arXiv subjects

Sara L. Ellison

Publications and source records attributed to Sara L. Ellison.

At least 19 recordsLinked to original sources

arXiv2026

The Mass Dependence of the Fundamental Metallicity Relation in Observations and Simulations

The metal content of galaxies provides direct insight into the underlying physical processes that drive galaxy evolution. An example of this is the three-parameter relationship between stellar mass, gas-phase metallicity, and star formation rate, commonly referred to as the Fundamental Metallicity Relation (FMR). Previous studies have suggested that the FMR is redshift-invariant (at $z \lesssim 4$) and fully accounts for the scatter in the mass-metallicity relation (MZR). In this work, we test this 'fundamental' relation in both cosmological simulations (EAGLE, SIMBA, Illustris, IllustrisTNG) and Sloan Digital Sky Survey (SDSS) observations. We find that the canonical anti-correlation between metallicity and specific star formation rate (sSFR) inverts in massive galaxies ($M_\star \gtrsim 10^{10.5} \mathrm{M}_\odot$) in EAGLE, IllustrisTNG, and SDSS. When including lower star forming galaxies, the positive correlation appears for all four simulations and SDSS. We speculate that this inversion may being driven by strong nuclear outflows (from, e.g., active galactic nuclei or stellar feedback), which quench star formation while simultaneously expelling preferentially enriched gas from the center of the galaxy. We also find that this 'inversion' appears in a number of metallicity diagnostics in observations (though the details depend on diagnostic) and persists out to $z \sim 1$ in the simulations. These results demonstrate that these strong nuclear outflows challenge simple gas regulator-type models and provide a new framework to test models of the baryon cycle in both future simulations and observations.

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arXiv2026

The effect of galaxy interactions on star formation rates in the COLIBRE simulations

Observations and theory indicate that galaxy interactions enhance star formation rates (SFRs). However, the degree of enhancement and its dependence on the properties of the interacting galaxies vary across different studies. In this work, we use the COLIBRE simulations of galaxy formation to investigate the effect of interactions on the SFRs of star-forming galaxies at redshift $z\approx0$. The COLIBRE simulations capture the multiphase nature of the interstellar medium and have volumes up to $200^3$ and $400^3$ cMpc$^3$ at m6 (gas and dark-matter particle mass $\sim10^6~\mathrm{M_\odot}$) and m7 ($\sim10^7~\mathrm{M_\odot}$) resolutions, respectively. After constructing samples of interacting galaxies (with mass ratios $>0.1$) and isolated controls, matched in stellar mass, large- and small-scale environment, and redshift, we show that the average specific SFR (sSFR) of interacting galaxies is enhanced by up to a factor of $\approx2$ for separations of $\approx10$ kpc. The enhancement decreases with pair separation but remains significant out to $\approx200$ kpc. The enhancement increases with increasing numerical resolution, is more pronounced in the central regions of galaxies, and decreases with increasing stellar mass at fixed separation. Mergers with higher mass ratios induce stronger sSFR enhancement. We compare our results with observational data from the SDSS, finding good agreement in the dependence of the mean sSFR enhancement on separation, but underpredicting its normalisation by a factor of $\approx2$. Finally, we show that the pre-merger sSFR enhancement of resolved interactions accounts for $\approx2$ per cent of the $z\approx0$ cosmic SFR density.

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arXiv2026

Merger Driven or Internal Evolution? A New Morphological Study of Tidal Disruption Event Host Galaxies

The host galaxies of tidal disruption events (TDEs) show enhanced central stellar concentration and are preferentially found in poststarburst and green valley populations. This connection has led to the proposal that TDE host galaxies likely have gone through recent mergers. We conduct a new morphological study of 14 TDE host galaxies, using the r-band images from the Sloan Digital Sky Survey (SDSS), Dark Energy Camera Legacy Survey, and Ultraviolet Near-Infrared Optical Northern Survey, with the images from the latter two surveys having much higher depth and resolution than SDSS. We examine galaxy structures using conventional methods and also apply diagnostics of merger activity from a suite of machine learning models. Consistent with previous studies, our results show that TDE host galaxies are ~16% more centrally concentrated when compared to non-TDE-host controls. However, surprisingly, TDE hosts lack any indication of significant recent merger activity from both morphological analysis and the machine learning merger classifier. Instead, our results reveal that TDE host galaxies in the green valley are approximately 1.5-3 times more likely to have bar-like or ringlike structures compared to their controls. Based on these results, we propose that bar-driven secular evolution, instead of mergers, likely dominates the recent evolution of the TDE hosts found in the green valley, which can simultaneously explain their distinctive nuclear properties and enhanced TDE rates.

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arXiv2026

MAUVE-MUSE: When Metallicity Follows or Fights Star Formation -- A Mass-Dependent Inversion in Virgo Galaxies

Although globally-integrated studies often find that, at fixed stellar mass, high star formation rate (SFR) galaxies are relatively metal-poor while lower-SFR systems are more metal-rich, the corresponding coupling between gas-phase metallicity ($Z_{\rm gas}$) and star formation on sub-galactic scales remains poorly constrained. In this study, we analyse 14 Virgo spirals from the MAUVE-MUSE survey to revisit the resolved mass-metallicity relation (rMZR) and its secondary dependence on SFR surface density ($Σ_\mathrm{SFR}$) at $\sim 100$\,pc scales. We construct co-spatial maps of stellar mass surface density ($Σ_*$), $Σ_\mathrm{SFR}$, and oxygen abundance. MAUVE-MUSE galaxies follow a standard rMZR, but when binned by $Σ_*$, we find a mass-dependent inversion in the $Z_\mathrm{gas}$-$Σ_\mathrm{SFR}$ relation using O3N2 calibrations: the standard anti-correlation is confined to low-$Σ_*$ bins, while high-$Σ_*$ regions show a positive correlation, inverting at $\log_{10}(Σ_*/M_\odot\,\mathrm{kpc}^{-2})\simeq 7.5$-8.0. Correlated and anti-correlated \ion{H}{ii} regions coexist within the same discs; the mass dependence emerges only when grouping spaxels by $Σ_*$. We develop a spatially resolved gas-regulator model showing this $Z_\mathrm{gas}$-$Σ_\mathrm{SFR}$ (anti-)correlation arises from competition between star-formation-driven and gas-supply-driven variability. This framework naturally extrapolates to the integrated scenario, providing a unified explanation for resolved and global relations. However, the presence and strength of the $Z_\mathrm{gas}$-$Σ_\mathrm{SFR}$ (anti-)correlation depend strongly on the metallicity indicator used, highlighting the challenge of disentangling physical secondary trends within metallicity scaling relations.

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arXiv2026

Galaxy evolution in the post-merger regime. IV -- The long-term effect of mergers on galactic stellar mass growth and distribution

Galaxy mergers are known to trigger bursts of central star formation, which should therefore lead to stellar mass growth in their inner regions. However, observational measurements of this `burst mass fraction' are scant. Here, we assemble a large sample of ~14,000 post-coalescence galaxies that have recently completed their merger-induced star formation, and compare various measurements of central stellar mass with a matched control sample. Specifically, we quantify (at fixed redshift, star formation rate and total stellar mass) the stellar mass enhancement within a fixed angular aperture (Delta M_{star,fibre}) and in the galactic bulge (Delta M_{star,bulge}), finding burst mass fractions of 10 -- 20%. 61 galaxies in our sample are at z<0.05 and have integral field unit data from the Mapping Galaxies at Apache Point (MaNGA) survey, allowing further kpc-scale assessment of excess stellar mass and radial gradients. When assessed within apertures defined in units of kpc we again find a ~ 15 -- 20% excess of stellar mass in the central regions of the post-mergers compared with matched controls. However, within apertures defined in units of effective radius this stellar mass enhancement increases to 40%, suggesting that the relative structure/size of the galaxy is important for regulating the location of the merger induced star formation. Moreover, we find that these stellar mass enhancements are spatially extended, out to ~7 kpc or around 1 R/R_e, although the small sample size of the MaNGA overlap limits our radial sampling. Our work represents the first direct measurement of merger-induced stellar mass growth that is independent of stellar population modelling, or fitting light profiles, demonstrating significant and extended mass build-up in late stage post-mergers.

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arXiv2026

A Comprehensive Analysis of WISE Mid-Infrared Colors for Obscured AGN Selection

In this paper, we investigate the robustness of WISE mid-IR color selection (W1-W2) for identifying obscured (Type 2) active galactic nuclei (AGNs) at low redshift (z<0.3), using a sample of ~360,000 SDSS galaxies classified via emission lines into Seyfert 2 (Sy2), LINER, and star-forming (BPT-SF) galaxies. We find that the K-correction is essential to remove non-AGN contamination, and once applied the simple W1-W2>0.5 selection emerges as optimal in terms of purity and completeness of AGN selection. However, we confirm that even this lenient cut selects only ~13% of Sy2 galaxies and that achieving W1-W2>0.5 requires AGN contributing >75% of the total infrared luminosity, which is uncommon. Although mid-IR-selected Sy2s tend to be luminous, the high [OIII] luminosity does not guarantee red W1-W2 (nor does any other tested global or NLR-scale parameter), suggesting the critical role of obscuration on smaller scales. <1% of BPT-SF systems (but making ~20% of all mid-IR selected galaxies) exhibit W1-W2>0.5 colors. Such colors cannot be reproduced by models of star-heated dust alone. Red BPT-SFs tend to have higher W4 luminosities than expected from SF, indicating true AGNs. Intriguingly, mid-IR AGNs in massive bulges ($M_{\mathrm{bulge}} \gtrsim 10^{10} M_{\odot}$) predominantly (84%) manifest themselves as BPT-AGNs, whereas those in low-mass bulges ($\lesssim 10^{10} M_{\odot}$) mostly (60%) manifest as BPT-SF. This BPT-AGN vs.\ BPT-SF dichotomy does not extend to total stellar mass. We conclude that although the mid-IR AGN selection is incomplete, its strength lies in identifying optically inconspicuous AGNs with low-mass bulges, regardless of the total mass.

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arXiv2026

EMBERS I: Low redshift post-starburst galaxies are frequently depleted in molecular gas relative to star forming progenitors

The cold gas content of post-starburst galaxies (PSBs) provides important insight into the mechanisms that drive rapid quenching, but a multiphase assessment of both the atomic and molecular gas in PSBs does not yet exist. We introduce the Ensemble of Multiphase Baryons Evolving in Rapidly-quenching Systems, or EMBERS, a homogeneously selected, nearly mass- and redshift-complete survey of the global atomic (HI) and molecular gas (H2) in PSBs, observed with the Five Hundred-metre Aperture Spherical Telescope (FAST) and the Institut de radioastronomie millimetrique (IRAM) 30m telescope. We present new CO(1-0) observations for 52 PSBs with the IRAM 30m, which, combined with 9 archival observations, gives a total H2 sample of 61, of which 58/61 have ancillary HI measurements. We detect CO(1-0) in 34/61 galaxies, corresponding to molecular gas fractions (fH2 = MH2/M*) ranging from two to 250 per cent. By comparing with a stellar-mass matched star-forming (SF) control sample from xCOLD GASS, we find that PSBs on average are 0.3-0.6 dex depleted in H2. However, considering both HI and H2, individual PSBs host diverse gas reservoirs ranging from gas-rich in both phases, elevated in one phase, or gas-poor, the latter of which is common at lower stellar mass. The existence of gas-normal and gas-depleted PSBs in both phases suggests that some PSBs may rejuvenate their star formation, but the rapid shutdown of star formation in others is likely terminal. Despite this diversity, the majority of EMBERS PSBs are gas-poor compared to SF controls, with the typical PSB hosting gas reservoirs intermediate to those found in star-forming and quenched galaxies.

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arXiv2026

The ALMaQUEST Survey XVII: Unveiling Multiple Quenching Pathways in Green Valley Galaxies via Molecular Gas and Quenching Timescale Analyses

Statistically, green valley (GV) galaxies exhibit lower molecular gas fractions ($f_{gas}$) and reduced star formation efficiency (SFE) compared to star-forming galaxies. However, it remains unclear whether quenching is primarily driven by one factor or results from a combination of mechanisms in individual GV galaxies. In this study, we address this question by examining the spatial distributions of star formation and molecular gas in 28 GVs selected from the ALMaQUEST survey and additional literature samples. For each galaxy, we identify regions with suppressed specific star formation rate (sSFR) and measure $Δf_{gas}$ and $Δ$SFE-offsets from the resolved scaling relations of the star-forming main sequence galaxies. By comparing the fraction of regions with negative $Δf_{gas}$ and $Δ$SFE, we classify 35.7$\pm$13.2\% (57.1$\pm$17.9\%) of GV galaxies as $f_{gas}$-driven, 39.3$\pm$14.0\% (39.3$\pm$14.0\%) as SFE-driven, and 25.0$\pm$10.6\% (3.6$\pm$3.6\%) as mixed mode when adopting a fixed (variable) CO-to-$\rm H_{2}$ conversion factor ($α_{CO}$). These results indicate that GVs undergo quenching through multiple pathways. As sSFR decreases from the main sequence to the green valley, we observe a transition toward predominantly SFE-driven quenching, possibly linked to internal processes such as morphological quenching or AGN activity. We further estimate the quenching timescale ($τ_{decay}$), defined as the time from the peak SFR to 1/e (approximately 37\%) of its value, using integrated MaNGA spectra. SFE-driven quenching is typically associated with short $τ_{decay}$ , while $f_{gas}$-driven quenching shows a broader range. Overall, 75\% of GVs exhibit $τ_{decay}$ shorter than 1 Gyr, suggesting that quenching in most GVs proceeds rapidly, challenging purely slow-quenching scenarios like starvation.

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arXiv2026

Galaxy Mergers in UNIONS -- II: Predicting Timescales in the Post-Merger Regime

Galaxy mergers are critical events that influence galaxy evolution by driving processes such as enhanced star formation, quenching, and active galactic nucleus (AGN) activity. However, constraining the timescales over which these processes occur in the post-merger phase has remained a significant challenge. This study extends the MUlti-Model Merger Identifier (\textsc{Mummi}) framework to predict post-merger timescales ($T_{PM}$) for galaxies, leveraging machine learning models trained on realism-enhanced mock observations derived from the IllustrisTNG simulations. By classifying post-merger galaxies into four temporal bins spanning 0 to 1.76 Gyr after coalescence, \textsc{Mummi} achieves time classification accuracies exceeding 70 per cent. We apply this framework to the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), yielding a catalog of 8,716 post-merger galaxies with $T_{PM}$ predictions and stellar masses $\log(M_*/M_\odot) \geq 10$ at redshifts 0.03 < z < 0.3. These results provide a robust methodology to connect galaxy interaction timescales with physical processes, enabling detailed studies of galaxy evolution in the post-merger regime.

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arXiv2025

SDSS-ALMA Legacy Value Archival Gas Exploration (SALVAGE) -- I: global star formation is governed by central (not global) molecular gas

Star-forming galaxies form tight relations between their stellar mass, star-formation rate, and molecular gas reservoir on global and resolved scales. On the path to quiescence, the exchange between gas and stars must inevitably be broken. Understanding the mechanisms governing star formation and quenching therefore requires observations of both the stellar and molecular gas components. To this end, we have assembled a sample of 277 galaxies ($0.02 \lesssim z \lesssim 0.25$) with semi-resolved optical and millimetre $^{12}$CO(1-0) data, wherein the properties of the inner $\thicksim$2 kpc can be distinguished from the outer regions. This effort was made possible by the Sloan Digital Sky Survey (SDSS) catalogues and the maturing archive of the Atacama Large (sub-)Millimetre Array (ALMA). We call this dataset the SDSS-ALMA Legacy Value Archival Gas Exploration (SALVAGE). In this work, we leverage SALVAGE to provide a semi-resolved perspective on global scaling relations and why some galaxies deviate from them. In agreement with previous work, we find that the offset of a galaxy from the global star-forming main sequence (SFMS) is driven by its inner star formation rate. With the relative inner and outer distributions of molecular gas fraction and star formation efficiency, we investigate whether the central star formation driving global changes is due to fuel availability or efficiency. We find that the position of a galaxy within the SFMS is largely due to the inner star-formation efficiency, while departure from the SFMS is driven by availability of central gas. The central few kpc are thus the most consequential region for galaxy evolution at low redshift.

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arXiv2025

Interacting galaxies in the IllustrisTNG simulations - IX: Mini mergers trigger AGN in cosmological simulations

Galaxy mergers are transformative events that can cause gaseous inflows capable of triggering active galactic nuclei (AGN). Previous studies of AGN in simulations have mainly focused on major interactions (i.e. between approximately equal mass galaxies), which produce the strongest inflows and, therefore, would be the most likely to trigger AGN activity. However, minor interactions are far more common and may still enhance accretion onto supermassive black holes. We present an analysis of post-merger galaxies from the IllustrisTNG simulation with stellar mass ratios of $μ>$1:100. We select post-mergers from the TNG50-1 simulation, from redshifts $0\leq z< 1$, with stellar masses greater than $10^{10}M_{\odot}$. We find an AGN excess in post-mergers with a stellar mass ratio as low as 1:40. The AGN excess is mass ratio and luminosity dependent, with 1.2-1.6 times more AGN found in post-mergers of 1:40$\leq μ< $1:4 than in matched non-merger controls, and as many as 6 times more AGN found in major $μ\geq$1:4 post-mergers. The AGN excess is long lived, between 500 Myr to 1 Gyr post-coalescence, across all of the mass ratio regimes. We demonstrate that the most luminous AGN in the simulation overwhelmingly occur in either post-mergers or pairs (with $μ\geq $1:40). Finally, we demonstrate that mini mergers are likely to be overlooked in observational studies due to the weakness of features usually associated with recent merger activity, such as tidal streams and shells, making it challenging to completely account for merger-induced AGN activity even in deep galaxy surveys.

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arXiv2025

Star Formation Rates, Metallicities, and Stellar Masses on kpc-scales in TNG50

Integral field units (IFU) have extended our knowledge of galactic properties to kpc (or, sometimes, even smaller) patches of galaxies. These scales are where the physics driving galaxy evolution (feedback, chemical enrichment, etc.) take place. Quantifying the spatially-resolved properties of galaxies, both observationally and theoretically, is therefore critical to our understanding of galaxy evolution. To this end, we investigate spatially-resolved scaling relations within galaxies of $M_\star>10^{9.0}$ at $z=0$ in IllustrisTNG. We examine both the resolved star-forming main sequence (rSFMS) and the resolved mass-metallicity relation (rMZR) using $1~{\rm kpc}\times1~{\rm kpc}$ maps. We find that the rSFMS in IllustrisTNG is well-described by a power-law, but is significantly shallower than the observed rSFMS. However, the disagreement between the rSFMS of IllustrisTNG and observations is likely driven by an overestimation of AGN feedback in IllustrisTNG for the higher mass hosts. Conversely, the rMZR for IllustrisTNG has very good agreement with observations. Furthermore, we argue that the rSFMS is an indirect result of the Schmidt-Kennicutt (SK) law and local gas relation, which are both independent of host galaxy properties. Finally, we expand upon a localized leaky-box model to study the evolution of idealized spaxels and find that it provides a good description of these resolved relations. The degree of agreement, however, between idealized spaxels and simulated spaxels depends on the `net' outflow rate for the spaxel, and the IllustrisTNG scaling relations indicate a preference for a low net outflow rate.

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arXiv2025

Unions with UNIONS: Using galaxy-galaxy lensing to probe galaxy mergers

We use galaxy-galaxy lensing to investigate how the dark matter (DM) haloes and stellar content of galaxies with $0.012 \leq z \leq 0.32$ and $10 \leq \log_{10}(M_\star/\mathrm{M}_\odot) \leq 12$ change as a result of the merger process. To this end, we construct two samples of galaxies obtained from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), comprising 1 623 post-mergers and $\sim$30 000 non-merging controls, that live in low-density environments to use as our lenses. These samples are weighted to share the same distributions of stellar mass, redshift, and geometric mean distance to a galaxy's three nearest neighbours to ensure differences in the lensing signal are due to the merger process itself. We do not detect a statistically significant difference in the excess surface density profile of post-mergers and non-merging controls with current data. Fitting haloes composed of a point-like stellar mass component and an extended DM structure described by a Navarro-Frenk-White profile to the lensing measurements yields, for both samples, halo masses of $M_\text{halo} \sim 4\times10^{12}\,\mathrm{M}_\odot$ and a moderately negative correlation between $M_\text{halo}$ and concentration $c$. This allows us to rule out, at the 95% confidence level, merger-induced starbursts in which more than 60% of the stellar mass is formed in the burst. The application of our methods to upcoming surveys that are able to provide samples $\sim$10$\times$ larger than our current catalogue are expected to detect the weak-lensing signatures of mergers and further constrain their properties.

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arXiv2025

Decoding the Single-peaked HI Spectra of Low Redshift Post-starburst Galaxies

Recent observations with the Five-hundred-meter Aperture Spherical Telescope (FAST) have revealed abundant reservoirs of neutral hydrogen (HI) in low redshift post-starburst galaxies (PSBs), raising the question of why star formation ceases rapidly in these systems. In this study, we present a detailed analysis of the shape of the integrated HI spectra of 67 PSBs. We find that PSBs exhibit significantly higher HI spectral concentration values ($K$) compared to a matched sample from xGASS, and are more comparable to those of starburst galaxies. By extending our analysis to spatially resolved HI data from THINGS and ATLAS$^{\rm 3D}$, we show that both centrally concentrated HI distributions and dynamically unsettled HI can effectively increase $K$, while non-axisymmetric structures only contribute to the scatter of the $K$ distribution. Distinguishing between central concentration and dynamically unsettled gas as the origin of high $K$ can be achieved by measuring the spectral asymmetry ($A_{\rm F}$), making the $K$-$A_{\rm F}$ plane a powerful diagnostic tool for identifying galaxies with unsettled HI using integrated spectra alone. Based on their location in the $K$-$A_{\rm F}$ plane, we find that most PSBs are not dominated by unsettled HI, but rather exhibit elevated central gas concentration. Both modes of gas redistribution in PSBs may eventually contribute to their quenching.

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arXiv2025

Metallicity Gradients in Modern Cosmological Simulations I: Tension Between Smooth Stellar Feedback Models and Observations

The metallicity of galaxies, and its variation with galactocentric radius, provides key insights into the formation histories of galaxies and the physical processes driving their evolution. In this work, we analyze the radial metallicity gradients of star forming galaxies in the EAGLE, Illustris, IllustrisTNG, and SIMBA cosmological simulations across a broad mass ($10^{8.0}M_\odot\leq M_\star \lesssim10^{12.0}M_\odot$) and redshift ($0\leq z\leq8$) range. We find that all simulations predict strong negative (i.e., radially decreasing) metallicity gradients at early cosmic times, likely due to their similar treatments of relatively smooth stellar feedback not providing sufficient mixing to quickly flatten gradients. The strongest redshift evolution occurs in galaxies with stellar masses of $10^{10.0}-10^{11.0}M_\odot$, while galaxies with stellar masses $< 10^{10}M_\odot$ and $>10^{11}M_\odot$ exhibit weaker redshift evolution. Our results of negative gradients at high-redshift contrast with the many positive and flat gradients in the $1 6$, the negative gradients observed with JWST and ALMA are flatter than those in simulations, albeit with closer agreement than at lower redshift. Overall, we suggest that these smooth stellar feedback galaxy simulations may not sufficiently mix their metal content radially, and that either stronger stellar feedback or additional subgrid turbulent metal diffusion models may be required to better reproduce observed metallicity gradients.

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arXiv2025

Unmixed metals: Variations in the enrichment of z~4 sub-damped Lyman alpha systems

The chemical abundance patterns of near-pristine objects provide important constraints on the properties of the first generations of stars in the Universe. We present the chemical abundances of five very metal-poor ([M/H]<-2.5) sub damped Lyman alpha systems (subDLAs) covering the redshift range $3.6<z<4.3$, identified with the XQ-100 survey. We find that the subDLAs in our sample show consistent chemical abundance patterns (in particular [C/O], [Al/O], and [Fe/O]) with those of very metal-poor DLAs. Based on Voigt profile fitting, the chemical abundance ratios [C/O], [Al/O], and [Si/O] of individual velocity components in at least three of the subDLAs shows some intrinsic scatter. In order to verify these chemical inhomogeneities in absorption components, we present a novel method for computing ionization corrections (ICs) on a component-by-component basis and show that ICs alone cannot explain the variations in [C/O], [Al/O], and [Si/O] between components of the same absorber at ~2 sigma significance. Comparing the observed abundance ratios to the simulated core-collapse supernovae yields of early stellar populations, we find that all individual components of the subDLAs appears to be enriched by progenitor masses of <30 M_sol. The observed inhomogeneities between components can be reproduced by differences in the progenitor mass or supernova explosion energy. As such, the observed chemical inhomogeneities between components can be explained by poorly mixed gas from different nucleosynthetic events.

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arXiv2025

Low redshift post-starburst galaxies host abundant HI reservoirs

Studying the gas content of post-starburst (PSB) galaxies can provide valuable clues regarding the process of fast quenching. Although previous works have studied the molecular gas content of PSBs, only a handful of HI measurements exist. Here, we present new Five hundred metre Aperture Spherical Telescope (FAST) 21cm observations of 44 PSBs, leading to 43 detections or sensitive upper limits of HI, which we combine with 25 archival measurements, for a total sample of 68 PSB MHI measurements. HI is detected in 57/68 galaxies, with HI masses ranging from MHI ~10^8.5 up to 10^10 Msun and gas fractions (fHI = MHI/M*) from a few percent up to almost 30 percent. Post-starbursts therefore retain ample atomic gas reservoirs, despite no longer forming stars. By comparing with a stellar mass-matched sample of star-forming galaxies in xGASS, we find that PSBs have, on average, gas fractions lower by ~0.2-0.4 dex, consistent with a mild reduction compared with their progenitor population. However, PSBs show a diversity of HI properties; about half have HI gas masses within the expected scatter of the star-forming population with the remaining 50 per cent up to a factor of 10 more gas-poor. Compared with galaxies in the green valley, about two thirds of PSBs have gas fractions within the expected range, with the remaining third up to a factor of 10 more gas-rich. Our results demonstrate that quenching in PSBs is not the result of wholesale removal of the atomic gas reservoir and that the population has atomic gas fractions that span the range from star-forming to green valley galaxies. We find no correlation between HI gas mass and time since burst; even galaxies a Gyr past their burst can remain HI-normal. The significant gas reservoirs remaining in many PSBs leaves open the possibility for future rekindling of star formation.

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arXiv2025

The First Large Absorption Survey in HI (FLASH): II. Pilot Survey data release and first results

The First Large Absorption Survey in HI (FLASH) is a large-area radio survey for neutral hydrogen in the redshift range 0.4 1$, and appear to be a mixture of intervening and associated systems. Interestingly, around two-thirds of the lines found in this untargeted sample are detected against sources with a peaked-spectrum radio continuum, which are only a minor (5-20%) fraction of the overall radio-source population. The overall detection rate for HI absorption lines in the Pilot Surveys (0.3 to 0.5 lines per ASKAP field) is a factor of two below the expected value. One possible reason for this is the presence of a range of spectral-line artefacts in the Pilot Survey data that have now been mitigated and are not expected to recur in the full FLASH survey. A future paper will discuss the host galaxies of the HI absorption systems identified here.

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