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We present and discuss the results of the Giant Metrewave Radio Telescope H  i 21-cm line mapping for five isolated low-mass (M |$_{\rm bary}$| |$\sim$| (2–8) |$\times 10^7$|  M |$\odot$|⁠) eXtremely Metal Poor dwarfs [12+ |$\log$| (O/H) = 7.13–7.28], selected from the nearby void galaxy sample. All the studied void dwarfs show disturbed morphology in the H  i maps with the angular resolutions of |$\sim 11$| to |$\sim 40\,{\rm arcsec}$|⁠. We examine the H  i morphology and velocity field and the relative orientation of their stellar and gas body spins. We discuss the overall non-equilibrium state of their gas and the possible origin and evolution of the studied void dwarfs. The most straightforward interpretation of the ubiquitous phenomenon of the gas component non-equilibrium state in these and similar void dwarfs is the cold accretion from the void filaments and/or minor mergers. The cold gas accretion in voids could be linked to the presence of small filaments that constitute the substructure of voids. [ABSTRACT FROM AUTHOR]

The Local Void is one of the nearest large voids, located at a distance of 23 Mpc. It lies largely behind the Galactic Bulge and is therefore extremely difficult to observe. We use H  i 21 cm emission observations from the SARAO MeerKAT Galactic Plane Survey (SMGPS) to study the Local Void and its surroundings over the Galactic longitude range 329° < ℓ < 55°, Galactic latitude | b | < 1.5°, and redshift cz < 7500 |$\rm km \, s^{-1}$|⁠. We have detected 291 galaxies to median rms sensitivity of 0.44 mJy per beam per 44 |$\rm km \, s^{-1}$| channel. We find 17 galaxies deep inside the Void, 96 at the border of the Void, while the remaining 178 galaxies are in average density environments. The extent of the Void is ∼58 Mpc. It is severely underdense for the longitude range 350° < ℓ < 35° up to redshift z < 4500 |$\rm km \, s^{-1}$|⁠. The galaxies in the Void tend to have H  i masses that are lower (by approximately 0.25 dex) than their average density counterparts. We find several potential candidates for small groups of galaxies, of which two groups (with 3 members and 5 members) in the Void show signs of filamentary substructure within the Void. [ABSTRACT FROM AUTHOR]

One of the major science goals of square kilometre array (SKA) is to understand the role played by atomic hydrogen (Hi) gas in the evolution of galaxies throughout cosmic time. The hyperfine transition line of the hydrogen atom at 21-cm is one of the best tools to detect and study the properties of Hi gas associated with galaxies. In this paper, we review our current understanding of Hi gas and its relationship with galaxies through observations of the 21-cm line both in emission and absorption. In addition, we provide an overview of the Hi science that will be possible with SKA and its precursors and pathfinders, i.e., Hi 21-cm emission and absorption studies of galaxies from nearby to high redshifts that will trace various processes governing galaxy evolution. [ABSTRACT FROM AUTHOR]

We study the relationship between the H  i specific angular momentum (⁠|$\rm j_{g}$|⁠) and the H  i mass (⁠|$\rm M_{g}$|⁠) for a sample of galaxies with well-measured H  i rotation curves. We find that the relation is well described by an unbroken power law |$\rm j_{g}$| ∝ |$\rm M_{g}$| α over the entire mass range (107–1010.5 M⊙), with α = 0.89 ± 0.05 (scatter 0.18 dex). This is in reasonable agreement with models which assume that evolutionary processes maintain H  i discs in a marginally stable state. The slope we observe is also significantly different from both the j  ∝ M 2/3 relation expected for dark matter haloes from tidal torquing models and the observed slope of the specific angular momentum–mass relation for the stellar component of disc galaxies. Our sample includes two H  i -bearing ultra-diffuse galaxies, and we find that their angular momentum follows the same relation as other galaxies. The only discrepant galaxies in our sample are early-type galaxies with large rotating H  i discs, which are found to have significantly higher angular momentum than expected from the power-law relation. The H  i discs of all these early-type galaxies are misaligned or counter-rotating with respect to the stellar discs, consistent with the gas being recently accreted. We speculate that late-stage wet mergers, as well as cold flows play a dominant role in determining the kinematics of the baryonic component of galaxies as suggested by recent numerical simulations. [ABSTRACT FROM AUTHOR]

Here we report the discovery with the Giant Metrewave Radio Telescope of an extremely large (∼115 kpc in diameter) H  i ring, located around a massive quenched galaxy, AGC 203001, but off-centred with respect to it. This ring does not have any bright extended optical counterpart unlike several other known ring galaxies. Our deep g -, r- , and i -band optical imaging of the H  i ring, using the MegaCam instrument on the Canada–France–Hawaii Telescope, shows however several regions with faint optical emission at a surface brightness level of ∼28 mag arcsec−2. Such an extended H  i structure is very rare with only one other case known so far – the Leo ring. Conventionally, off-centred rings have been explained by a collision with an 'intruder' galaxy leading to expanding density waves of gas and stars in the form of a ring. However, in such a scenario the impact also leads to large amounts of star formation in the ring that is not observed in the ring presented in this paper. We discuss possible scenarios for the formation of such H  i -dominated rings. [ABSTRACT FROM AUTHOR]

We construct mass models of eight gas rich dwarf galaxies that lie in the Lynx–Cancer void. From NFW fits to the dark matter halo profile, we find that the concentration parameters of haloes of void dwarf galaxies are similar to those of dwarf galaxies in normal density regions. We also measure the slope of the central dark matter density profiles, obtained by converting the rotation curves derived using 3D (fat) and 2D (ROTCUR) tilted ring fitting routines, into mass densities. We find that the average slope (α  = −1.39 ± 0.19), obtained from 3D fitting is consistent with that expected from an NFW profile. On the other hand, the average slope measured using the 2D approach is closer to what would be expected for an isothermal profile. This suggests that systematic effects in velocity field analysis have a significant effect on the slope of the central dark matter density profiles. Given the modest number of galaxies we use for our analysis, it is important to check these results using a larger sample. [ABSTRACT FROM AUTHOR]

We present high resolution H  i 21 cm Giant Meterwave Radio Telescope observations of the superthin galaxy FGC 1540 with a spatial resolution of 10 arcsec × 8 arcsec and a spectral resolution of 1.73 km s−1 and an rms noise of 0.9 mJy per beam. We obtain its rotation curve as well as deprojected radial H  i surface density profile by fitting a 3D tilted-ring model directly to the H  i data cubes using the publicly available software, Fully Automated tirrific. We also present the rotation curve of FGC 1540 derived from its optical spectroscopy study using the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We use the rotation curve, the H  i surface density profile together with Spitzer 3.6  $$\mu$$ m, and the SDSS i -band data to construct the mass models for FGC 1540. We find that both the Pseudo-isothermal () as well as Navarro–Frenk–White dark matter (DM) haloes, fit the observed rotation curve equally well. The PIS model indicates a compact DM halo (R C/ R D < 2), with the best-fitting core radius (R C) approximately half the exponential stellar disc scale length (R D), which is in agreement with the mass models of superthin galaxies studied earlier in the literature. Since the vertical thickness of the galactic stellar disc is determined by a balance between the net gravitational field and the velocity dispersion in the vertical direction, the compact DM halo may be primarily responsible in regulating the superthin vertical structure of the stellar disc in FGC 1540 as was found in case of the superthin galaxy UGC7321. [ABSTRACT FROM AUTHOR]

We conducted a survey of seven magnetic O-type stars and eleven B-type stars with masses above 8M⊙ using the Very Large Array in the 1, 3 and 13 cm bands. The survey resulted in a detection of two O- and two B-type stars. While the detected O-type stars - HD 37742 and HD 47129 - are in binary systems, the detected B-type stars, HD 156424 and ALS 9522, are not known to be in binaries. All four stars were detected at 3 cm, whereas three were detected at 1 cm and only one star was detected at 13 cm. The detected B-type stars are significantly more radio luminous than the non-detected ones, which is not the case for O-type stars. The non-detections at 13 cm are interpreted as due to thermal free-free absorption. Mass-loss rates were estimated using 3 cm flux densities and were compared with theoretical mass-loss rates, which assume free-free emission. For HD 37742, the two values of the massloss rates were in good agreement, possibly suggesting that the radio emission for this star is mainly thermal. For the other three stars, the estimated mass-loss rates from radio observations were much higher than those expected from theory, suggesting either a possible contribution from non-thermal emission from the magnetic star or thermal or non-thermal emission due to interacting winds of the binary system, especially for HD 47129. All the detected stars are predicted to host centrifugal magnetospheres except HD 37742, which is likely to host a dynamical magnetosphere. This suggests that non-thermal radio emission is favoured in stars with centrifugal magnetospheres. [ABSTRACT FROM AUTHOR]

We conducted a search for |$\textrm {H}\, \scriptstyle \mathrm{I}$| emission of the gas-rich galaxies in the Vela region (260° ≤ ℓ ≤ 290°, −2° ≤ b ≤ 1°) to explore the Vela Supercluster (VSCL) at V hel ∼ 18 000  |$\rm km\, s^{-1}$|⁠ , largely obscured by Galactic dust. Within the mostly Radio Frequency Interference-free band (250 < V hel < 25 000  |$\rm km\, s^{-1}$|⁠) of MeerKAT, the analysis focuses on 157 hexagonally distributed pointings extracted from the South African Radio Astronomy Observatory MeerKAT Galactic Plane Survey located in the Vela region (Vela−SMGPS). These were combined into 10 contiguous mosaics, covering an ∼90 square degrees area. Among the 843 |$\textrm {H}\, \scriptstyle \mathrm{I}$| detected sources, 39 were previously discovered in the H  i Parkes Zone of Avoidance survey (V hel < 12 000  |$\rm km\, s^{-1}$|⁠ ; rms ∼ 6  |$\rm mJy\, beam^{-1}$|⁠). With the improved rms level of the Vela−SMGPS, i.e. 0.29–0.56  |$\rm mJy\, beam^{-1}$|⁠ , our study unveils nearly 12 times more detections (471 candidates) in that same velocity range. We furthermore could identify 187 galaxy candidates with an |$\textrm {H}\, \scriptstyle \mathrm{I}$| mass limit reaching |$\log (M_{\rm HI}/\rm {\rm M}_{\odot }) = 9.44$| in the VSCL velocity range V hel ∼ 19 500 ± 3500  |$\rm km\, s^{-1}$|⁠. We find indications of two wall-like overdensities that confirm the original suspicion that these walls intersect at low latitudes around longitudes of ℓ ∼ 272°–278°. We also find a strong signature most likely associated with the Hydra/Antlia extension and evidence of a previously unknown narrow filament at V hel ∼ 12 000 |$\rm km\, s^{-1}$|⁠. This paper demonstrates the efficiency of systematic |$\textrm {H}\, \scriptstyle \mathrm{I}$| surveys with the Square Kilometre Array (SKA) precursor MeerKAT, even in the most obscured part of the Zone of Avoidance. [ABSTRACT FROM AUTHOR]

We present the first measurement of H i mass of star-forming galaxies in different large scale structure environments from a blind survey at z ∼ 0.37. In particular, we carry out a spectral line stacking analysis considering 2875 spectra of colour-selected star-forming galaxies undetected in H i at 0.23 < z < 0.49 in the COSMOS field, extracted from the MIGHTEE-H i Early Science data cubes, acquired with the MeerKAT radio telescope. We stack galaxies belonging to different subsamples depending on three different definitions of large-scale structure environment: local galaxy overdensity, position inside the host dark matter halo (central, satellite, or isolated), and cosmic web type (field, filament, or knot). We first stack the full star-forming galaxy sample and find a robust H i detection yielding an average galaxy H i mass of |$M_{\rm H \, {\small I}}=(8.12\pm 0.75)\times 10^9\, {\rm M}_\odot$| at ∼11.8σ. Next, we investigate the different subsamples finding a negligible difference in M H i as a function of the galaxy overdensity. We report an H i excess compared to the full sample in satellite galaxies (M H i  = (11.31 ± 1.22) × 109, at ∼10.2σ) and in filaments (M H i  = (11.62 ± 0.90) × 109. Conversely, we report non-detections for the central and knot galaxies subsamples, which appear to be H i -deficient. We find the same qualitative results also when stacking in units of H i fraction (f H i ). We conclude that the H i amount in star-forming galaxies at the studied redshifts correlates with the large-scale structure environment. [ABSTRACT FROM AUTHOR]

This paper presents the first H  i results extracted from the South African Radio Astronomy Observatory (SARAO) MeerKAT Galactic Plane Survey (SMGPS) – a narrow strip (Δ b ∼ 3°) along the southern Milky Way. The primary goal consisted in tracing the great attractor (GA) wall across the innermost zone of avoidance. We reduced a segment spanning the longitude range 302° ≤ ℓ ≤ 332° for the redshift range z ≤ 0.08. The superb SMGPS sensitivity (rms = 0.3–0.5 mJy beam−1 per 44 km s−1 channel) and angular resolution (∼31″ × 26″) lead to a detection limit of log(M H  i /M⊙) ≥ 8.5 at the GA distance (⁠|$\mbox{$V_{\rm {hel}}$}\, \sim 3500{\!-\!}6500$|  km s−1). A total of 477 galaxy candidates were identified over the full redshift range. A comparison of the few H  i detections with counterparts in the literature (mostly HIZOA) found the H  i fluxes and other H  i parameters to be highly consistent. The continuation of the GA wall is confirmed through a prominent overdensity of N  = 214 detections in the GA distance range. At higher latitudes, the wall moves to higher redshifts, supportive of a possible link with the Ophiuchus cluster located behind the Galactic Bulge. This deep interferometric H  i survey demonstrates the power of the SMGPS in improving our insight of large-scale structures at these extremely low latitudes, despite the high obscuration and continuum background. [ABSTRACT FROM AUTHOR]

We study the |$M_{\rm H\, {\small {I}}}-M_{\star }$| relation over the last billion years using the MIGHTEE-H  i sample. We first model the upper envelope of the |$M_{\rm H\, {\small {I}}}-M_{\star }$| relation with a Bayesian technique applied to a total number of 249 H  i -selected galaxies, without binning the data sets, while taking account of the intrinsic scatter. We fit the envelope with both linear and non-linear models, and find that the non-linear model is preferred over the linear one with a measured transition stellar mass of log10(M ⋆/M⊙) = 9.15 ± 0.87, beyond which the slope flattens. This finding supports the view that the lack of H  i gas is ultimately responsible for the decreasing star formation rate observed in the massive main-sequence galaxies. For spirals alone, which are biased towards the massive galaxies in our sample, the slope beyond the transition mass is shallower than for the full sample, indicative of distinct gas processes ongoing for the spirals/high-mass galaxies from other types with lower stellar masses. We then create mock catalogues for the MIGHTEE-H  i detections and non-detections with two main galaxy populations of late- and early-type galaxies to measure the underlying |$M_{\rm H\, {\small {I}}}-M_{\star }$| relation. We find that the turnover in this relation persists whether considering the two galaxy populations as a whole or separately. We note that an underlying linear relation could mimic this turnover in the observed scaling relation, but a model with a turnover is strongly preferred. Measurements on the logarithmic average of H  i masses against the stellar mass are provided as a benchmark for future studies. [ABSTRACT FROM AUTHOR]

We present the first measurement of the H  i mass function (HIMF) using data from MeerKAT, based on 276 direct detections from the MeerKAT International GigaHertz Tiered Extragalactic Exploration (MIGHTEE) Survey Early Science data covering a period of approximately a billion years (0 ≤ |$z$| ≤ 0.084). This is the first HIMF measured using interferometric data over non-group or cluster field, i.e. a deep blank field. We constrain the parameters of the Schechter function that describes the HIMF with two different methods: |$1/V_{\rm max}$| and modified maximum likelihood (MML). We find a low-mass slope |$\alpha =-1.29^{+0.37}_{-0.26}$| , 'knee' mass |$\log _{10}(M_{*}/{\rm {\rm M}_{\odot }}) = 10.07^{+0.24}_{-0.24}$| and normalization |$\log _{10}(\phi _{*}/\rm Mpc^{-3})=-2.34^{+0.32}_{-0.36}$| (H 0 = 67.4 km s−1 Mpc−1) for |$1/V_{\rm max}$| , and |$\alpha =-1.44^{+0.13}_{-0.10}$| , 'knee' mass |$\log _{10}(M_{*}/{\rm {\rm M}_{\odot }}) = 10.22^{+0.10}_{-0.13}$| and normalization |$\log _{10}(\phi _{*}/\rm Mpc^{-3})=-2.52^{+0.19}_{-0.14}$| for MML. When using |$1/V_{\rm max}$| we find both the low-mass slope and 'knee' mass to be consistent within 1σ with previous studies based on single-dish surveys. The cosmological mass density of H  i is found to be slightly larger than previously reported: |$\Omega _{\rm H\,{\small I}}=5.46^{+0.94}_{-0.99} \times 10^{-4}\,h^{-1}_{67.4}$| from |$1/V_{\rm max}$| and |$\Omega _{\rm H\,{\small I}}=6.31^{+0.31}_{-0.31} \times 10^{-4}\,h^{-1}_{67.4}$| from MML but consistent within the uncertainties. We find no evidence for evolution of the HIMF over the last billion years. [ABSTRACT FROM AUTHOR]

We report results from a Giant Metrewave Radio Telescope search for 'associated' redshifted HI 21 cm absorption from 24 active galactic nuclei (AGNs), at 1.1 < z < 3.6, selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. 22 out of 23 sources with usable data showed no evidence of absorption, with typical 3σ optical depth detection limits of ≈0.01 at a velocity resolution of ≈30 km s-1. A single tentative absorption detection was obtained at z ≈ 3.530 towards TXS 0604+728. If confirmed, this would be the highest redshift at which HI 21 cm absorption has ever been detected. Including 29 CJF sources with searches for redshifted HI 21 cm absorption in the literature, mostly at z < 1, we construct a sample of 52 uniformly selected flat-spectrum sources. A Peto-Prentice two-sample test for censored data finds (at ≈3σ significance) that the strength of HI 21 cm absorption is weaker in the high-z sample than in the low-z sample; this is the first statistically significant evidence for redshift evolution in the strength of HI 21 cm absorption in a uniformly selected AGN sample. However, the two-sample test also finds that the HI 21 cm absorption strength is higher in AGNs with low ultraviolet or radio luminosities, at ≈3.4σ significance. The fact that the higher luminosity AGNs of the sample typically lie at high redshifts implies that it is currently not possible to break the degeneracy between AGN luminosity and redshift evolution as the primary cause of the low HI 21 cm opacities in high-redshift, high-luminosity AGNs. [ABSTRACT FROM AUTHOR]