We present the latest results from the First Large Absorption Survey in H i (FLASH), an untargeted survey searching for 21-cm line absorption within the redshift range of 0.4 < z < 1.0 using the ASKAP. 21-cm absorption provides a unique and unbiased probe of H i beyond the local Universe. The survey has successfully completed two phases of Pilot Surveys and commenced the Full Survey toward the wide-field southern sky with δ < +15 degrees and |b| > 8.5 degrees. Throughout the Pilot Surveys, we detected 30 genuine candidates of associated and intervening absorbers, revealing a diverse range of line properties across all redshifts. These absorbers are linked either to nearby host galaxies or distant background radio sources, respectively. We will highlight some of the newly discovered spectra and provide an update on the progress of the ongoing Full Survey, which will continue until 2028.

The redshifted 21cm line of Neutral Hydrogen (HI), detected by radio telescopes can probe the large-scale structure of the post-reionization Universe via Intensity Mapping techniques. This makes HI one of the most promising observables to transform our understanding of dark matter and dark energy. We present here what we hope to achieve once the full SKAO is operational. We also discuss the recent development on HI intensity mapping with the SKAO precursor MeerKAT and a key challenge in these types of experiments: the subtraction of the bright foregrounds. The subtraction can be achieved with component separation techniques, but the quality of the results needs to be carefully checked against simulations. Fortunately, the cross-correlation of the HI signal with galaxy survey data which trace the same underlying dark matter distribution allows a strong reduction of systematics down to the detection of the HI signal in current MeerKAT data.

Since the pioneering work of Arp and collaborators in the 1970s, it has been known that the field between Stephan’s Quintet and NGC 7331 contains much diffuse gas and many filamentary structures. Little to no work on the nature of these structures was done in the subsequent forty years, although a new deep colour image was published by Duc et al. (2018) confirming the existence of the structures and showing them in fine detail. Here we carry out a detailed study of the structures using the data of Duc et al., and some of our deeper exposures of selected fields, and look for detections in other wavebands.

The circumgalactic HI might hold the clue to the missing accretion: the mismatch between the HI accretion rate and star formation rate of local (z∼0) galaxies. Using the Green Bank Telescope, we probed the 21-cm emission from HI in the CGM of two local galaxies out to 100 kpc impact parameter at an unprecedented sensitivity of 1016 cm−2. We detected HI of diverse spectral shapes, velocity widths, and column densities. Interferometric maps from the Westerbork Synthesis Radio Telescope cannot explain >50% of these single-dish detections. This excess circumgalactic HI along major axes pointings co-rotates with the HI disks and inflows along the minor axes pointings. Whether it accounts for the missing accretion is subject to individual targets.

The WALLABY HI survey on the Australian SKA Pathfinder is measuring Tully-Fisher distances and peculiar velocities for about 50,000 galaxies in the nearby universe. We have developed a new Bayesian methodology for simultaneously fitting the parameters of the Tully-Fisher relation and a model of the velocity field reconstructed from a redshift survey density field. This approach deals effectively with selection effects and biases and yields significantly more precise distances and peculiar velocities for individual galaxies. We demonstrate the methodology on the HI Tully-Fisher subsample of the CosmicFlows-4 dataset and obtain an improved estimate of the growth rate of structure in the present-day Universe from a comparison of the predicted and observed velocity fields, as well as an improved estimate of the Hubble constant. We simulate the HI Tully-Fisher observations from the WALLABY survey and forecast the constraints it will place on the growth rate of structure.

One of the key questions in the field of galaxy evolution is “How do galaxies assemble and evolve?”. The MeerKAT Hi Observations of Nearby Galactic Objects – Observing Southern Emitters (MHONGOOSE) survey aims to address this question by imaging the diffuse gas at the outskirts of galaxies down to Hi column densities as low as ∼ 5×1017 cm−2 for the first time, looking for direct evidence of accretion, among other science goals. In these proceedings I will show results from one of the MHONGOOSE targets, UGCA320. UGCA320 is an edge-on dwarf galaxy (M*∼109M⊙) with a remarkable Hi disc. It contains extraplanar gas that can only be detected with these highly sensitive observations, as well as asymmetries and velocity structures that cannot be explained with simple models. UGCA320 is part of a small galaxy group, and has possibly interacted with one or more of its neighbours. These are also included in the MeerKAT FoV, and have resulted in some very interesting serendipitous Hi detections that warrant detailed, individual studies of their own. In these proceedings I will show an analysis of the Hi content of UGCA320, and discuss possible interpretations in the context of the group environment. The surprising low column density features we detect with these ultra-deep observations showcase excellently what unprecendented detections of the neutral hydrogen in and around galaxies the SKA might bring, not only in groups of dwarf galaxies, but in a wide range of galaxy types and masses.

Numerical simulations have proven crucial tools for understanding environmental effects that influence the HI content of galaxies. They have provided a variety of new tools and diagnostics that can be applied to observations to greatly deepen our understanding of the processes at play. Furthermore, recent results from cosmological simulations make it increasingly clear that we cannot ignore the close connection between dense environments and their surrounding large scale structure. However, while hydrodynamical cosmological simulations have been shown to be highly successful at matching global gas scaling relations of central galaxies, it remains challenging to accurately match the properties of satellite galaxies. Thus, the complex physics at play on satellite galaxies may potentially provide valuable constraints on the sub-grid physics employed in the next generation of simulations.

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.