arXiv subjects

D. Kleiner

Publications and source records attributed to D. Kleiner.

At least 19 recordsLinked to original sources

arXiv2026

Deep HI observations of cold gas inflow and outflow

A major question in galaxy evolution is how galaxies acquire sufficient gas to sustain their star formation rates. HI observations with high angular resolution and sensitivity to very low column densities are some of the important observational ingredients that are currently still missing. Answers to these questions are necessary for a correct interpretation of observations of galaxy evolution in the high-redshift universe and will provide crucial input for the sub-grid physics in hydrodynamical simulations of galaxy evolutions. In this chapter we discuss the progress that has been made over the past years, describe the various processes that lead to inflow and outflow of gas, and discuss how SKA-Mid AA4 observations can contribute to further understanding these important aspects of galaxy evolution using deep observations of nearby individual disk and dwarf galaxies.

astro-ph.GA↗

arXiv2026

The ultra-deep HI radial profiles of late-type galaxies from MHONGOOSE

Galaxy discs have a finite extent, yet how and where their neutral atomic hydrogen (HI) components end is not fully understood. The existence of a break in the HI disc at column densities of $\sim10^{19}$ cm$^{-2}$ has been debated since early 21-cm observations of the spiral galaxy NGC 3198. We present the HI radial profiles of 16 star-forming, late-type galaxies from the MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters (MHONGOOSE) survey spanning a range of HI mass from $10^{7}$ M$_\odot$ to $10^{10}$ M$_\odot$. We probed via spectral stacking their HI discs down to inclination-corrected column densities of a few times $10^{17}$ cm$^{-2}$ at kpc resolution. The HI radial profiles of high-mass (M$_{\rm {HI}}>10^{9}$ M$_\odot$) galaxies are characterised by a inner plateau, followed by a knee at column densities of $\sim5\times10^{20}$ cm$^{-2}$, but no edges are unambiguously identified. The HI radial profiles of low-mass (M$_{\rm {HI}}>10^{9}$ M$_\odot$) galaxies shows a steeper decline and also no edges. We found that the profiles are self-similar when normalised to the radius at which a mass surface density of 0.01 M$_\odot$ pc$^{-2}$ is reached, in agreement with recent literature results, but we also found a possible correlation between the normalisation radius and the environment, suggesting that environment contributes to the shaping of the HI distribution in galaxies. The emerging picture is that the diverse morphology of the HI radial profiles is difficult to interpret, and future studies with a larger sample are necessary for quantifying the contribution of internal and external processes acting at different levels for different galaxies.

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arXiv2026

HI absorption in MHONGOOSE -- Spin temperatures and cold neutral medium in nearby disk galaxies

Combined HI emission-absorption studies constrain the spin temperature and phase structure of the neutral atomic hydrogen interstellar medium (ISM), but have largely been limited to the Milky Way and the Local Group. We extend this technique to galaxies at distances of 7-22 Mpc using deep data from the MeerKAT HI Observations of Nearby Galactic Objects - Observing Southern Emitters (MHONGOOSE) survey, and quantify the detection fraction and Cold Neutral Medium (CNM) properties at these distances. We search for HI absorption toward 56 background continuum sources in 21 out of the 30 MHONGOOSE galaxies (with nine galaxies lacking suitable background sources), and detect absorption associated with the galaxies' HI disks in three cases: one sight line in NGC 289 and two in NGC 7424. This corresponds to detection rates of 3/56 (5 percent) for the full sample and 3/31 (10 percent) for a clean sub-sample of sight lines, considering only unresolved background sources behind 14 low-inclination galaxies. Detections occur only where both the continuum flux and the foreground \HI column density are high, with optical-depth sensitivity as the primary limiting factor. For the detected sight lines, we model the absorption and emission spectra to derive spin temperatures and CNM fractions using the standard combined emission-absorption method. The CNM spin temperatures and line widths are comparable to Local Group measurements, but the inferred CNM fractions are systematically lower. We argue that this difference is primarily a resolution effect: at the distances of our galaxies, the emission spectra average over several hundred parsecs, diluting structured CNM relative to the smoother Warm Neutral Medium (WNM). This demonstrates that emission-absorption analyses can be extended beyond the Local Group, provided that care is taken in constructing representative emission spectra.

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arXiv2026

FEASTS and MHONGOOSE: HI Column Density Distribution at $z=0$ for $N_\mathrm{HI}>10^{17.8}\, \mathrm{cm}^{-2}$

We present the first $z=0$ HI column density distribution function, $f(N_\mathrm{HI})$, extending down to $\log (N_\mathrm{HI}/\mathrm{cm}^{-2})=17.8$. This was derived from high-sensitivity 21-cm emission-line imaging at $\sim$1 kpc resolution. At high-column-densities (19.8$< \log (N_\mathrm{HI}/\mathrm{cm}^{-2}) <$21.3), our results align with earlier $z=0$ studies but benefit from 100 times greater sensitivity. Comparisons with $z\sim3$ quasar absorption-line studies reveal that $f(N_\mathrm{HI})$ at $z=0$ is systematically lower by 0.1-0.4 dex for $19.2< \log (N_\mathrm{HI}/\mathrm{cm}^{-2}) <21$. However, the distributions become comparable at $17.8< \log (N_\mathrm{HI}/\mathrm{cm}^{-2}) <19.2$, suggesting weak evolution in this regime. Extrapolating the length incidence ($\mathrm{d}N/\mathrm{d}X$) for $\log (N_\mathrm{HI}/\mathrm{cm}^{-2}) >17.5$ implies a covering fraction ($f_\mathrm{cov}$) of $\sim0.7$ within 1-kpc-scale HI-detected pixels at $z=0$. Notably, for $17.8< \log (N_\mathrm{HI}/\mathrm{cm}^{-2}) <20$, impact parameters at a given $N_\mathrm{HI}$ are significantly lower than previous $z\sim0$ absorption-line results and TNG50 simulation predictions. This discrepancy indicates challenges in identifying galaxy counterparts for absorbers and in recovering low-column-density HI within cosmological simulations. Finally, we derive a covering fraction of 0.006 for $\log (N_\mathrm{HI}/\mathrm{cm}^{-2}) >17.8$ gas within the virial radius around Milky-Way-like galaxies. These findings provide new constraints on the baryonic flows and gaseous dynamics governing galaxy evolution.

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arXiv2025

The MeerKAT Fornax Survey VI. The collapse of the galaxy HI Mass Function in Fornax

We present the deepest HI mass Function (HIMF) ever measured, outside the Local Group. The observations are part of the MeerKAT Fornax Survey and cover a 4 x 4 deg^2 field, corresponding to ~ Rvir. The 3$\sigma$ detection limit is log(MHI/Msun) = 5.7 for a 50 km/s-wide point source. We detect HI in 35 galaxies and 44 clouds with no optical counterparts. Using deep optical images from the Fornax Deep Survey, we show that the clouds are a distinct population, separated by a four magnitude gap from the faintest HI-detected galaxies. The majority (33 out of 44) of the clouds are associated with the two galaxies with the most HI in the cluster -- NGC 1365 and NGC 1427A, although the clouds contribute a negligible amount to the total MHI budget. By performing a SNR analysis and computing the Rauzy statistic on the HI detections, we demonstrate that our catalogue is complete down log(MHI/Msun) = 6, and we are therefore able to probe the HIMF down to this level. We find an abrupt drop of the number density of HI-detected galaxies at log(MHI/Msun) = 7, signifying a clear absence of galaxies between 6 < log(MHI/Msun) < 7. We use the modified maximum likelihood method to fit a Schechter function down to log(MHI/Msun) > 7, the range where the HIMF follows a power-law. The measured low-mass slope is $\alpha$ = -1.31 $\pm$ 0.13, with a characteristic knee mass of log(M*/Msun) = 10.52 $\pm$ 1.89. The low-mass slope matches the slope in the field, while the knee is defined by a single galaxy and is unconstrained. Below log(MHI/Msun) = 7, there is a sharp departure from a Schechter function, and we report the first robust measurement of the collapse of a HIMF. For the HIMF below log(MHI/Msun) = 7 to follow a power-law, tens of galaxies are needed -- a factor ~ six higher than what is observed. The collapse of the Fornax HIMF is likely due to the rapid removal of HI from low-mass galaxies.

astro-ph.GA↗

arXiv2025

The inside-out quenching of the MHONGOOSE galaxy NGC 1371

We present the deepest 21-cm spectral line and 1.4 GHz broad-band continuum observations of nearby early-type spiral galaxy NGC 1371 as part of the MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters (MHONGOOSE) survey. We found the neutral atomic hydrogen (HI) mostly distributed in a regularly rotating disc with a hole $\sim5$ kpc wide around the galactic centre. The continuum observations reveal, within the HI hole, emission from one of the lowest luminosity AGN known to date and from two unique $\sim10$-kpc wide bipolar bubbles never observed before in this galaxy. The properties of the bubbles suggest that they may result from the impact of the low-power radio jet propagating within the gaseous disk instead of perpendicular to it. We found indication for jet-induced ionised outflows within the HI hole but no molecular gas (upper limit of $M_{\text{H$_2$}}<2\times10^5\text{ M$_\odot$}$) is detected. The emerging picture is that the gas in the central regions has been rapidly depleted by the stellar bar or, despite its low power, the AGN in NGC 1371 is efficiently heating and/or removing the gas through the jets and possibly by radiative winds, leading to the inside-out quenching of the galaxy.

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arXiv2025

The MeerKAT Fornax Survey V. H i kinematics and Fornax cluster membership of the dwarf galaxy ESO 358-60

The MeerKAT Fornax Survey (MFS) is a large survey project mapping the HI in the Fornax cluster. Most of the cluster members detected in HI show significant signs of interaction with the intra-cluster medium or other galaxies. The galaxy ESO 358-60 however stands out as its large HI disk appears regular and undisturbed. A possible explanation for this undisturbed disk is that the galaxy is not in Fornax. We analyze the HI distribution within and around ESO 358-60 based on the MFS observations. We visually inspect the low resolution data in order to study the HI disk from the center to its outskirts and look for low column density gas that could reveal recent interactions. We then construct a detailed parameterization of the HI disk by fitting a tilted ring model to the high resolution data cube. We use the fitted rotational velocity to place the galaxy on the baryonic Tully-Fisher relationship. By equating the galaxy's HI and 3.6 $\mu$m fluxes to the thus retrieved baryonic mass, we obtain a redshift independent distance. Our modeling confirms the regularity of the HI disk in ESO 358-60 but also shows that the galaxy contains a significant line of sight warp and contains radial motions, of the order of 10 km s$^{-1}$, that cover the extent of the optical disk. From the modeling we obtain a velocity V$_{\rm flat} = $48.1 $\pm$ 1.4 km s$^{-1}$ for the best fit rotation curve. This leads to a distance from the baryonic Tully-Fisher relation of 9.4 $\pm$ 2.5 Mpc,$\sim$ 10 Mpc less than the distance to the Fornax cluster. This distance not only fits better with V$_{\rm flat}$ but also with the overall HI distribution of low mass galaxies and the fact that the galaxy appears undisturbed and reasonably symmetric. At 9.4 Mpc ESO 358-60 cannot be a member of the Fornax cluster but is a foreground field galaxy.

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arXiv2025

HI within and around observed and simulated galaxy discs -- Comparing MeerKAT observations with mock data from TNG50 and FIRE-2

Atomic hydrogen (HI) is an ideal tracer of gas flows in and around galaxies, and it is uniquely observable in the nearby Universe. Here we make use of wide-field (~1 square degree), spatially resolved (down to 22"), high-sensitivity (~$10^{18}$ cm$^{-2}$) HI observations of 5 nearby galaxies with stellar mass of $5\times10^{10}$ M$_\odot$, taken with the MeerKAT radio telescope. Four of these were observed as part of the MHONGOOSE survey. We characterise their main HI properties and compare these with synthetic HI data from a sample of 25 similarly massive star-forming galaxies from the TNG50 (20) and FIRE-2 (5) suites of cosmological hydrodynamical simulations. Globally, the simulated systems have HI and molecular hydrogen (H$_2$) masses in good agreement with the observations, but only when the H$_2$ recipe of Blitz & Rosolowsky (2006) is employed. The other recipes that we tested overestimate the H$_2$-to-HI mass fraction by up to an order of magnitude. On a local scale, we find two main discrepancies between observed and simulated data. First, the simulated galaxies show a more irregular HI morphology than the observed ones due to the presence of HI with column density $<10^{20}$ cm$^{-2}$ up to ~100 kpc from the galaxy centre, in spite of the fact that they inhabit more isolated environments than the observed targets. Second, the simulated galaxies and in particular those from the FIRE-2 suite, feature more complex and overall broader HI line profiles than the observed ones. We interpret this as being due to the combined effect of stellar feedback and gas accretion, which lead to a large-scale gas circulation that is more vigorous than in the observed galaxies. Our results indicate that, with respect to the simulations, gentler processes of gas inflows and outflows are at work in the nearby Universe, leading to more regular and less turbulent HI discs.

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arXiv2024

Searching for HI around MHONGOOSE Galaxies via Spectral Stacking

The observed star formation rates of galaxies in the Local Universe suggests that they are replenishing their gas reservoir across cosmic time. Cosmological simulations predict that this accretion of fresh gas can occur in a hot or a cold mode, yet the existence of low column density ($\sim10^{17}$ cm$^{-2}$) neutral atomic hydrogen (HI) tracing the cold mode has not been unambiguously confirmed by observations. We present the application of unconstrained spectral stacking to attempt to detect the emission from this HI in the Circum-Galactic Medium (CGM) and Inter-Galactic Medium (IGM) of 6 nearby star forming galaxies from the MHONGOOSE sample for which full-depth observations are available. Our stacking procedure consists of a standard spectral stacking algorithm coupled with a one-dimensional spectral line finder designed to extract reliable signal close to the noise level. In agreement with previous studies, we found that the amount of signal detected outside the HI disk is much smaller than implied by simulations. Furthermore, the column density limit that we achieve via stacking ($\sim10^{17}$ cm$^{-2}$) suggests that direct detection of the neutral CGM/IGM component might be challenging in the future, even with the next generation of radio telescopes.

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arXiv2024

The MeerKAT Fornax Survey. III. Ram-pressure stripping of the tidally interacting galaxy NGC 1427A in the Fornax cluster

We present MeerKAT Fornax Survey HI observations of NGC 1427A, a blue irregular galaxy with a stellar mass of 2e+9 Msun located near the centre of the Fornax galaxy cluster. Thanks to the excellent resolution (1 to 6 kpc spatially, 1.4 km/s in velocity) and HI column density sensitivity (4e+19/cm^2 to 1e+18/cm^2 depending on resolution), our data deliver new insights on the long-debated interaction of this galaxy with the cluster environment. We confirm the presence of a broad, one-sided, starless HI tail stretching from the outer regions of the stellar body and pointing away from the cluster centre. We find the tail to have 50% more HI (4e+8 Msun) and to be 3 times longer (70 kpc) than in previous observations. In fact, we detect scattered HI clouds out to 300 kpc from the galaxy in the direction of the tail -- possibly the most ancient remnant of the passage of NGC 1427A through the intracluster medium of Fornax. Both the velocity gradient along the HI tail and the peculiar kinematics of HI in the outer region of the stellar body are consistent with the effect of ram pressure given the line-of-sight motion of the galaxy within the cluster. However, several properties cannot be explained solely by ram pressure and suggest an ongoing tidal interaction. This includes: the close match between dense HI and stars within the disturbed stellar body; the abundant kinematically-anomalous HI; and the inversion of the HI velocity gradient near the base of the HI tail. We rule out an interaction with the cluster tidal field, and conclude that NGC 1427A is the result of a high-speed galaxy encounter or of a merger started at least 300 Myr ago, where ram pressure shapes the distribution and kinematics of the HI in the perturbed outer stellar body and in the tidal tails.

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arXiv2024

MHONGOOSE discovery of a gas-rich low-surface brightness galaxy in the Dorado Group

We present the discovery of a low-mass gas-rich low-surface brightness galaxy in the Dorado Group, at a distance of 17.7 Mpc. Combining deep MeerKAT 21-cm observations from the MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters (MHONGOOSE) survey with deep photometric images from the VST Early-type Galaxy Survey (VEGAS) we find a stellar and neutral atomic hydrogen (HI) gas mass of $M_\star = 2.23\times10^6$ M$_\odot$ and $M_{\rm HI}=1.68\times10^6$ M$_\odot$, respectively. This low-surface brightness galaxy is the lowest mass HI detection found in a group beyond the Local Universe ($D\gtrsim 10$ Mpc). The dwarf galaxy has the typical overall properties of gas-rich low surface brightness galaxies in the Local group, but with some striking differences. Namely, the MHONGOOSE observations reveal a very low column density ($\sim 10^{18-19}$ cm$^{-2}$) HI disk with asymmetrical morphology possibly supported by rotation and higher velocity dispersion in the centre. There, deep optical photometry and UV-observations suggest a recent enhancement of the star formation. Found at galactocentric distances where in the Local Group dwarf galaxies are depleted of cold gas (at $390$ projected-kpc distance from the group centre), this galaxy is likely on its first orbit within the Dorado group. We discuss the possible environmental effects that may have caused the formation of the HI disk and the enhancement of star formation, highlighting the short-lived phase (a few hundreds of Myr) of the gaseous disk, before either SF or hydrodynamical forces will deplete the gas of the galaxy.

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arXiv2024

MHONGOOSE -- A MeerKAT Nearby Galaxy HI Survey

The MHONGOOSE (MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters) survey maps the distribution and kinematics of the neutral atomic hydrogen (HI) gas in and around 30 nearby star-forming spiral and dwarf galaxies to extremely low HI column densities. The HI column density sensitivity (3 sigma over 16 km/s) ranges from ~ 5 x 10^{17} cm^{-2} at 90'' resolution to ~4 x 10^{19} cm^{-2} at the highest resolution of 7''. The HI mass sensitivity (3 sigma over 50 km/s) is ~5.5 X 10^5 M_sun at a distance of 10 Mpc (the median distance of the sample galaxies). The velocity resolution of the data is 1.4 km/s. One of the main science goals of the survey is the detection of cold, accreting gas in the outskirts of the sample galaxies. The sample was selected to cover a range in HI masses, from 10^7 M_sun to almost 10^{11} M_sun, to optimally sample possible accretion scenarios and environments. The distance to the sample galaxies ranges from 3 to 23 Mpc. In this paper, we present the sample selection, survey design, and observation and reduction procedures. We compare the integrated HI fluxes based on the MeerKAT data with those derived from single-dish measurement and find good agreement, indicating that our MeerKAT observations are recovering all flux. We present HI moment maps of the entire sample based on the first ten percent of the survey data, and find that a comparison of the zeroth- and second-moment values shows a clear separation between the physical properties of the HI in areas with star formation and areas without, related to the formation of a cold neutral medium. Finally, we give an overview of the HI-detected companion and satellite galaxies in the 30 fields, five of which have not previously been catalogued. We find a clear relation between the number of companion galaxies and the mass of the main target galaxy.

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arXiv2024

Possible origins of anomalous H$\,$I gas around MHONGOOSE galaxy, NGC 5068

The existing reservoirs of neutral atomic hydrogen gas (H$\,$I) in galaxies are insufficient to have maintained the observed levels of star formation without some kind of replenishment. {This refuelling of the H$\,$I reservoirs} is likely to occur at column densities an order of magnitude lower than previous observational limits (N$_{\rm{H\,I}\, limit} \sim 10^{19}\,$cm$^{-2}$ at 30$''$ resolution over a linewidth of $20\,$km/s). In this paper, we present recent deep H$\,$I observations of NGC 5068, a nearby isolated star-forming galaxy observed by MeerKAT as part of the MHONGOOSE survey. With these new data, we are able to detect low column density H$\,$I around NGC 5068 with a $3\sigma$ detection limit of N$_{\rm{H\,I}} = 6.4 \times 10^{17}\,$cm$^{-2}$ at 90$''$ resolution over a $20\,$km/s linewidth. The high sensitivity and resolution of the MeerKAT data reveal a complex morphology of the H$\,$I in this galaxy -- a regularly rotating inner disk coincident with the main star-forming disk of the galaxy, a warped outer disk of low column density gas (N$_{\rm{H\,I}} < 9 \times 10^{19}\,$cm$^{-2}$), in addition to clumps of gas on the north west side of the galaxy. We employ a simple two disk model that describe the inner and outer disks, and are able to identify anomalous gas that deviates from the rotation of the main galaxy. The morphology and the kinematics of the anomalous gas suggest a possible extra-galactic origin. We explore a number of possible origin scenarios that may explain the anomalous gas, and conclude that fresh accretion is the most likely scenario.

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arXiv2023

The star formation histories of galaxies in different stages of pre-processing in the Fornax A group

We study the recent star formation histories of ten galaxies in the Fornax A galaxy group, on the outskirts of the Fornax cluster. The group galaxies are gas-rich, and their neutral atomic hydrogen (HI) was studied in detail with observations from the MeerKAT telescope. This allowed them to be classified into different stages of pre-processing (early, ongoing, advanced). We use long-slit spectra obtained with the South African Large Telescope (SALT) to analyse stellar population indicators to constrain quenching timescales and to compare these to the HI gas content of the galaxies. The H$α$ equivalent width, EW(H$α$), suggest that the pre-processing stage is closely related to the recent (< 10 Myr) specific Star Formation Rate (sSFR). The early-stage galaxy (NGC 1326B) is not yet quenched in its outer parts, while the ongoing-stage galaxies mostly have a distributed population of very young stars, though less so in their outer parts. The galaxies in the advanced stage of pre-processing show very low recent sSFR in the outer parts. Our results suggest that NGC 1326B, FCC 35 and FCC 46 underwent significantly different histories from secular evolution during the last Gyr. The fact that most galaxies are on the secular evolution sequence implies that pre-processing has a negligible effect on these galaxies compared to secular evolution. We find EW(H$α$) to be a useful tool for classifying the stage of pre-processing in group galaxies. The recent sSFR and HI morphology show that galaxies in the Fornax A vicinity are pre-processing from the outside in.

astro-ph.GA↗

arXiv2023

The MeerKAT Fornax Survey II. The rapid removal of HI from dwarf galaxies in the Fornax cluster

We present MeerKAT Fornax Survey atomic hydrogen (HI) observations of the dwarf galaxies located in the central ~2.5 x 4 deg$^2$ of the Fornax galaxy cluster. The HI images presented in this work have a $3σ$ column density sensitivity between 2.7 and 50 x 10$^{18}$ cm$^{-2}$ over 25 km s$^{-1}$ for spatial resolution between 4 and 1 kpc. We are able to detect an impressive MHI = 5 x 10$^{5}$ Msun 3$σ$ point source with a line width of 50 km s$^{-1}$ at a distance of 20 Mpc. We detect HI in 17 out of the 304 dwarfs in our field -- 14 out of the 36 late type dwarfs (LTDs), and 3 of the 268 early type dwarfs (ETDs). The HI-detected LTDs have likely just joined the cluster and are on their first infall as they are located at large clustocentric radii, with comparable MHI and mean stellar surface brightness at fixed luminosity as blue, star-forming LTDs in the field. The HI-detected ETDs have likely been in the cluster longer than the LTDs and acquired their HI through a recent merger or accretion from nearby HI. Eight of the HI-detected LTDs host irregular or asymmetric HI emission and disturbed or lopsided stellar emission. There are two clear cases of ram-pressure shaping the HI, with the LTDs displaying compressed HI on the side closest to the cluster centre and a one-sided, starless tail pointing away from the cluster centre. The HI-detected dwarfs avoid the most massive potentials, consistent with massive galaxies playing an active role in the removal of HI. We create a simple toy model to quantify the timescale of HI stripping in the cluster. We find that a MHI = 10$^{8}$ Msun dwarf will be stripped in ~ 240 Myr. The model is consistent with our observations, where low mass LTDs are directly stripped of their HI from a single encounter and more massive LTDs can harbour a disturbed HI morphology due to longer times or multiple encounters being required to fully strip their HI.

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arXiv2023

ALMA/ACA CO Survey of the IC 1459 and NGC 4636 Groups: Environmental Effects on the Molecular Gas of Group Galaxies

We present new results of a 12CO(J=1-0) imaging survey using the Atacama Compact Array (ACA) for 31 HI detected galaxies in the IC 1459 and NGC 4636 groups. This is the first CO imaging survey for loose galaxy groups. We obtained well-resolved CO data (~0.7-1.5 kpc) for a total of 16 galaxies in two environments. By comparing our ACA CO data with the HI and UV data, we probe the impacts of the group environment on the cold gas components (CO and HI gas) and star formation activity. We find that CO and/or HI morphologies are disturbed in our group members, some of which show highly asymmetric CO distributions (e.g., IC 5264, NGC 7421, and NGC 7418). In comparison with isolated galaxies in the xCOLD GASS sample, our group galaxies tend to have low star formation rates and low H2 gas fractions. Our findings suggest that the group environment can change the distribution of cold gas components, including the molecular gas, and star formation properties of galaxies. This is supporting evidence that preprocessing in the group-like environment can play an important role in galaxy evolution.

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arXiv2023

WALLABY Pilot Survey: The diversity of HI structural parameters in nearby galaxies

We investigate the diversity in the sizes and average surface densities of the neutral atomic hydrogen (HI) gas discs in ~280 nearby galaxies detected by the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY). We combine the uniformly observed, interferometric HI data from pilot observations of the Hydra cluster and NGC 4636 group fields with photometry measured from ultraviolet, optical and near-infrared imaging surveys to investigate the interplay between stellar structure, star formation and HI structural parameters. We quantify the HI structure by the size of the HI relative to the optical disc and the average HI surface density measured using effective and isodensity radii. For galaxies resolved by >1.3 beams, we find that galaxies with higher stellar masses and stellar surface densities tend to have less extended HI discs and lower HI surface densities: the isodensity HI structural parameters show a weak negative dependence on stellar mass and stellar mass surface density. These trends strengthen when we limit our sample to galaxies resolved by >2 beams. We find that galaxies with higher HI surface densities and more extended HI discs tend to be more star forming: the isodensity HI structural parameters have stronger correlations with star formation. Normalising the HI disc size by the optical effective radius (instead of the isophotal radius) produces positive correlations with stellar masses and stellar surface densities and removes the correlations with star formation. This is due to the effective and isodensity HI radii increasing with mass at similar rates while, in the optical, the effective radius increases slower than the isophotal radius. Our results demonstrate that with WALLABY we can begin to bridge the gap between small galaxy samples with high spatial resolution HI data and large, statistical studies using spatially unresolved, single-dish data.

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arXiv2023

The AGN fuelling/feedback cycle in nearby radio galaxies - V. The cold atomic gas of NGC 3100 and its group

We present Australia Compact Telescope Array (ATCA) 21-cm observations of the nearby low-excitation radio galaxy (LERG) NGC 3100. This is the brightest galaxy of a loose group and hosts a young ($\sim 2$ Myr) radio source. The ATCA observations reveal for the first time the presence of neutral hydrogen (HI) gas in absorption in the centre of this radio galaxy, and in emission in two low-mass galaxies of the group and in a diffuse dark cloud in the proximity of NGC 3100. The sensitivity to low-column density gas ($N_{\rm HI}\sim 10^{19}$ cm$^{-2}$) allows us to reveal asymmetries in the periphery of most the HI-detected galaxies, suggesting that tidal interactions may be on-going. The diffuse cloud does not show a stellar counterpart down to $27$ mag/arcsec$^2$ and could be the remnant of these interactions. The analysis of the HI absorption complex in NGC 3100 indicates that the atomic phase of the hydrogen is distributed as its molecular phase (observed at arcsecond resolution through several carbon monoxide emission lines). We suggest that the interactions occurring within the group are causing turbulent cold gas clouds in the intra-group medium to be slowly accreted towards the centre of NGC 3100. This caused the recent formation of the cold circum-nuclear disk which is likely sustaining the young nuclear activity.

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