We present the fourth data release (DR4) of the SkyMapper Southern Survey (SMSS), the last major step in our hemispheric survey with six optical filters: u, v, g, r, i, z. SMSS DR4 covers 26 000 deg$^{2}$ from over 400 000 images acquired by the 1.3 m SkyMapper telescope between 2014-03 and 2021-09. The 6-band sky coverage extends from the South Celestial Pole to $\delta=+16^{\circ}$, with some images reaching $\delta\sim +28^{\circ}$. In contrast to previous DRs, we include all good-quality images from the facility taken during that time span, not only those explicitly taken for the public Survey. From the image dataset, we produce a catalogue of over 15 billion detections made from $\sim$700 million unique astrophysical objects. The typical 10$\sigma$ depths for each field range between 18.5 and 20.5 mag, depending on the filter, but certain sky regions include longer exposures that reach as deep as 22 mag in some filters. As with previous SMSS catalogues, we have cross-matched with a host of other imaging and spectroscopic datasets to facilitate additional science outcomes. SMSS DR4 is now available to the worldwide astronomical community.

We combine integral field unit (IFU) and atomic hydrogen (Hi) high resolution observations to investigate the effects of the cluster environment on the interstellar medium and star formation properties of a sample of star-forming dwarf galaxies (SFDG) in the Virgo cluster. The dwarfs are located in different regions of the cluster and present different star formation properties. In some cases, the star-forming emission traced by the Hα line is centrally concentrated indicating that the cluster environment is gradually quenching the galaxies outside-in; in other dwarfs the asymmetry of the location of the star-forming regions and the presence of tails in the Hi component suggest that ram pressure stripping is temporarily triggering star formation in the interface region between the interstellar and the intracluster medium. Moreover, some dwarfs present an enhanced star formation activity, indicating that gas accretion or, most likely, a dwarf-dwarf merger scenario may have occurred, as suggested by the evidence of inverted metallicity gradients in two galaxies of the sample. We analyse Hi and Hα emission morphologies and velocity fields, and the metallicity radial profiles to better understand the stage of interaction of the dwarfs with the cluster environment and to investigate the effects on the quenching and boosting of the star formation process in dwarf galaxies.

Detection of 21-cm signal of neutral hydrogen in high redshift is one of the promising probe to study the physical processes in the early universe, from the Cosmic Dawn (CD) to the Epoch of Reionization (EoR). The domination of foregrounds (bright or diffuse) in the radio sky at low-frequency presents a significant challenge. Aside from foregrounds, there are additional obstacles which include ionospheric corruption, radio frequency interference (RFI), and instrumental systematics. Here, we are presenting radio continuum image from first upgraded Giant meterwave Radio Telescope (uGMRT) observations of the ELAIS-N1 field within the band-2, corresponds to the redshift range of 4.7–10.8. A significant feat is achieved with substantial 32-hour observations and a usable bandwidth of approximately 100 MHz. The final image reached an RMS noise level of 236 μJy/beam, the deepest image so far in this band of uGMRT.

The MeerKAT Fornax Survey is performing a detailed study of the nearby Fornax galaxy cluster to understand how galaxies lose their cold gas and stop forming stars in low-mass clusters (Mvir ≤ 1014 M⊙). We are doing so through very deep (down to ∼1018 cm-2) and high resolution (up to ∼1 kpc and 1 km s-1) MeerKAT observations of neutral atomic hydrogen gas (HI) in a 1× 2 Mpc2 region centred on Fornax. At the time of writing, the survey is 88% complete. The data analyzed so far focus on the central region of the Fornax cluster and reveal, for the first time, the ubiquitous presence of tails and clouds of HI. Some of the HI is clearly being removed from Fornax galaxies as they interact with one another or with the intra-cluster medium. We present a sample of galaxies with long, one-sided, star-less HI tails (of which only one was previously known) radially oriented within the cluster. The properties of these tails represent the first unambiguous evidence of ram pressure shaping the distribution of HI in the Fornax cluster. Furthermore, interactions within the Fornax environment shape the HI mass function, the HI content of dwarf galaxies and determines how the HI sustains the nuclear activity of some cluster members and neighbouring galaxies.

The ΛCDM model predicts that voids harbour a population of gas-rich dwarf galaxies with very low metallicities. According to this model, dwarf galaxies are the first step in the build-up of more massive galaxies. Their growth can stem from dark matter halo mergers or gas accretion from the cosmic web, although the dominant mechanism remains unclear. Some studies suggest that the chemical evolution of dwarf galaxies in voids is dominated by internal processes linked simultaneously to stellar mass growth and enrichment of these galaxies. Others, mostly based on the analysis of Hi gas morphology, find evidence of gas accretion and galaxy interaction. Thus, investigating dwarf galaxies in voids offers the chance to investigate how galaxies evolve free from the influences of massive galaxies within a mass range representative of earlier stages in galaxy formation and evolution. We aim to trace how these galaxies evolve in low-density environments by analysing the dynamics and morphology of their neutral gas content. From WALLABY DR1 – an ongoing survey at 21 cm of the Southern Hemisphere carried out with the ASKAP telescope -, we selected 20 low-mass galaxies in the Local Void. The Hi masses of the sample span the range between 3 × 107 M⊙ < MHI < 1010 M⊙. We analyse the Hi morphology, line-of-sight velocity and velocity dispersion maps of the sample. We find evidence of perturbed Hi morphology and velocity fields in at least 32% of our sample, suggesting evidence of ongoing interactions and/or gas accretion. In particular, we analyse the nearest collisional ring galaxy, ESO179-031, composed of three known interacting dwarf galaxies. Its 21 cm data analysis shows clear evidence of tidal interactions and regions of increased turbulent speeds, suggesting the possible presence of an additional fourth object interacting with the other galaxies.

As stellar winds and supernovae drive interstellar gas and cause turbulent flows, while turbulence affects star formation by compressing and disrupting gas clouds, this feedback process is still awaiting answers. Where does star formation feedback end up? Does it dissipate locally or does it reach the whole thickness of the galaxy? We describe one of the smallest nearby galaxies, revisiting the LITTLE THINGS data: the high-quality HI 21 cm VLA spectra provide a resolved view of DDO 43’s gas content. We look for environmental processes traced by HI, and our spectral analysis results reveal the characteristics of local and global neutral gas flows in DDO 43.

The Wide-field ASKAP L-band Legacy All-sky Blind Survey (WALLABY) is a large H<sc>i</sc> survey being carried out with the Australian SKA Pathfinder (ASKAP). WALLABY will image about 14,000 square degrees of the sky in the 21-cm line of neutral hydrogen and is expected to detect approximately 200,000 galaxies out to a redshift of about 0.1. What makes WALLABY unique is the combination of a large galaxy sample with sufficiently high angular resolution (30″ FWHM) to spatially resolve a significant fraction of those galaxies. This enables entirely new scientific studies, such as the search for H<sc>i</sc> polar ring galaxy candidates or the extraction of resolved galaxy H<sc>i</sc> scaling relations for statistical samples. While the full survey is still underway, catalogues, images and spectra of nearly 2400 galaxies detected during the pilot phase have already been publicly released. More data releases are anticipated over the next few years as the survey progresses.

New wide-field Hi surveys have produced exciting results about galaxy evolution. The WALLABY pilot survey (∼400 deg2 in total) captures snapshots of Hi-rich galaxies infalling into massive groups and interacting in pairs. We statistically characterize the shrinking of Hi disks and changing in star formation patterns due to ram pressure, harassment, and tidal interactions in distinct environments. The FEASTS (∼60 deg2 in total) maps Hi around 55 galaxies down to a column density level of 1017.7 cm-2. The diffuse Hi is found to be similar to the fountain-driven thick disk Hi in the inner region, and closely related to tidal interactions in the outer region. Beyond the optical disk, it has an almost constant volume density implying its cooling role. The 1017.7 cm-2 Hi extends far both along and perpendicular to the disk, and carries clues about gas accretion.

The distribution of neutral atomic hydrogen (H i) extends beyond galaxies into the circumgalactic (CGM) and intergalactic medium (IGM) due to its diffuse nature. Comprehensive mapping of H i gas on multiple scales requires combining the capabilities of both single-dish and interferometric radio telescopes. In this context, we introduce a FAST-MeerKAT synergy project aimed at mapping H i sources in the COSMOS field by leveraging the unparalleled sensitivity of FAST and the high resolution of MeerKAT, allowing for detailed studies of H i distribution across diverse galactic environments. Our observations demonstrate that the FAST-H i sources are marginally more H i massive than their MIGHTEE-H i counterparts by a median fraction of ∼ 7% for isolated galaxies and ∼ 18% for galaxy pairs, indicating that the majority of the H i gas in simple galactic environments is concentrated within galaxies, rather than dispersed throughout the surrounding CGM and IGM in our local Universe.

The Cosmic Neutral Hydrogen Density (ΩHI) is a parameter which measures the quantity of neutral hydrogen gas in the universe across cosmic time. Because HI, through its transformation into molecular hydrogen, is the key ingredient for the eventual star formation in galaxies, understanding its evolution is crucial for understanding the turnover in the cosmic star formation density at redshift z∼2. While ΩHI is well established in the local universe through direct HI emission measurements and at very high redshifts through observations of damped Lyman-alpha systems, there are very few measurements at intermediate redshifts and those that exist have quite large uncertainties. Taking advantage of the sensitivity of MeerKAT, techniques such as spectral stacking, and the depth of the Looking At the Distant Universe (LADUMA) survey, we present a preliminary measurement of ΩHI = 7.86 ± 1.57 ×10−4 for z = 0.55 using our first release of L-band data.

This paper presents the results of our recent Hi and OH absorption survey towards 40 low-intermediate radio luminosity (∼1023-1026 W Hz-1) active galactic nuclei (AGNs) with the Five-hundred-metre Aperture Spherical radio Telescope (FAST). Of the 40, we had good data at Hi observing frequencies for only 13 radio AGNs with 8 detections. Of these 8, five are discoveries including four of the lowest radio luminosity detections in the redshift range 0.25-0.35. We also had radio frequency interference (RFI) free spectra at OH observing frequencies for 23 radio AGNs with no detections. By stacking 23 spectra, we obtained the upper limit on OH column density to be 2.27×1014 cm-2 assuming excitation temperature, Tex = 10 K and covering factor of one. We also stacked 7 OH spectra with Hi absorption detections to achieve an upper limit on OH column density to be 3.47 × 1014 cm-2 and OH to Hi ratio to be 1.78 × 10-7.

Neutral Hydrogen (HI) gas reservoirs make up the bulk of the interstellar medium and the raw fuel out of which stars eventually form. The star-formation rate (SFR) density has been observed to decline sharply over the past 10 Gyr (since z ∼ 2). Through spectral stacking using the L-band dataset from the deep MeerKAT HI survey LADUMA, we have measured the average HI masses and characteristic HI depletion timescales of star-forming galaxies for different intermediate redshift sub-samples (0.246 ≤ z ≤ 0.48 and 0.52 ≤ z ≤ 0.62) and stellar mass sub-samples (log M* > 10 and log M* ≤ 10). We find lower HI masses and shorter gas depletion times when comparing to both higher redshift results at z ∼ 1 and Local Universe observations for all sub-samples. These results imply HI reservoir depletion and then subsequent replenishment over cosmic time however different selection effects and systematic variations in the samples need to be considered.

The study presented in this paper focuses on verifying cosmological simulations by searching for extended neutral hydrogen (HI) emission in the Fornax galaxy cluster. Using data from the MeerKAT Fornax Survey (MFS), which maps HI across the cluster, we compare observational results with predictions from the TNG50 cosmological simulations. The goal is to investigate the HI content of galaxies in dense environments, analyze gas stripping processes, and test the accuracy of simulations like TNG50 in modeling cold gas interactions. By comparing HI covering fractions and column densities, the study highlights both the successes and limitations of current models in representing the physical processes within galaxy clusters.

UGCA 250 is an edge-on, late-type spiral (Sd) galaxy part of the HI nearby galaxies legacy survey MHONGOOSE. This work looks for signatures of extraplanar gas associated with this galaxy with the aims of characterising the kinematics of the disk as well as its extensive gaseous layers. This is achieved due to the ability of MeerKAT to probe the innermost regions as well as the most extended regions associated with this galaxy. We perform a kinematic analysis on one of the full depth MHONGOOSE HI cubes using tilted-ring modelling and Gaussian decomposition. This is supplemented with high-resolution RSS longslit data from SALT, allowing for the detection of extraplanar gas in both its neutral and ionised states. We find interesting kinematics in both the optical and radio, hinting that the kinematics should be explained by more than a simple disk model. The end result is a multi-wavelength characterisation of the gas associated with this galaxy. Thus, we probe for multi-phase planar and extraplanar diffuse gas associated with this galaxy. This work illustrates how sensitive HI observations with MeerKAT can be combined with high-resolution spectroscopic SALT data to achieve a complete kinematic study of a nearby galaxy.

Understanding what shapes the cold gas component of galaxies, which both provides the fuel for star formation and is strongly affected by the subsequent stellar feedback, is a crucial step towards a better understanding of galaxy evolution. This contribution will present detailed analysis of the atomic hydrogen (HI) in a sample of 46 Milky Way-mass galaxies from cosmological simulations. This set of simulations comprises galaxies evolved self-consistently across cosmic time with different baryonic sub-grid physics: three different star formation models and two different stellar feedback prescriptions. We discuss the complex interplay between star formation, feedback and HI properties of the galaxies and show that while the HI properties (mass, extent, density profile, scale height, morphology) of the simulated galaxies are in broad agreement with current observational results, they vary significantly between the different baryonic physics models. The morphology of HI discs is particularly sensitive to the different physics models.

The aim of this research is to utilise data from the IMAGINE survey to probe deeper into the circumgalactic medium (CGM) of spiral galaxy NGC7424 and identify large scale HI structures using the high column densities achievable with ATCA. We intend to make systematic comparison of measurable HI properties such as HI flux and and linewidths of NGC7424 in both IMAGINE and MHONGOOSE (observed with MeerKAT) surveys as well as data obtained from single dish telescopes. The future goal is to combine both interferomentric and single dish data to obtain images with both high column density and resolution to search for evidence of cold mode accretion in the CGM.

Ultra-diffuse galaxies (UDGs) have stellar masses typical to dwarf galaxies but physical sizes more akin to Milky Way-type galaxies. The reason for their extended sizes is an open question, with several internal and environmental mechanisms theorised. UDG in the field are generally observed to be star-forming and gas-rich, making HI an efficient tool to obtain distances and therefore estimate their physical sizes. We compare gas-rich UDGs and dwarf galaxies from the an extensive HI follow-up survey of optically-selected UDG candidates from the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) catalogue. Moreover, we compare HI-detected UDGs and dwarfs in SMUDGes to those formed in two state-of-the-art cosmological simulations: Numerical Investigation of a Hundred Astrophysical Objects (Nihao), in which UDGs are a result of early bursty star formation, and Romulus25, in which UDGs form due to high spin major mergers. We find that the present-day, global properties of gas-rich UDGs formed in both simulations are consistent with each other, and with our observed UDG sample. Moreover, we find no distinct difference between UDGs and dwarfs in neither simulations, nor observations, including in the gas richness – optical size plane. This study suggests that the simplest explanation for the consistent properties between the two populations is that gas-rich UDGs are an extreme subset of dwarf galaxies, and not a distinct population.

We present here a study of the broadband spectral properties of 33 sources detected in HI absorption as part of the ASKAP-FLASH Pilot Surveys. We outline our approach to spectral classification and discuss the correlation seen between spectral shape and the detection of HI absorption. We further consider the implications of the observed correlation on the spatial distribution of the neutral gas, and on the jet-gas interactions. Our results are evaluated in the context of the forthcoming, full ASKAP-FLASH survey and other large, untargeted searches of the radio sky.

The Small and Large Magellanic Clouds (SMC and LMC) provide perfect laboratories to study cold neutral medium (CNM) and warm neutral medium (WNM) phases of atomic hydrogen (HI) in low-metallicity environments. Recent GASKAP-HI survey using the ASKAP radio telescope shows detailed HI structure in these galaxies. This study focuses on the HI properties in the extreme outskirts of the SMC and LMC. We find that most CNM structures have spin temperatures below 40 K, colder than what is found for HI in the Milky Way. The CNM fraction in the SMC is below 20%, while some LMC regions exceed 70%. We identify a unique cold cloud in the LMC, entirely composed of the CNM, near a filamentary structure. Additionally, we find the presence of supergiant shells near most of the cold gas in the LMC outskirts, suggesting that large shell expansion likely promotes more cold cloud formation, compared to what is found in the SMC.

We employed HI data together with Planck polarization data to estimate the foreground magnetic field and intensity toward the Large Magellanic (Milky) Cloud. For the magnetic field structure, we applied the Velocity Gradients Technique on HI spectra while for the intensity contribution separation we correlated the atomic gas column density with the Planck intensity at 353 GHz, around the galaxy. Our study highlights the importance of neutral hydrogen measurements in and around galaxies and suggests a method for foreground subtraction.