%, 90% CI = 1% to 20%), 25% of patients with multiple histologic diagnoses experienced prolonged stable disease of 6 months and over (range = 9-26 cycles), suggesting a potential effect of tazemetostat on disease stabilization. [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]

The kinematic disturbances associated with major galaxy mergers are known to produce gas inflows, which in turn may trigger accretion onto the supermassive black holes (SMBH) of the participant galaxies. While this effect has been studied in galaxy pairs, the frequency of active galactic nuclei (AGNs) in fully coalesced post-merger systems is poorly constrained due to the limited size or impurity of extant post-merger samples. Previously, we combined convolutional neural network (CNN) predictions with visual classifications to identify a highly pure sample of 699 post-mergers in deep r -band imaging. In the work presented here, we quantify the frequency of AGNs in this sample using three metrics: optical emission lines, mid-infrared (mid-IR) colour, and radio detection of low-excitation radio galaxies (LERGs). We also compare the frequency of AGNs in post-mergers to that in a sample of spectroscopically identified galaxy pairs. We find that AGNs identified by narrow-line optical emission and mid-IR colour have an increased incidence rate in post-mergers, with excesses of ~4 over mass- and redshift-matched controls. The optical and mid-IR AGN excesses in post-mergers exceed the values found for galaxy pairs, indicating that AGN activity in mergers peaks after coalescence. Conversely, we recover no significant excess of LERGs in post-mergers or pairs. Finally, we find that the [O  iii ] luminosity (a proxy for SMBH accretion rate) in post-mergers that host an optical AGN is ~0.3 dex higher on average than in non-interacting galaxies with an optical AGN, suggesting that mergers generate higher accretion rates than secular triggering mechanisms. [ABSTRACT FROM AUTHOR]

We present an analysis of the instantaneous supermassive black hole (SMBH) accretion rates in a collection of 1563 post-merger galaxies drawn from the IllustrisTNG simulation. Our sample consists of galaxies that have experienced a merger in the last simulation snapshot (within ∼ 160 Myrs of coalescence) in the redshift range 0 < z < 1, with merger stellar mass ratios >1: 10 and post-merger stellar masses >1010M⊙. We find that, on average, the accretion rates of the post-mergers are ∼1.7 times higher than in a control sample and that post-mergers are three to four times more likely to experience a luminous active galactic nuclei (AGN) phase than isolated galaxies. SMBH accretion rate enhancements persist for ∼2 Gyrs after coalescence, significantly exceeding the ∼500 Myr lifetime of star formation rate enhancements. We find that the presence of simultaneous enhancements in both the star formation and SMBH accretion rates depends on both the mass ratio of the merger and on the gas mass of the post-merger galaxy. Despite these accretion rate enhancements, only ∼35 per cent of post-mergers experience a luminous AGN (L bol > 1044erg/s) within 500 Myrs after coalescence, and fewer than 10 per cent achieve a luminosity in excess of L bol > 1045erg/s. Moreover, only ∼10 per cent of the highest luminosity (L bol > 1045erg/s) AGN in the IllustrisTNG galaxy sample are recent mergers. Our results are therefore consistent with a picture in which mergers can (but do not always) trigger AGN activity, but where the majority of galaxies hosting high luminosity AGN are not recent mergers. [ABSTRACT FROM AUTHOR]

We investigate the role of galaxy mergers on supermassive black hole (SMBH) accretion and star formation quenching in three state-of-the-art cosmological simulations with contrasting physics models: EAGLE, Illustris, and IllustrisTNG (TNG). We find that recently coalesced 'post-mergers' in all three simulations have elevated SMBH accretion rates by factors of ∼2–5. However, rapid (within 500 Myr of coalescence) quenching of star formation is rare, with incidence rates of |$0.4{{\ \rm per\ cent}}$| in Illustris, |$4.5{{\ \rm per\ cent}}$| in EAGLE, and |$10{{\ \rm per\ cent}}$| in TNG. The rarity of quenching in post-mergers results from substantial gas reservoirs that remain intact after the merger. The post-mergers that do successfully quench tend to be those that had both low pre-merger gas fractions as well as those that experience the largest gas losses. Although rare, the recently quenched fraction of post-mergers is still elevated compared to a control sample of non-mergers by factors of two in TNG and 11 in EAGLE. Conversely, quenching is rarer in Illustris post-mergers than in their control. Recent observational results by Ellison et al. have found rapid quenching to be at least 30 times more common in post-mergers, a significantly higher excess than found in any of the simulations. Our results therefore indicate that whilst merger-induced SMBH accretion is a widespread prediction of the simulation, its link to quenching depends sensitively on the physics models, and that none of the subgrid models of the simulations studied here can fully capture the connection between mergers and rapid quenching seen in observations. [ABSTRACT FROM AUTHOR]

Galaxy mergers trigger both star formation and accretion on to the central supermassive black hole. As a result of subsequent energetic feedback processes, it has long been proposed that star formation may be promptly extinguished in galaxy merger remnants. However, this prediction of widespread, rapid quenching in late stage mergers has been recently called into question with modern simulations and has never been tested observationally. Here we perform the first empirical assessment of the long-predicted end phase in the merger sequence. Based on a sample of ∼500 post-mergers identified from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), we show that the frequency of post-merger galaxies that have rapidly shutdown their star formation following a previous starburst is 30–60 times higher than expected from a control sample of non-merging galaxies. No such excess is found in a sample of close galaxy pairs, demonstrating that mergers can indeed lead to a rapid halt to star formation, but that this process only manifests after coalescence. [ABSTRACT FROM AUTHOR]

Galaxy mergers are known to trigger both extended and central star formation. However, what remains to be understood is whether this triggered star formation is facilitated by enhanced star formation efficiencies (SFEs), or an abundance of molecular gas fuel. This work presents spatially resolved measurements of CO emission collected with the Atacama Large Millimetre Array (ALMA) for 20 merging galaxies (either pairs or post-mergers) selected from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. Eleven additional merging galaxies are selected from the ALMA MaNGA QUEnching and STar formation (ALMaQUEST) survey, resulting in a set of 31 mergers at various stages of interaction and covering a broad range of star formation rates (SFRs). We investigate galaxy-to-galaxy variations in the resolved Kennicutt–Schmidt relation, (rKS: |$\Sigma _{\textrm {H}_2}$| versus ΣSFR), the resolved molecular gas main sequence (rMGMS: Σ⋆ versus |$\Sigma _{\textrm {H}_2}$|⁠), and the resolved star-forming main sequence (rSFMS: Σ⋆ versus ΣSFR). We quantify offsets from these resolved relations to determine if SFR, molecular gas fraction, or/and SFE is/are enhanced in different regions of an individual galaxy. By comparing offsets in all three parameters, we can discern whether gas fraction or SFE powers an enhanced ΣSFR. We find that merger-induced star formation can be driven by a variety of mechanisms, both within a galaxy and between different mergers, regardless of interaction stage. [ABSTRACT FROM AUTHOR]

Powerful outflows are thought to play a critical role in galaxy evolution and black hole growth. We present the first large-scale systematic study of ionized outflows in paired galaxies and post-mergers compared to a robust control sample of isolated galaxies. We isolate the impact of the merger environment to determine if outflow properties depend on merger stage. Our sample contains ∼4000 paired galaxies and ∼250 post-mergers in the local universe (0.02 ≤ z ≤ 0.2) from the Sloan Digital Sky Survey Data Release 7 (SDSS DR 7) matched in stellar mass, redshift, local density of galaxies, and [O  iii ] λ5007 luminosity to a control sample of isolated galaxies. By fitting the [O  iii ] λ5007 line, we find ionized outflows in ∼15 per cent of our entire sample. Outflows are much rarer in star-forming galaxies compared to active galactic nuclei (AGNs), and outflow incidence and velocity increase with [O  iii ] λ5007 luminosity. Outflow incidence is significantly elevated in the optical + mid-infrared selected AGN compared to purely optical AGN; over 60 per cent show outflows at the highest luminosities (⁠|$L_{\mathrm{[OIII]~\lambda 5007}}\, \gtrsim$| 1042 erg s−1), suggesting mid-infrared AGN selection favours galaxies with powerful outflows, at least for higher [O  iii ] λ5007 luminosities. However, we find no statistically significant difference in outflow incidence, velocity, and luminosity in mergers compared to isolated galaxies, and there is no dependence on merger stage. Therefore, while interactions are predicted to drive gas inflows and subsequently trigger nuclear star formation and accretion activity, when the power source of the outflow is controlled for, the merging environment has no further impact on the large-scale ionized outflows as traced by [O  iii ] λ5007. [ABSTRACT FROM AUTHOR]

The importance of the post-merger epoch in galaxy evolution has been well documented, but post-mergers are notoriously difficult to identify. While the features induced by mergers can sometimes be distinctive, they are frequently missed by visual inspection. In addition, visual classification efforts are highly inefficient because of the inherent rarity of post-mergers (~1 per cent in the low-redshift Universe), and non-parametric statistical merger selection methods do not account for the diversity of post-mergers or the environments in which they appear. To address these issues, we deploy a convolutional neural network (CNN) that has been trained and evaluated on realistic mock observations of simulated galaxies from the IllustrisTNG simulations, to galaxy images from the Canada France Imaging Survey, which is part of the Ultraviolet Near Infrared Optical Northern Survey. We present the characteristics of the galaxies with the highest CNN-predicted post-merger certainties, as well as a visually confirmed subset of 699 post-mergers. We find that post-mergers with high CNN merger probabilities [ p (x) > 0.8] have an average star formation rate that is 0.1 dex higher than a mass- and redshift-matched control sample. The SFR enhancement is even greater in the visually confirmed post-merger sample, a factor of 2 higher than the control sample. [ABSTRACT FROM AUTHOR]

One of the central challenges to establishing the role of mergers in galaxy evolution is the selection of pure and complete merger samples in observations. In particular, while large and reasonably pure interacting galaxy pair samples can be obtained with relative ease via spectroscopic criteria, automated selection of post-coalescence merger remnants is restricted to the physical characteristics of remnants alone. Furthermore, such selection has predominantly focused on imaging data – whereas kinematic data may offer a complimentary basis for identifying merger remnants. Therefore, we examine the theoretical utility of both the morphological and kinematic features of merger remnants in distinguishing galaxy merger remnants from other galaxies. Deep classification models are calibrated and evaluated using idealized synthetic images and line-of-sight stellar velocity maps of a heterogeneous population of galaxies and merger remnants from the TNG100 cosmological hydrodynamical simulation. We show that even idealized stellar kinematic data have limited utility compared to imaging and underperforms by |$2.1 \pm 0.5 {{\ \rm per\ cent}}$| in completeness and |$4.7 \pm 0.4 {{\ \rm per\ cent}}$| in purity for our fiducial model architecture. Combining imaging and stellar kinematics offers a small boost in completeness (by |$1.8 \pm 0.4 {{\ \rm per\ cent}}$|⁠ , compared to |$92.7 \pm 0.2 {{\ \rm per\ cent}}$| from imaging alone) but no change in purity (⁠|$0.1\pm 0.3 {{\ \rm per\ cent}}$| improvement compared to |$92.7 \pm 0.2 {{\ \rm per\ cent}}$|⁠ , evaluated with equal numbers of merger remnant and non-remnant control galaxies). Classification accuracy of all models is particularly sensitive to physical companions at separations ≲ 40 kpc and to time-since-coalescence. Taken together, our results show that the stellar kinematic data have little to offer in compliment to imaging for merger remnant identification in a heterogeneous galaxy population. [ABSTRACT FROM AUTHOR]

Background This is a phase II subprotocol of the NCI-COG Pediatric MATCH study evaluating vemurafenib, a selective oral inhibitor of BRAF V600 mutated kinase, in patients with relapsed or refractory solid tumors harboring BRAF V600 mutations. Methods Patients received vemurafenib at 550 mg/m2 (maximum 960 mg/dose) orally twice daily for 28-day cycles until progression or intolerable toxicity. The primary aim was to determine the objective response rate and secondary objectives included estimating progression-free survival and assessing the tolerability of vemurafenib. Results Twenty-two patients matched to the subprotocol and 4 patients (18%) enrolled. Primary reasons for non-enrollment were ineligibility due to exclusions of low-grade glioma (n  = 7) and prior BRAF inhibitor therapy (n  = 7). Enrolled diagnoses were one each of histiocytosis, ameloblastoma, Ewing sarcoma, and high-grade glioma, all with BRAF V600E mutations. Treatment was overall tolerable with mostly expected grade 1/2 adverse events (AE). Grade 3 or 4 AE on treatment were acute kidney injury, hyperglycemia, and maculopapular rash. One patient came off therapy due to AE. One patient (glioma) had an objective partial response and remained on protocol therapy for 15 cycles. Conclusion There was a low accrual rate on this MATCH subprotocol, with only 18% of those who matched with BRAFV600 mutations enrolling, resulting in early termination, and limiting study results (ClinicalTrials.gov Identifier: NCT03220035). [ABSTRACT FROM AUTHOR]

Background The National Cancer Institute-Children's Oncology Group Pediatric Molecular Analysis for Therapy Choice (MATCH) precision oncology platform trial enrolled children aged 1-21 years with treatment-refractory solid tumors and predefined actionable genetic alterations. Patients with tumors harboring alterations in DNA damage repair (DDR) genes were assigned to receive olaparib. Methods Tumor and blood samples were submitted for centralized molecular testing. Tumor and germline sequencing were conducted in parallel. Olaparib was given twice daily for 28-day cycles starting at a dose 30% lower than the adult recommended phase 2 dose (RP2D). The primary endpoint was the objective response. Results Eighteen patients matched (1.5% of those screened) based on the presence of a deleterious gene alteration in BRCA1/2, RAD51C/D, or ATM detected by tumor sequencing without germline subtraction or analysis of loss of heterozygosity (LOH). Eleven (61%) harbored a germline mutation, with only one exhibiting LOH. Six patients enrolled and received the olaparib starting dose of 135 mg/m2/dose. Two participants were fully evaluable; 4 were inevaluable because <85% of the prescribed dose was administered during cycle 1. There were no dose-limiting toxicities or responses. Minimal hematologic toxicity was observed. Conclusion Most DDR gene alterations detected in Pediatric MATCH were germline, monoallelic, and unlikely to confer homologous recombination deficiency predicting sensitivity to olaparib monotherapy. The study closed due to poor accrual. ClinicalTrials.gov Identifier NCT03233204. IRB approved: initial July 24, 2017. [ABSTRACT FROM AUTHOR]

Galaxy mergers are traditionally one of the favoured mechanisms for the transformation of spiral galaxies to spheroids and for quenching star formation. To test this paradigm in the context of modern cosmological simulations, we use the IllustrisTNG simulation to investigate the impact of individual merger events on quenching star formation [i.e. star formation rate (SFR) at least 3σ below the star-forming main sequence] within 500 Myr after the coalescence phase. The rate of quenching amongst recently merged galaxies is compared with a control sample that is matched in redshift, stellar mass, SFR, black hole mass, and environment. We find quenching to be uncommon among the descendants of post-merger galaxies, with only |${\sim} 5{{\ \rm per\ cent}}$| of galaxies quenching within 500 Myr after the merger. Despite this low absolute rate, we find that quenching occurs in post-mergers at twice the rate of the control galaxies. The fraction of quenched post-merger descendants 1.5 Gyr after the merger become statistically indistinguishable from that of non-post-mergers, suggesting that mergers could speed up the quenching process in those post-mergers whose progenitors had physical conditions able to sustain effective active galactic nuclei (AGN) kinetic feedback, thus capable of removing gas from galaxies. Our results indicate that although quenching does not commonly occur promptly after coalescence, mergers none the less do promote the cessation of star formation in some post-mergers. We find that, in IllustrisTNG, it is the implementation of the AGN kinetic feedback that is responsible for quenching post-mergers, as well as non-post-merger controls. As a result of the released kinetic energy, galaxies experience gas loss and eventually they will quench. Galaxies with an initially low gas fraction show a preferable pre-disposition towards quenching. The primary distinguishing factor between quenched and star-forming galaxies is gas fraction, with a sharp boundary at f gas ∼ 0.1 in TNG. [ABSTRACT FROM AUTHOR]

We investigate the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy-merger simulations (stellar mass ratio = 2.5:1), which employs the 'Feedback In Realistic Environments-2' model (fire-2). This framework resolves star formation, feedback processes, and the multiphase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (H  i) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR enhancement in the central kiloparsec is fuel-driven (55 per cent for the secondary, 71 per cent for the primary) – while central SFR suppression is efficiency-driven (91 per cent for the secondary, 97 per cent for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially resolved galaxy surveys – capable of examining vast and diverse samples of interacting systems – coupled with multiwavelength campaigns aimed to capture their internal ISM structure. [ABSTRACT FROM AUTHOR]

We use the IllustrisTNG cosmological hydrodynamical simulations to investigate how the specific star formation rates (sSFRs) of massive galaxies (M * > 1010 M⊙) depend on the distance to their closest companions. We estimate sSFR enhancements by comparing with control samples that are matched in redshift, stellar mass, local density, and isolation, and we restrict our analysis to pairs with stellar mass ratios of 0.1 to 10. At small separations (∼15 kpc), the mean sSFR is enhanced by a factor of 2.0 ± 0.1 in the flagship (110.7 Mpc)3 simulation (TNG100-1). Statistically significant enhancements extend out to 3D separations of 280 kpc in the (302.6 Mpc)3 simulation (TNG300-1). We find similar trends in the EAGLE and Illustris simulations, although their sSFR enhancements are lower than those in TNG100-1 by about a factor of two. Enhancements in IllustrisTNG galaxies are seen throughout the redshift range explored (0 ≤ |$z$| < 1), with the strength of the enhancements decreasing with increasing redshift for galaxies with close companions. In order to more closely compare with observational results, we separately consider 2D projected distances between galaxies in IllustrisTNG. We detect significant sSFR enhancements out to projected separations of 260 kpc in TNG300-1, with projection effects diluting the size of the enhancements by about 20 per cent below 50 kpc. We find similar sSFR enhancements in TNG100-1 and Sloan Digital Sky Survey galaxies, with enhancements extending out to projected separations of about 150 kpc for star-forming galaxies at |$z$| < 0.2. Finally, by summing over all separations, we estimate that the presence of closest companions boosts the average sSFR of massive galaxies in TNG100-1 by 14.5 per cent. [ABSTRACT FROM AUTHOR]

In recent observational studies, star-forming galaxies have been shown to follow a relation often dubbed the fundamental metallicity relation (FMR). This relation links the stellar mass of a galaxy with its star formation rate (SFR) and its gas-phase metallicity. Specifically, the FMR predicts that galaxies, at a given stellar mass, exhibit lower metallicities for higher SFRs. This trend is qualitatively consistent with observations of galaxy pairs, which have been robustly shown to experience increasing gas-phase metallicity dilution and enhanced star formation activity with decreasing projected separation. In this work, we show that, despite the qualitative consistency with FMR expectations, the observed O/H dilution in galaxy pairs of the Sloan Digital Sky Survey is stronger than what is predicted by the FMR. We conclude that the evolutionary phase of galaxies interacting with companions is not encoded in the FMR, and thus, mergers constitute a clearly defined population of outliers. We find that galaxies in pairs are consistent with the FMR only when their separation is larger than 110 kpc. Finally, we also quantify the local environment of the pairs using the number of galaxy neighbours within 2 Mpc, N 2, and the projected separation to the second closest galaxy, r 2. We find that pairs are more sensitive to a second companion than to the local galaxy density, displaying less elevated SFRs with smaller values of r 2. [ABSTRACT FROM AUTHOR]

Galaxy mergers are a major evolutionary transformation whose effects are borne out by a plethora of observations and numerical simulations. However, most previous simulations have used idealized, isolated, binary mergers and there has not been significant progress on studying statistical samples of galaxy mergers in large cosmological simulations. We present a sample of 27 691 post-merger (PM) galaxies (0 c ≤ z ≤ 1) identified from IllustrisTNG: a cosmological, large box, magnetohydrodynamical simulation suite. The PM sample spans a wide range of merger and galaxy properties (M ⋆, μ, f gas). We demonstrate that star-forming (SF) PMs exhibit enhanced star formation rates (SFRs) on average by a factor of ∼2, while the passive PMs show no statistical enhancement. We find that the SFR enhancements: (1) show no dependence on redshift, (2) anticorrelate with the PM's stellar mass, and (3) correlate with the gas fraction of the PM's progenitors. However, SF PMs show stronger enhancements which may indicate other processes being at play (e.g. gas phase, feedback efficiency). Although the SFR enhancement correlates mildly with the merger mass ratio, the more abundant minor mergers (0.1 ≤ μ < 0.3) still contribute |${\sim}50{{\ \rm per\ cent}}$| of the total SFR enhancement. By tracing the PM sample forward in time, we find that galaxy mergers can drive significant SFR enhancements which decay over ∼0.5 Gyr independent of the merger mass ratio, although the decay time-scale is dependent on the simulation resolution. The strongest merger-driven starburst galaxies evolve to be passive/quenched on faster time-scales than their controls. [ABSTRACT FROM AUTHOR]

The question of whether galaxy mergers are linked to the triggering of active galactic nuclei (AGN) continues to be a topic of considerable debate. The issue can be broken down into two distinct questions: (1) Can galaxy mergers trigger AGN? (2) Are galaxy mergers the dominant AGN triggering mechanism? A complete picture of the AGN-merger connection requires that both of these questions are addressed with the same data set. In previous work, we have shown that galaxy mergers selected from the Sloan Digital Sky Survey (SDSS) show an excess of both optically selected, and mid-IR colour-selected AGN, demonstrating that the answer to the first of the above questions is affirmative. Here, we use the same optical and mid-IR AGN selection to address the second question, by quantifying the frequency of morphological disturbances in low-surface brightness r -band images from the Canada France Imaging Survey (CFIS). Only ∼30 per cent of optical AGN host galaxies are morphologically disturbed, indicating that recent interactions are not the dominant trigger. However, almost 60 per cent of mid-IR AGN hosts show signs of visual disturbance, indicating that interactions play a more significant role in nuclear feeding. Both mid-IR and optically selected AGN have interacting fractions that are a factor of two greater than a mass and redshift matched non-AGN control sample, an excess that increases with both AGN luminosity and host galaxy stellar mass. [ABSTRACT FROM AUTHOR]

We present a comprehensive suite of high-resolution (parsec-scale), idealized (non-cosmological) galaxy merger simulations (24 runs, stellar mass ratio ∼2.5:1) to investigate the connection between interaction-induced star formation and the evolution of the interstellar medium (ISM) in various temperature–density regimes. We use the gizmo code and the second version of the 'Feedback in Realistic Environments' model (FIRE-2), which captures the multiphase structure of the ISM. Our simulations are designed to represent galaxy mergers in the local Universe. In this work, we focus on the 'galaxy-pair period' between first and second pericentric passage. We split the ISM into four regimes: hot, warm, cool, and cold-dense, motivated by the hot, ionized, atomic and molecular gas phases observed in real galaxies. We find that, on average, interactions enhance the star formation rate of the pair (⁠|${\sim }30{{\ \rm per\ cent}}$|⁠, merger-suite sample average) and elevate their cold-dense gas content (⁠|${\sim }18{{\ \rm per\ cent}}$|⁠). This is accompanied by a decrease in warm gas (⁠|${\sim }11{{\ \rm per\ cent}}$|⁠), a negligible change in cool gas (⁠|${\sim }4{{\ \rm per\ cent}}$| increase), and a substantial increase in hot gas (⁠|${\sim }400{{\ \rm per\ cent}}$|⁠). The amount of cold-dense gas with densities above 1000 cm−3 (the cold ultra-dense regime) is elevated significantly (⁠|${\sim }240{{\ \rm per\ cent}}$|⁠), but only accounts for |${\sim }0.15{{\ \rm per\ cent}}$| (on average) of the cold-dense gas budget. [ABSTRACT FROM AUTHOR]

We investigate the dependence of galaxy structure on a variety of galactic and environmental parameters for ∼500 000 galaxies at z < 0.2, taken from the Sloan Digital Sky Survey data release 7 (SDSS-DR7). We utilize bulge-to-total stellar mass ratio (B/T)* as the primary indicator of galactic structure, which circumvents issues of morphological dependence on waveband. We rank galaxy and environmental parameters in terms of how predictive they are of galaxy structure, using an artificial neural network approach. We find that distance from the star-forming main sequence (ΔSFR), followed by stellar mass (M *), are the most closely connected parameters to (B/T)*, and are significantly more predictive of galaxy structure than global star formation rate (SFR), or any environmental metric considered (for both central and satellite galaxies). Additionally, we make a detailed comparison to the Illustris hydrodynamical simulation and the LGalaxies semi-analytic model. In both simulations, we find a significant lack of bulge-dominated galaxies at a fixed stellar mass, compared to the SDSS. This result highlights a potentially serious problem in contemporary models of galaxy evolution. [ABSTRACT FROM AUTHOR]