In this contribution we present our recent study of the resonant structure of M51, using kinematic mapping with the CO molecule from PAWS supplemented with our own velocity map in Hα, which is complemented with the analysis of the morphology of M51 in terms of the variation of the pitch angle along the spiral arms, which unveil the existence of two kinks in each arm. We demonstrate the role of its interacting satellite NGC 5195 in stimulating the formation of the spiral arms and kinks, and demonstrate that it has twice passed through the plane of M51, calculating the time interval between passages. Our results are strengthened by comparison with simulations of this interaction.

Atomic hydrogen (HI) gas is a key fuel reservoir for star formation in galaxies and provides an excellent tracer of their dynamical masses. The Square Kilometre Array (SKA) precursor observatories are already hinting at the incredible HI observations that we can expect from the SKA. We are also undergoing a quieter but equally exciting revolution in simulations of HI in galaxies. Up to now, we have had to resort to labelling HI in cosmological galaxy formation simulations in post-processing, subject to uncertain calibration and assumptions. New simulations are implementing cooling to 10 K and non-equilibrium ionization and chemistry, enabling on-the-fly calculation of HI fractions. We will soon have for the first time fully self-consistent, spatially-resolved predictions for the atomic hydrogen content, structure and kinematics of galaxies across cosmic history. I give an overview of the state of the art in simulation-based predictions for HI including recent breakthroughs and current challenges.

Understanding the thicknesses of disk galaxies is crucial to further determine galaxy properties such as the volumetric star formation (VSF) law and the shape of dark matter halo. The neutral hydrogen (HI) component stands out due to its ubiquity in disk galaxies and its tight connection to star formation and feedback in galaxies. In this project, we will present the results obtained from examining the HI scale heights of four edge-on MHONGOOSE galaxies at, z ∼ 0 discuss how they relate to other properties of the galaxies, including the scale height in the IR and optical, and discuss the implications for star formation and feedback in these galaxies.

We explore the inner core of the enigmatic Vela Supercluster (VSCL) at cz ∼ 1800 km S−1 across the inner Zone of Avoidance. We mapped the morphology and inner structure of VSCL by identifying its gas-rich spiral population using two MeerKAT surveys that jointly cover an area of ∼300 deg2 within 263° ≤ l ≤ 284°, |b| < 6.7°, and are sensitive to galaxies throughout the probed volume (cz < 25000 km s-1). Here we present highlights of the new findings from the over 1500 newly discovered HI-rich galaxies. Various distinct structures could be clearly identified. Most importantly for this study are the discoveries of the prominence of the two walls that link to the previously identified higher-latitude VSCL. The HI mass functions at these wall distances show mass density ratios exceeding 1.2 when compared to ALFALFA 100, with possible crossings of the Galactic Plane at two locations.

The Surface Photometry and Accurate Rotation Curves (SPARC) database has provided the community with mass models for 175 nearby galaxies, allowing different research teams to test different dark matter models, galaxy evolution models, and modified gravity theories. Extensive tests, however, are hampered by the somewhat heterogeneous nature of the Hi rotation curves and the limited sample size of SPARC. To overcome these limitations, we are working on BIG-SPARC, a new database that consists of about 4000 galaxies with Hi datacubes from public telescope archives (APERTIF, ASKAP, ATCA, GMRT, MeerKAT, VLA, and WSRT) and near infrared photometry from WISE. For these galaxies, we will provide homogeneously derived Hi rotation curves, surface brightness profiles, and mass models. BIG-SPARC is expected to increase the size of its predecessor by a factor of more than 20. This is a necessary step to prepare for the additional order of magnitude increase in sample size expected from ongoing and future Hi surveys with the Square Kilometre Array (SKA) and its pathfinders.

Post-starburst galaxies (PSBs) are key to understanding how galaxies transition from active star formation to quiescence. We identify 36 PSBs by spectroscopic selection in the COSMOS field at z < 0.45. An analysis of their colours, star formation rates, and morphologies, suggests that they are in an early-stage post-starburst transitional phase. Additionally, we use neutral hydrogen and radio continuum data to investigate their gas content and their potential for star formation.

The properties of galaxies follow scaling relations related to the physics that govern galaxy evolution. Based on these, we can identify galaxies undergoing specific evolutionary processes such as H i-excess galaxies, which have relatively high H i mass compared to their stellar mass. The possible reasons for this could be either recent gas accretion or an inefficient conversion of the H i to molecular gas. Since recent gas accretion is difficult to prove conclusively, we investigated gas conversion by analysing the molecular gas content of five extremely H i rich galaxies from the H ix galaxy sample. For this, we obtained CO observations of the sample galaxies with the Atacama Large Millimeter Array (ALMA). While we find that our sample galaxies have relatively regularly rotating molecular gas disks, their molecular gas fraction is significantly lower than what is expected from scaling relations and the CO gas has relatively high velocity dispersion.

The deep SARAO MeerKAT Galactic Plane Legacy Survey (SMGPS) covers the southern Milky Way along a narrow strip of ∆b ∼ 3°. While primarily aimed at exploring the Galaxy, the SMGPS provides a unique opportunity to trace the large-scale distribution of galaxies through the redshifted spectral 21cm line emission of their gas. The SMGPS H I survey is sensitive to galaxies with H I masses to 6·109 M⊙ throughout the probed volume (cz < 25 000 km s-1), and to 3·108 M⊙ at the Great Attractor distance. We will first summarize highlights of newly uncovered structures, then show preliminary results from deep MeerKAT64 follow-up observations of a hugely extended low H I mass galaxy disk identified in the SMGPS. The new observations, being sensitive to much lower H I column densities, trace the disk out even further (∼ 260 kpc).

The surroundings of galaxies influence their evolution through multiple entangled mechanisms awaiting further dissection. For the nearby merging-pair M51 group, we jointly deconvolve VLA and FAST (FEASTS) data. The strong tidal interaction produces large amounts of diffuse. Its turbulent mixing with the CGM could bring about a cooling flow rate comparable to the SFR, implying tidal-interaction–induced cooling to be an important gas-accretion channel. For the intermediate-mass NGC 4636 group, we identify the dominant mechanism for each satellite, through quantifying the strength and -stripping extent of ram-pressure stripping and tidal interaction. We preliminarily break the degeneracy of these two on gas removal, and estimate the timescale of gas stripping, which decreases with distance to the group centre, ranging from ∼1Gyr to 10Myr. Our studies quantify in a physically motivated way the details and processes of environmental-effects–driven galaxy evolution.

The role of diffuse Hi gas in galaxy formation and evolution is crucial for understanding aspects of cosmic structure growth. Diffuse atomic hydrogen serves as the secondary fuel for star formation and spans a wide range of column densities, contributing to the baryonic content of galaxies, the circumgalactic medium (CGM), and the intragroup medium (IGrM). However, constructing a clear picture of how gas flows between these different components, especially in group environments, remains a challenge. Here, we present a new H i-selected group sample from the Looking at the Distant Universe with the MeerKAT Array (LADUMA) large survey project with the MeerKAT interferometer. We have identified several loose and compact Hi galaxy groups within our LADUMA DR1 catalog (Kazemi-Moridani et al. in prep). This sample provides a unique opportunity to investigate environmental effects and how galaxy interactions impact observed Hi scaling relations.

HI emission allows velocity curves of galaxies to be constrained out to the dark matter halo. Therefore the structure of dark matter halos can be determined from spiral galaxy rotation curves if the contribution from gas and stars can be properly understood. This work presents the structure, column density distribution, flux and mass of HI in and around the galaxy NGC 45 provided by the IMAGINE survey, with the objective of performing mass modelling of this object. The total flux density obtained is 207.70 Jy km s-1 corresponding to an HI mass of 2.13 × 109 M⊙ for a distance of 6.6 Mpc. These values are comparable with the literature as well as the obtained global HI profile and the velocity field. The galaxy is now ready to have the HI, stellar and dark matter mass modelled.

We present the results of our search for Hi emission from gas-rich galaxies in the Ophiuchus Supercluster region. The study covers a Galactic longitude range of 329° ≤ l ≤ 55° (excluding 358° ≤ l ≤ 2°) and a Galactic latitude range of |b| < 1.5°, within a redshift range of 7500 km s−1 < cz < 25000 km s-1. As part of the SARAO MeerKAT Galactic Plane Survey (SMKT-GPS), our primary objective is to map the large-scale structure of this region, investigate overdense areas, and explore possible connections between the Ophiuchus Supercluster and larger cosmic structures, such as the Great Attractor. Here we used 440 MeerKAT pointings, which were combined into 28 mosaics, each consisting of 22 pointings. These mosaics cover an area of approximately 252 deg2 with a median rms sensitivity of 0.46 mJy beam−1 per 44 km s−1 channel. Among the 589 Hi sources detected, 286 fall within the redshift range of 7500 km s−1 < cz < 12000 km s−1, which corresponds to the Ophiuchus Supercluster. Several overdense regions have been identified at redshifts of cz ∼ 9000, 11500, and 13500 km s−1.

Using 21 cm line emission data from the SARAO MeerKAT Galactic Plane Survey (SMGPS), we conducted a deep exploration of the southern Milky Way to trace large-scale structures of galaxies in the most opaque region of the sky (|b|<2°). We focused on the Great Attractor wall and its extension toward the Crux region. The high sensitivity (rms = 0.3-0.5 mJy beam−1) and angular resolution (∼31″ × 26″) enabled us to map structures across the Zone of Avoidance, as well as identify new ones. In the GA region we detected 477 galaxies, of 214 are within the GA velocity range of 3500-6500 km s−1. In the Crux extension we found an unexpected overdensity around 6000 km s−1, seemingly unrelated to the GA. For the first time, we present a redshift wedge of the local universe (≤10000 kms−1) at these extreme latitudes, enhancing the map of the local universe and demonstrating the power of Hi emission in revealing galaxies hidden from optical surveys.

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.