This chapter introduces the contributions of the HI galaxy science in this volume reviewing the latest developments and urgent questions in HI galaxy science, providing guiding principles for a layered set of future key science projects. The key science will include: a complete censuses of HI morphologies and kinematics at sub-kpc and 1 km/s resolution within and around galaxies in the nearby Universe; a measurement of the cosmic HI mass density and HI mass function evolution at least up to z 1; an improved understanding of the Universe at z>1, particularly the balance between cold molecular and cool atomic gas. We also provide a view of the synergistic multi-wavelength surveys available in 2028+ in the southern hemisphere. This effort will improve our understanding of the baryon cycle across a significant fraction of the cosmic history, including the processes of gas accretion, consumption and removal as well as AGN and star formation feedback. Based on these science goals, the earlier proposed three-tiered survey strategy remains, but survey parameters and predictions are adjusted according to AA* and AA4 developments. This chapter is an update of the earlier "Advancing Astrophysics with the Square Kilometre Array" chapter 'HI Science with the SKA' by Staveley-Smith Oosterloo.

We present MeerKAT observations searching for H I absorption in a sample of 17 powerful (L-1.4 GHz > 10(27) W Hz(-1)) radio sources at intermediate redshifts (0.25 < z < 0.7). The sample is well characterised at radio and optical wavelengths, allowing us to connect the presence (or absence) of H I to the properties of the active galactic nucleus (AGN) and its host galaxy. The sample consists mostly of core-dominated sources and quasars. Half of the targets have a UV luminosity below the limit of L-UV = 10(23) W Hz(-1), whereby at values above this limit, the gas would be expected to be ionised by this radiation. We obtained 15 spectra free (or almost free) of radio frequency interference, reaching extremely low optical depths (tau(peak) < 0.005) resulting in three new H I absorption detections. Two are associated H I absorptions, giving a detection rate of such systems of 13%+/- 7%. Both are found in a young radio source (PKS 1151-34 and PKS 1306-09), confirming the trend that this type of sources are more often detected in H I compared to more evolved ones. The UV luminosity of both these sources is below 10(23) W Hz(-1). Surprisingly, one of the detections (PKS 1151-34) is hosted by a quasar, suggesting that the radio lobes of this source are still embedded in the circumnuclear disc. In the second source (PKS 1306-09), the H I is highly blueshifted and likely part of the jet-driven outflow earlier observed in the warm ionised gas. This represents a new addition to the group of young radio AGNs, where multi-phased outflows have been observed as predicted by numerical simulations. A third detection is a 'local intervening' system, caused by a galaxy in the local environment of PKS 0405-12 and located in front of the southern radio lobe of this source, about 100 kpc in projection from this quasar. More such cases are expected to show up in large, blind surveys and our results show the need for high spatial resolution and good ancillary data to separate associated from intervening absorption. Overall, the results indicate a variety of plausible situations, which resemble what is seen at low redshifts. For the associated absorption, a combination of evolutionary status of the radio sources, physical conditions, and geometry of the gas structure determine the detection rate of H I absorption. The data also show the excellent capabilities of MeerKAT for obtaining very low optical depth detections, revealing the presence of an otherwise missed group of absorptions.

The brightest cluster galaxy NGC 1275, at the centre of the Perseus cluster, hosts the radio-loud AGN 3C 84 and a circumnuclear disc (CND) of cold molecular gas onto which large-scale CO filaments accrete, yet the physical and chemical conditions of the gas in the CND remain poorly constrained. We present a spatially resolved analysis of ALMA CO(2-1), HCN(3-2), and HCO^+(3-2) observations at 72 pc. The data are partitioned into beam-sized hexagonal regions and modelled with a Bayesian framework that couples a neural network emulator of time-dependent chemistry (UCLCHEM) with non-LTE radiative transfer (SpectralRadex) to infer the gas density, kinetic temperature, and cosmic ray ionisation rate in each region. All three line ratios peak in the inner disc and decline with radius. The inference shows radial gradients in density (log_10 n( H_2) ≈ 6.3 to ∼ 5), kinetic temperature ( 200 K to 160 K), and cosmic ray ionisation rate (log_10(ζ/ζ_0) ≈ 4.9 to ∼ 3). Despite the powerful radio AGN, the observed HCN(3-2)/HCO^+(3-2) ratio remains ≲ 1 across the disc. Our modelling attributes this to optical depth saturation of HCN(3-2) (τ∼ 1-3), which suppresses the intensity ratio even when the HCN abundance exceeds that of HCO^+ by a factor of three or more. The HCN/HCO^+ intensity ratio therefore cannot be used as an abundance diagnostic without accounting for optical depth, and a low ratio does not necessarily imply weak AGN influence on the chemistry. Azimuthally resolved profiles suggest a localised HCO^+/CO enhancement at the western disc boundary, coinciding with the filament-disc accretion interface and consistent with shock processing by velocity shear between the infalling filaments and the rotating disc. These results indicate that the CND is shaped by accretion from its filamentary environment.

We present NOEMA CO(2–1) observations of a nearby, young, low-luminosity radio source, B2 0258+35. Our earlier CO(1–0) study had shown the presence of strong jet-ISM interaction and a massive molecular gas outflow involving 75% of the circumnuclear gas. Our follow-up CO(2–1) observations have revealed even more complex gas kinematics, where the southern radio jet is driving out molecular gas in the form of a swiftly expanding bubble, with velocities up to almost 400 km s−1. We found highly elevated CO(2–1)/CO(1–0) line ratios for the gas belonging to the bubble and also further away from the radio jets. Previous observations have shown that the active galactic nucleus (AGN) in the host galaxy, NGC 1167, is in a very low-accretion state. Thus, we attribute the high line ratios to the high gas excitation caused by the jet–ISM interaction. The radio jets, despite exhibiting a relatively low luminosity (1.3 × 1044 erg s−1), are solely responsible for the observed extreme gas kinematics. This is one of the clearest detections of an expanding cold gas bubble in such a type of source, showing that the jets are affecting both the kinematics and physicals conditions of the gas. Our study adds to the growing store of evidence that low-luminosity radio sources can also affect the kinematics and physical conditions of the cold gas, which fuels star formation, in their host galaxies to a significant extent. Hence, such sources should be considered in models seeking to quantify feedback from radio AGN.

The First Large Absorption Survey in H i (FLASH) is a large-area radio survey for neutral hydrogen in and around galaxies in the intermediate redshift range $0.4\lt z\lt1.0$ , using the 21-cm H i absorption line as a probe of cold neutral gas. The survey uses the ASKAP radio telescope and will cover 24,000 deg $^2$ of sky over the next five years. FLASH breaks new ground in two ways – it is the first large H i absorption survey to be carried out without any optical preselection of targets, and we use an automated Bayesian line-finding tool to search through large datasets and assign a statistical significance to potential line detections. Two Pilot Surveys, covering around 3000 deg $^2$ of sky, were carried out in 2019-22 to test and verify the strategy for the full FLASH survey. The processed data products from these Pilot Surveys (spectral-line cubes, continuum images, and catalogues) are public and available online. In this paper, we describe the FLASH spectral-line and continuum data products and discuss the quality of the H i spectra and the completeness of our automated line search. Finally, we present a set of 30 new H i absorption lines that were robustly detected in the Pilot Surveys, almost doubling the number of known H i absorption systems at $0.4\lt z\lt1$ . The detected lines span a wide range in H i optical depth, including three lines with a peak optical depth $\tau\gt1$ , and appear to be a mixture of intervening and associated systems. Interestingly, around two-thirds of the lines found in this untargeted sample are detected against sources with a peaked-spectrum radio continuum, which are only a minor (5–20%) fraction of the overall radio-source population. The detection rate for H i absorption lines in the Pilot Surveys (0.3 to 0.5 lines per 40 deg $^2$ ASKAP field) is a factor of two below the expected value. One possible reason for this is the presence of a range of spectral-line artefacts in the Pilot Survey data that have now been mitigated and are not expected to recur in the full FLASH survey. A future paper in this series will discuss the host galaxies of the H i absorption systems identified here.

Extragalactic gas accretion and outflows driven by stellar and active galactic nucleus (AGN) feedback are expected to influence the distribution and kinematics of gas in and around galaxies. Atomic hydrogen (H I) is an ideal tracer of these processes, and it is uniquely observable in nearby galaxies. Here we made use of wide-field (1 degrees x1 degrees), spatially resolved (down to 22 ''), high-sensitivity (similar to 10(18) cm(-2)) H I observations of five nearby spiral galaxies with stellar mass of similar to 5 x 10(10) M-circle dot, taken with the MeerKAT radio telescope. Four of these were observed as part of the MHONGOOSE survey. We characterise the main H I properties in regions of a few hundred kiloparsecs around the discs of these galaxies, and compare them with synthetic H I data from a sample of 25 similarly massive star-forming galaxies from the TNG50 (20) and FIRE-2 (5) suites of cosmological hydrodynamical simulations. Overall, the simulated systems have H I and molecular hydrogen (H-2) masses in good agreement with the observations, but only when a pressure-based H-2 recipe is employed. The other recipes that we tested overestimate the H-2-to-H I mass fraction by up to an order of magnitude. On a local scale, we find two main discrepancies between the observed and simulated data. First, the simulated galaxies show a more irregular H I morphology than the observed galaxies, due to the presence of H I with column density < 10(20) cm(-2) up to similar to 100 kpc from the galaxy centre, even though 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 H I line profiles than the observed galaxies. 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 H I discs.

We present ALMA CO(1-0) and CO(3-2) observations of a powerful young radio galaxy, PKS 0023-26, hosted by a far-infrared bright galaxy. The galaxy has a luminous optical AGN and a very extended distribution of molecular gas. We used these observations (together with available CO(2-1) data) to trace the impact of the AGN across the extent of the radio emission and beyond on scales of a few kpc. Despite the strength of the optical AGN, the kinematics of the cold molecular gas is strongly affected only in the central kpc, and is more weakly affected around the northern lobe. We found other signatures of the substantial impact of the radio AGN, however. Most notably, extreme line ratios of the CO transitions in a region aligned with the radio axis indicate conditions very different from those observed in the undisturbed gas at large radii. The non-detection of CO(1-0) at the location of the core of the radio source implies extreme conditions at this location. Furthermore, on the scale of a few kpc, the cold molecular gas appears to be wrapped around the northern radio lobe. This suggests that a strong jet-cloud interaction has depleted the northern lobe of molecular gas, perhaps as a result of the hot wind behind the jet-induced shock that shreds the clouds via hydrodynamic instabilities. The higher gas velocity dispersion and molecular excitation that we observed close to this location may then be the result of a milder interaction in which the expanding jet cocoon induces turbulence in the surrounding interstellar medium. These results highlight that the impact of an AGN can manifest itself not only in the kinematics of the gas, but also in molecular line ratios and in the distribution of the gas. Although the radio plasma and the cold molecular gas are clearly coupled, the kinetic energy that is transferred to the ISM is only a small fraction of the energy available from the AGN.

Using the Karl G. Jansky Very Large Array (VLA), we have detected absorption lines due to carbon-monoxide, CO(J=0-1), and the cyano radical, CN(N=0-1), associated with radio galaxy B2 0902+34 at redshift z=3.4. The detection of millimeter-band absorption observed 1.5 Gyr after the Big Bang facilitates studying molecular clouds down to gas masses inaccessible to emission-line observations. The CO absorption in B2 0902+34 has a peak optical depth of τ ≥ 8.6 redshift as previously detected 21-cm absorption of neutral hydrogen (HI) gas. Each CO component traces an integrated H_2 column density of N(H2) ≥ 3x10^20 cm^-2. CN absorption is detected for both CO components, as well as for a blueshifted component not detected in CO, with CO/CN line ratios ranging from ≤0.4 to 2.4. We discuss the scenario that the absorption components originate from collections of small and dense molecular clouds that are embedded in a region with more diffuse gas and high turbulence, possibly within the influence of the central Active Galactic Nucleus or starburst region. The degree of reddening in B2 0902+34, with a rest-frame color B-K   4.2, is lower than the very red colors (B-K > 6) found among other known redshifted CO absorption systems at z<1. Nevertheless, when including also the many non-detections from the literature, a potential correlation between the absorption-line strength and B-K color is evident, giving weight to the argument that the red colors of CO absorbers are due to a high dust content.

ABSTRACT We construct the Schwarzschild dynamical models for 11 early-type galaxies with the SAURON and Mitchell stellar IFUs out to 2–4Re, and construct dynamical models with combined stellar and H i kinematics for a subsample of four galaxies with H i velocity fields out to 10Re obtained from the Westerbork Synthesis Radio Telescope, thus robustly obtaining the dark matter content out to large radii for these galaxies. Adopting a generalized-NFW dark matter profile, we measure an NFW-like density cusp in the dark matter inner slopes for all sample galaxies, with a mean value of 1.00 ± 0.04 (rms scatter 0.15). The mean dark matter fraction for the sample is 0.2 within 1Re, and increases to 0.4 at 2Re, and 0.6 at 5Re. The dark matter fractions within 1Re of these galaxies are systematically lower than the predictions of both the TNG-100 and EAGLE simulations. For the dark matter fractions within 2Re and 5Re, 40 and 70 per cent galaxies are 1σ consistent with either the TNG-100 or the EAGLE predictions, while the remaining 60 and 30 per cent galaxies lie below the 1σ region. Combined with 36 galaxies with dark matter fractions measured out to 5Re in the literature, about 10 per cent of these 47 galaxies lie below the 3σ region of the TNG-100 or EAGLE predictions.

Gas accretion by a galaxy’s central supermassive black hole (SMBH) and the resultant energetic feedback by the accreting active galactic nucleus (AGN) on the gas in and around a galaxy are two tightly intertwined but competing processes that play a crucial role in the evolution of galaxies. Observations of galaxy clusters have shown how the plasma jets emitted by an AGN heat the intracluster medium, preventing cooling of the cluster gas and thereby the infall of this gas onto the central galaxy. On the other hand, outflows of multiphase gas, driven by the jets, can cool as they rise into the intracluster medium, leading to filaments of colder gas. The fate of this cold gas is unclear, but it has been suggested that it plays a role in feeding the central SMBH. We present the results of reprocessed CO(2-1) Atacama Large Millimeter/submillimeter Array observations of the cold molecular gas in the central regions of NGC 1275, the central galaxy of the Perseus cluster and which hosts the radio-loud AGN 3C 84 (Perseus A). These data show in detail how kiloparsec-sized cold gas filaments resulting from the jet-induced cooling of cluster gas are flowing towards the galaxy centre and how they feed the circum-nuclear accretion disk (100 pc diameter) of the SMBH. Thus, cooled gas can, in this way, play a role in feeding the AGN. These results complete our view of the feedback loop of how an AGN can impact its surroundings and how the effects of this impact maintain the AGN activity. ALMA observations show the streams of molecular gas blown from the centre of a galaxy by the energy released by an active supermassive black hole are falling back onto the black hole, making sure it stays active.

ABSTRACT The ultraluminous infrared galaxy F13451+1232 is an excellent example of a galaxy merger in the early stages of active galactic nucleus (AGN) activity, a phase in which AGN-driven outflows are expected to be particularly important. However, previous observations have determined that the mass outflow rates of the warm ionized and neutral gas phases in F13451+1232 are relatively modest, and there has been no robust detection of molecular outflows. Using high-spatial resolution Atacama Large Millimeter/submillimeter Array CO(1–0) observations, we detect a kiloparsec-scale circumnuclear disc, as well as extended (r ∼ 440 pc), intermediate-velocity (300 < |v| < 400 km s−1) cold molecular gas emission that cannot be explained by rotational disc motions. If interpreted as AGN-driven outflows, the mass outflow rates associated with this intermediate-velocity gas are relatively modest ($\dot{M}_\mathrm{out}=22$–27 M⊙ yr−1); however, we also detect a compact (rout < 120 pc), high-velocity (400 < v < 680 km s−1) cold molecular outflow near the primary nucleus of F13451+1232, which carries an order of magnitude more mass ($\dot{M}_\mathrm{out}$ ∼ 230 M⊙ yr−1) than (and several times the kinetic power of) the previously detected warmer phases. Moreover, the similar spatial scales of this compact outflow and the radio structure indicate that it is likely accelerated by the small-scale (r ∼ 130 pc) AGN jet in the primary nucleus of F13451+1232. Considering the compactness of the nuclear outflow and intermediate-velocity non-rotating gas that we detect, we argue that high-spatial resolution observations are necessary to properly quantify the properties of AGN-driven outflows and their impacts on host galaxies.

We construct the Schwarzschild dynamical models for 11 early-type galaxies with the SAURON and Mitchell stellar IFUs out to 2-4 R_e, and construct dynamical models with combined stellar and HI kinematics for a subsample of 4 galaxies with HI velocity fields out to 10 R_e obtained from the Westerbork Synthesis Radio Telescope, thus robustly obtaining the dark matter content out to large radii for these galaxies. Adopting a generalised-NFW dark matter profile, we measure an NFW-like density cusp in the dark matter inner slopes for all sample galaxies, with a mean value of 1.00±0.04 (rms scatter 0.15). The mean dark matter fraction for the sample is 0.2 within 1 R_e, and increases to 0.4 at 2 R_e, and 0.6 at 5 R_e. The dark matter fractions within 1 R_e of these galaxies are systematically lower than the predictions of both the TNG-100 and EAGLE simulations. For the dark matter fractions within 2 R_e and 5 R_e, 40 are 1-σ consistent with either the TNG-100 or the EAGLE predictions, while the remaining 60 Combined with 36 galaxies with dark matter fractions measured out to 5 R_e in the literature, about 10 3-σ region of the TNG-100 or EAGLE predictions.

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.

We present deep kiloparsec- and parsec-scale neutral atomic hydrogen (H I) absorption observations of a very young radio source (<= 5000 years), 4C 31.04, using the Westerbork Synthesis Radio Telescope (WSRT) and the Global Very Long Baseline Interferometry (VLBI) array. Using z = 0.0598, derived from molecular gas observations, we detect, at both kpc and pc scales, a broad absorption feature (FWZI = 360 km s(-1)) centred at the systemic velocity, and narrow absorption (FWZI = 6.6 km s(-1)) redshifted by 220 km s(-1), both previously observed. Additionally, we detect a new blueshifted, broad, shallow absorption wing. At pc scales, the broad absorption at the systemic velocity is detected across the entire radio source while the shallow wing is only seen against part of the eastern lobe. The gas has higher H I column density along the eastern lobe than along the western one. The velocity dispersion of the gas is high (>= 40 km s(-1)) along the entire radio continuum, and is highest (>= 60 km s(-1)) in the region including the outflow and the radio hot spot. While we detect a velocity gradient along the western lobe and parts of the eastern lobe at PA similar to 5 degrees -10 degrees, most of the gas along the rest of the eastern lobe exhibits no signs of rotation. Earlier optical spectroscopy suggests that the optical AGN is very weak. We therefore conclude that the radio lobes of 4C 31.04 are expanding into a circumnuclear disc, partially disrupting it and making the gas highly turbulent. The distribution of gas is predominantly smooth at the spatial resolution of similar to 4 pc studied here. However, clumps of gas are also present, particularly along the eastern lobe. This lobe appears to be strongly interacting with the clouds and driving an outflow similar to 35 pc from the radio core, with a mass-outflow rate of 0.3 <= M(over dot) <= 1.4 M-circle dot year(-1). It is likely that this interaction has caused the eastern lobe to be rebrightened, giving the source an asymmetric morphology. We compare our observations with the predictions of a recent analytical model regarding the survival of atomic gas clouds in radio-jet-driven outflows and find that the existence of a subkpc-scale outflow in this case could imply inefficient mixing of the cold gas with the hot medium and high gas density, leading to very short cooling times. Overall, our study provides further evidence of the strong impact of radio jets on the cold interstellar medium (ISM) in the early stages of their evolution and supports the predictions of numerical simulations regarding jet-ISM interactions and the nature of the circumnuclear gas into which the jets expand.

The jets of radio AGN provide one of the most important forms of active galactic nuclei (AGN) feedback, yet considerable uncertainties remain about how they are triggered. Since the molecular gas reservoirs of the host galaxies can supply key information about the dominant triggering mechanism(s), here we present Atacama Large Millimeter/sub-millimeter Array CO(1-0) observations of a complete sample of 29 powerful radio AGN (P-1.4GHz>10(25) W Hz(-1) and 0.05 < z < 0.3) with an angular resolution of about 2-3 arcsec (corresponding to 2-8 kpc). We detect molecular gas with masses in the range 10(8.9)<M-H2<10(10.2) M circle dot in the early-type host galaxies of ten targets, while for the other 19 sources, we derive upper limits. The detection rate of objects with such large molecular masses - 34 +/- 9 per cent - is higher than in the general population of non-active early-type galaxies (ETGs: <10 per cent). The kinematics of the molecular gas are dominated in most cases by rotating disc-like structures, with diameters up to 25 kpc. Compared with the results for samples of quiescent ETG in the literature, we find a larger fraction of more massive, more extended and less settled molecular gas structures. In most of the CO-detected sources, the results are consistent with triggering of the AGN as the gas settles following a merger or close encounter with a gas-rich companion. However, in a minority of objects at the centres of rich clusters of galaxies, the accretion of gas cooling from the hot X-ray haloes is a plausible alternative to galaxy interactions as a triggering mechanism.

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 H I 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 (H I) gas mass of M-star = 2.23 x 10(6) M-circle dot and M-H I = 1.68 x 10(6) M-circle dot, respectively. This low surface brightness galaxy is the lowest-mass H I detection found in a group beyond the local Universe (D greater than or similar to 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 (similar to 10(18 - 19) cm(-2)) H I 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 a projected distance of 390 kpc 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 H I disk and the enhancement of star formation (SF), highlighting the short-lived phase (a few hundreds million years) of the gaseous disk, before either SF or hydrodynamical forces will deplete the gas of the galaxy.

We present the discovery of a very extended (550 kpc) and low -surface -brightness (3.3 mu Jy arcsec(-2) at 144 MHz) radio emission region in Abell 1318. These properties are consistent with its characterisation as an active galactic nucleus (AGN) remnant radio plasma, based on its morphology and radio spectral properties. We performed a broad -band (54-1400 MHz) radio spectral index and curvature analysis using LOFAR, uGMRT, and WSRT-APERTIF data. We also derived the radiative age of the detected emission, estimating a maximum age of 250 Myr. The morphology of the source is remarkably intriguing, with two larger, oval -shaped components and a thinner, elongated, and filamentary structure in between, plausibly reminiscent of two aged lobes and a jet. Based on archival Swift as well as SDSS data we performed an X-ray and optical characterisation of the system, whose virial mass was estimated to be similar to 7.4x10(13) M-circle dot. This places A1318 in the galaxy group regime. Interestingly, the radio source does not have a clear optical counterpart embedded in it, thus, we propose that it is most likely an unusual AGN remnant of previous episode(s) of activity of the AGN hosted by the brightest group galaxy (similar to 2.6 x 10(12) M-circle dot), which is located at a projected distance of similar to 170 kpc in the current epoch. This relatively high offset may be a result of IGrM sloshing sourced by a minor merger. The filamentary morphology of the source may suggest that the remnant plasma has been perturbed by the system dynamics, however, only future deeper X-ray observations will be able to address this question.

We present MeerKAT Fornax Survey H I observations of NGC 1427A, a blue irregular galaxy with a stellar mass of similar to 2 x 10(9 )M(circle dot) located near the centre of the Fornax galaxy cluster. Thanks to the excellent resolution (1-6 kpc spatially, 1.4 km s(-1) in velocity) and H I column density sensitivity (similar to 4 x 10(19) to similar to 10(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 H I 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 H I (4 x 10(8) M-circle dot) and to be 3 times longer (70 kpc) than in previous observations. In fact, we detect scattered H I 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 H I tail and the peculiar kinematics of H I 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 H I and stars within the disturbed stellar body; the abundant kinematically anomalous H I; and the inversion of the H I velocity gradient near the base of the H I 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 H I in the perturbed outer stellar body and in the tidal tails.

Context. Understanding the dwarf galaxy population in low density environments (in the field) is crucial for testing the current Lambda Cold Dark Matter cosmological model. The increase in diversity toward low-mass galaxies is seen as an increase in the scatter of scaling relations, such as the stellar mass-size and the baryonic Tully-Fisher relation (BTFR), and is also demonstrated by recent in-depth studies of an extreme sub-class of dwarf galaxies with low surface brightnesses but large physical sizes called ultra-diffuse galaxies (UDGs). Aims. We aim to select dwarf galaxies independent of their stellar content and to make a detailed study of their gas and stellar properties. We selected galaxies from the APERture Tile In Focus (Apertif) H I survey and applied a constraint on their i-band absolute magnitude in order to exclude high-mass systems. The sample consists of 24 galaxies, 22 of which are resolved in H I by at least three beams, and they span H I mass ranges of 8.6 less than or similar to log(M-H I/M-circle dot) less than or similar to 9.7 and a stellar mass range of 8.0 less than or similar to log(M-star/M-circle dot)less than or similar to 9.7 (with only three galaxies having log (M-star/M-circle dot) > 9). Methods. We determined the geometrical parameters of the H I and stellar disks, built kinematic models from the H I data using (3D)Barolo, and extracted surface brightness profiles in the g-, r-, and i-bands from the Pan-STARRS 1 photometric survey. We used these measurements to place our galaxies on the stellar mass-size relation and the BTFR, and we compared them with other samples from the literature. Results. We find that at a fixed stellar mass, our H I-selected dwarfs have larger optical effective radii than isolated optically selected dwarfs from the literature, and we found misalignments between the optical and H I morphologies for some of our sample. For most of our galaxies, we used the H I morphology to determine their kinematics, and we stress that deep optical observations are needed to trace the underlying stellar disks. Standard dwarfs in our sample follow the same BTFR of high-mass galaxies, whereas UDGs are slightly offset toward lower rotational velocities, in qualitative agreement with results from previous studies. Finally, our sample features a fraction (25%) of dwarf galaxies in pairs that is significantly larger with respect to previous estimates based on optical spectroscopic data.