Galaxy star formation rates (SFR), Hi masses, and H2 masses all show strong correlations with stellar mass for star-forming systems. Moreover, galaxies with high/low SFR also have high/low gas contents. Here we compare observations of correlated deviations off the main sequence from Janowiecki et al. to those in three galaxy formation simulations: Simba, EAGLE, and IllustrisTNG. Simulations predict that H2 deviations are linearly correlated with SFR deviations, as expected since star formation directly arises in dense gas. However, Hi deviations predictions vary, with Simba producing a near-linear slope but EAGLE and IllustrisTNG predicting sub-linear slopes. At earlier cosmic epochs, a sub-linear slope is always present in EAGLE and IllustrisTNG, but only at z = 2 in Simba, becoming linear at z ≤ 1. This suggests that the linear slope owes to the onset of AGN feedback, and hence that such feedback must remove Hi from the galaxy to match the observed linear slope. This provides unique insights into baryon cycling processes that connect gas within galaxies and their surroundings.

We present a search for HI 21 cm absorption associated with active galactic nuclei (AGN) using the SKA Precursor telescope MeerKAT. We use the latest data release from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) HI survey [Heywood et al., 2024]. This data has a high spectral resolution of 26.1 kHz (∼8 kms−1) and wide bandwidth covering HI redshifts to z = 0.48. We have performed a targeted search for HI absorption along sight lines for the 181 radio galaxies in the COSMOS field within this redshift range. Our primary goal is to examine the interplay between gas, AGN fuelling and feedback, and the impact of these on the overall evolution of the host galaxies. The excellent sensitivity of both the MIGHTEE-HI and MIGHTEE-continuum data in this field allows us to examine lower radio power continuum sources than previously possible. We present 12 associated-HI absorption candidates.

The observed star formation rates in nearby galaxies suggest they are replenishing their gas reservoir over cosmic time. Cosmological simulations predict that this gas accretion can happen in two modes: hot and cold. However, direct observation of the low-column density (∼ 1017 cm−2) neutral atomic hydrogen (Hi) tracing the cold mode remains unconfirmed. Using the the spectral stacking technique on ultra-deep (∼1018 cm−2) Hi data provided by the MHONGOOSE survey we still found no clear evidence for the presence of the low-column density gas around nearby galaxies as predicted by simulations. We suggests that the gaseous disk has a sharp cut-off at large distances, consequently, detecting H i in the circum-galactic and inter-galactic medium could remain challenging, even with future advanced radio telescopes.

Ultra diffuse galaxies (UDGs) are a low surface brightness population of galaxies exhibiting large sizes. Their recent unexpected abundance in clusters has brought into question our understanding of galaxy evolution and the effect of the harsh cluster environment. Using MeerKAT HI observations of the Hydra I cluster, a nearby Southern galaxy cluster with ongoing galaxy interactions and evolution, we explore the neutral hydrogen content in low stellar mass galaxies, specifically investigating UDGs. Detailed observations of these galaxies in HI provide insights into the processes they experience in these harsh and dense environments. We present preliminary results on our search for UDGs in the Hydra cluster with MeerKAT, including HI detections of a UDG near the center of the cluster, a possible LSB galaxy and a dark cloud of neutral hydrogen.

We present the first MeerKAT observations of the brightest neutral hydrogen (HI) cloud in the M31-M33 HI filament [Braun Thilker(2004)] at a resolution of 40″. Our analysis unveils previously unseen details in the internal kinematic structure of the HI cloud. We measure NHI ≈ 1019 cm-2 in the cloud, an order of magnitude greater than previously observed, and recover approximately two-thirds of the previously measured MHI. We see signs of ram pressure stripping implying a particle number density of nHI ≈ 10-4 cm-3 in the circumgalactic medium (CGM) of M31. The cloud may also have been tidally disrupted by M31. Both results suggest that the HI cloud is being destroyed as it falls through the CGM of M31.

The advent of innovative new radio telescopes and instruments around the world has enabled HI observations of galaxies that are sharper, deeper, fainter and farther than ever before. This paper provides an overview of the extragalactic HI landscape today, with a focus on large surveys with SKA pathfinder facilities. Ongoing work to measure structural properties of HI disks from the WALLABY survey on the ASKAP telescope is also summarised, with an emphasis on the challenges of building statistical samples of the marginally detected and resolved systems that are characteristic of widefield untargetted surveys. This work paves the way to map HI disks across cosmic time with the SKA.

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.