Neutral Hydrogen Gas Research Articles

BreakBRD Galaxies: Evolutionary Clues through an Analysis of Gas Content

By combining newly obtained deep Green Bank Telescope 21 cm observations with optical spectroscopic data, we present an analysis of the gas content of break bulges in red disks (breakBRD) galaxies, a population denoted by their blue star-forming centers and red quenched disks that do not appear to follow the typical inside-out evolution of spiral galaxies. We confirm previous results that the neutral atomic hydrogen (H i) gas fractions of breakBRDs are, on average, lower than those of typical galaxies on the star-forming sequence (SFS), and find that their H i fractions are generally higher than green valley (GV) galaxies. H i depletion times for breakBRDs are roughly an order of magnitude lower than those of SFS galaxies, in stark contrast with GV galaxies that typically have much longer depletion times than SFS galaxies. The nuclear gas-phase metallicities of breakBRDs have a broader distribution than SFS galaxies and are skewed toward slightly above-average values. BreakBRDs are systematically offset from the baryonic Tully–Fisher relation toward lower baryonic mass at a given rotation velocity. They also have higher typical H i asymmetries than SFS galaxies, and of those galaxies with spatially resolved gas velocity fields from the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory survey, two-thirds are either highly distorted or completely misaligned relative to the stellar disk. Evidence supports a scenario where breakBRDs are in an early phase of quenching, and there is mixed evidence that their behavior is related to past merger activity.

MHONGOOSE: A MeerKAT nearby galaxy H I survey

The MHONGOOSE (MeerKAT H I Observations of Nearby Galactic Objects: Observing Southern Emitters) survey maps the distribution and kinematics of the neutral atomic hydrogen (H I) gas in and around 30 nearby star-forming spiral and dwarf galaxies to extremely low H I column densities. The H I column density sensitivity (3σ over 16 km s−1) ranges from ∼5 × 1017 cm−2 at 90″ resolution to ∼4 × 1019 cm−2 at the highest resolution of 7″. The H I mass sensitivity (3σ over 50 km s−1) is ∼5.5 × 105 M⊙ at a distance of 10 Mpc (the median distance of the sample galaxies). The velocity resolution of the data is 1.4 km s−1. 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 H I masses from 107 M⊙ to almost 1011 M⊙ in order 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 compared the integrated H I 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 H I 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 in the physical properties of the H I between areas with star formation and areas without related to the formation of a cold neutral medium. Finally, we give an overview of the H I-detected companion and satellite galaxies in the 30 fields, five of which have not previously been cataloged. We find a clear relation between the number of companion galaxies and the mass of the main target galaxy.

Possible origins of anomalous H I gas around MHONGOOSE galaxy, NGC 5068

The existing reservoirs of neutral atomic hydrogen gas (H I) in galaxies are insufficient to have maintained the observed levels of star formation without some kind of replenishment. This refuelling of the H I reservoirs is likely to occur at column densities an order of magnitude lower than previous observational limits (NH I, limit ∼ 1019 cm−2 at a 30″ resolution over a linewidth of 20 km s−1). In this paper, we present recent deep H I observations of NGC 5068, a nearby isolated star-forming galaxy observed by MeerKAT as part of the MHONGOOSE survey. With these new data, we were able to detect low column density H I around NGC 5068 with a 3σ detection limit of NH I = 6.4 × 1017 cm−2 at a 90″ resolution over a 20 km s−1 linewidth. The high sensitivity and resolution of the MeerKAT data reveal a complex morphology of the H I in this galaxy – a regularly rotating inner disk coincident with the main star-forming disk of the galaxy, a warped outer disk of low column density gas (NH I < 9 × 1019 cm−2), in addition to clumps of gas on the north-western side of the galaxy. We employed a simple two disk model that described the inner and outer disks, which enabled us to identify anomalous gas that deviates from the rotation of the main galaxy. The morphology and the kinematics of the anomalous gas suggest a possible extra-galactic origin. We explore a number of possible origin scenarios that may explain the anomalous gas, and conclude that fresh accretion is the most likely scenario.

Radiative transfer of 21-cm line through ionised cavities in an expanding universe

Abstract The optical depth parameterisation is typically used to study the 21-cm signals associated with the properties of the neutral hydrogen (H i) gas and the ionisation morphology during the Epoch of Reionisation (EoR), without solving the radiative transfer equation. To assess the uncertainties resulting from this simplification, we conduct explicit radiative transfer calculations using the cosmological 21-cm radiative transfer (C21LRT) code and examine the imprints of ionisation structures on the 21-cm spectrum. We consider a globally averaged reionisation history and implement fully ionised cavities (H ii bubbles) of diameters d ranging from 0.01 Mpc to 10 Mpc at epochs within the emission and the absorption regimes of the 21-cm global signal. The single-ray C21LRT calculations show that the shape of the imprinted spectral features are primarily determined by d and the 21-cm line profile, which is parametrised by the turbulent velocity of the H i gas. It reveals the spectral features tied to the transition from ionised to neutral regions that calculations based on the optical depth parametrisation were unable to capture. We also present analytical approximations of the calculated spectral features of the H ii bubbles. The multiple-ray calculations show that the apparent shape of a H ii bubble (of d = 5 Mpc at z = 8), because of the finite speed of light, differs depending on whether the bubble’s ionisation front is stationary or expanding. Our study shows the necessity of properly accounting for the effects of line-continuum interaction, line broadening and cosmological expansion to correctly predict the EoR 21-cm signals.

Mapper of the IGM spin temperature: instrument overview

ABSTRACT The observation of the global 21 cm signal produced by neutral hydrogen gas in the intergalactic medium (IGM) during the Dark Ages, Cosmic Dawn, and Epoch of Reionization requires measurements with extremely well-calibrated wideband radiometers. We describe the design and characterization of the Mapper of the IGM Spin Temperature (MIST), which is a new ground-based, single-antenna, global 21 cm experiment. The design of MIST was guided by the objectives of avoiding systematics from an antenna ground plane and cables around the antenna, as well as maximizing the instrument’s on-sky efficiency and portability for operations at remote sites. We have built two MIST instruments, which observe in the range 25–105 MHz. For the 21 cm signal, this frequency range approximately corresponds to redshifts 55.5 > z > 12.5, encompassing the Dark Ages and Cosmic Dawn. The MIST antenna is a horizontal blade dipole of 2.42 m in length, 60 cm in width, and 52 cm in height above the ground. This antenna operates without a metal ground plane. The instruments run on 12 V batteries and have a maximum power consumption of 17 W. The batteries and electronics are contained in a single receiver box located under the antenna. We present the characterization of the instruments using electromagnetic simulations and lab measurements. We also show sample sky measurements from recent observations at remote sites in California, Nevada, and the Canadian High Arctic. These measurements indicate that the instruments perform as expected. Detailed analyses of the sky measurements are left for future work.

Polarimetry of the Lyα envelope of the radio-quiet quasar SDSS J124020.91+145535.6

The radio quiet quasar SDSS J1240+1455 lies at a redshift of z = 3.11, is surrounded by a Lyα blob (LAB), and is absorbed by a proximate damped Lyα system. In order to better define the morphology of the blob and determine its emission mechanism, we gathered deep narrow-band images isolating the Lyα line of this object in linearly polarized light. We provide a deep intensity image of the blob, showing a filamentary structure extending up to 16″ (or 122 physical kpc) in diameter. No significant polarization signal could be extracted from the data, but 95% probability upper limits were defined through simulations. They vary between ∼3% in the central 0.75″ disk (after subtraction of the unpolarized quasar continuum) and ∼10% in the 3.8 − 5.5″ annulus. The low polarization suggests that the Lyα photons are emitted mostly in situ, by recombination and de-excitation in a gas largely ionized by the quasar ultraviolet light, rather than by a central source and scattered subsequently by neutral hydrogen gas. This blob shows no detectable polarization signal, contrary to LAB1, a brighter and more extended blob that is not related to the nearby active galactic nucleus (AGN) in any obvious way, and where a significant polarization signal of about 18% was detected.

Absorption of Millimeter-band CO and CN in the Early Universe: Molecular Clouds in the Radio Galaxy B2 0902+34 at Redshift 3.4

Using the Karl G. Jansky Very Large Array, 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% and consists of two components, one of which has the same redshift as previously detected 21 cm absorption of neutral hydrogen (H i) gas. Each CO component traces an integrated H2 column density of NH2 ≳ 3 × 1020 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 a starburst region. The degree of reddening in B2 0902+34, with 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 the many nondetections 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.

Tidally offset neutral gas in Lyman continuum emitting galaxy Haro 11

ABSTRACT Around 400 Myr after the big bang, the ultraviolet emission from star-forming galaxies reionized the Universe. Ionizing radiation (Lyman continuum, LyC) is absorbed by cold neutral hydrogen gas (H i) within galaxies, hindering the escape of LyC photons. Since the H i reservoir of LyC emitters has never been mapped, major uncertainties remain on how LyC photons escape galaxies and ionize the intergalactic medium. We have directly imaged the neutral gas in the nearby reionization-era analogue galaxy Haro 11 with the 21 cm line to identify the mechanism enabling ionizing radiation escape. We find that merger-driven interactions have caused a bulk offset of the neutral gas by about $6\,$ kpc from the centre of the galaxy, where LyC emission production sites are located. This could facilitate the escape of ionizing radiation into our line of sight. Galaxy interactions can cause both elevated LyC production and large-scale displacement of H i from the regions where these photons are produced. They could contribute to the anisotropic escape of LyC radiation from galaxies and the reionization of the Universe. We argue for a systematic assessment of the effect of environment on LyC production and escape.

Canonical Hubble-Tension-Resolving Early Dark Energy Cosmologies Are Inconsistent with the Lyman-α Forest.

Current cosmological data exhibit discordance between indirect and some direct inferences of the present-day expansion rate H_{0}. Early dark energy (EDE), which briefly increases the cosmic expansion rate prior to recombination, is a leading scenario for resolving this "Hubble tension" while preserving a good fit to cosmic microwave background (CMB) data. However, this comes at the cost of changes in parameters that affect structure formation in the late-time universe, including the spectral index of scalar perturbations n_{s}. Here, we present the first constraints on axionlike EDE using data from the Lyman-α forest, i.e., absorption lines imprinted in background quasar spectra by neutral hydrogen gas along the line of sight. We consider two independent measurements of the one-dimensional Lyα forest flux power spectrum from the Sloan Digital Sky Survey (SDSS eBOSS) and from the MIKE/HIRES and X-Shooter spectrographs. We combine these with a baseline dataset comprised of Planck CMB data and baryon acoustic oscillation (BAO) measurements. Combining the eBOSS Lyα data with the CMB and BAO dataset reduces the 95% confidence level (C.L.) upper bound on the maximum fractional contribution of EDE to the cosmic energy budget f_{EDE} from 0.07 to 0.03 and constrains H_{0}=67.9_{-0.4}^{+0.4} km/s/Mpc (68% C.L.), with maximum aposteriori value H_{0}=67.9 km/s/Mpc. Similar results are obtained for the MIKE/HIRES and X-Shooter Lyα data. Our Lyα-based EDE constraints yield H_{0} values that are in >4σ tension with the SH0ES distance-ladder measurement and are driven by the preference of the Lyα forest data for n_{s} values lower than those required by EDE cosmologies that fit Planck CMB data. Taken at face value, the Lyα forest severely constrains canonical EDE models that could resolve the Hubble tension.

Exploring the Structures and Substructures of the Andromeda Satellite Dwarf Galaxies Cassiopeia III, Perseus I, and Lacerta I

We present results from wide-field imaging of the resolved stellar populations of the dwarf spheroidal galaxies Cassiopeia III (And XXXII) and Perseus I (And XXXIII), two satellites in the outer stellar halo of the Andromeda galaxy (M31). Our WIYN pODI photometry traces the red giant star population in each galaxy to ∼2.5−3 half-light radii from the galaxy center. We use the tip of the red giant branch (TRGB) method to derive distances of (m − M)0 = 24.62 ± 0.12 mag (839 kpc, or kpc from M31) for Cas III and 24.47 ± 0.13 mag (738 kpc, or 351 kpc from M31) for Per I. These values are consistent within the errors with TRGB distances derived from a deeper Hubble Space Telescope study of the galaxies’ inner regions. For each galaxy, we derive structural parameters, total magnitude, and central surface brightness. We also place upper limits on the ratio of neutral hydrogen gas mass to optical luminosity, confirming the gas-poor nature of both galaxies. We combine our data set with corresponding data for the M31 satellite galaxy Lacerta I (And XXXI) from earlier work and search for substructure within the RGB star populations of Cas III, Per I, and Lac I. We find an overdense region on the west side of Lac I at a significance level of 2.5σ–3σ and a low-significance filament extending in the direction of M31. In Cas III, we identify two modestly significant overdensities near the center of the galaxy and another at two half-light radii. Per I shows no evidence for substructure in its RGB star population, which may reflect this galaxy’s isolated nature.

WALLABY pre-pilot survey: ultra-diffuse galaxies in the Eridanus supergroup

ABSTRACT We present a pilot study of the atomic neutral hydrogen gas (H i) content of ultra-diffuse galaxy (UDG) candidates. In this paper, we use the pre-pilot Eridanus field data from the Widefield ASKAP L-band Legacy All-sky Blind Survey to search for H i in UDG candidates found in the Systematically Measuring Ultra-diffuse Galaxies survey (SMUDGes). We narrow down to 78 SMUDGes UDG candidates within the maximum radial extents of the Eridanus subgroups for this study. Most SMUDGes UDGs candidates in this study have effective radii smaller than 1.5 kpc and thus fail to meet the defining size threshold. We only find one H i detection, which we classify as a low-surface-brightness dwarf. Six putative UDGs are H i-free. We show the overall distribution of SMUDGes UDG candidates on the size–luminosity relation and compare them with low-mass dwarfs on the atomic gas fraction versus stellar mass scaling relation. There is no correlation between gas-richness and colour indicating that colour is not the sole parameter determining their H i content. The evolutionary paths that drive galaxy morphological changes and UDG formation channels are likely the additional factors to affect the H i content of putative UDGs. The actual numbers of UDGs for the Eridanus and NGC 1332 subgroups are consistent with the predicted abundance of UDGs and the halo virial mass relation, except for the NGC 1407 subgroup, which has a smaller number of UDGs than the predicted number. Different group environments suggest that these putative UDGs are likely formed via the satellite accretion scenario.

Asymmetries in random motions of neutral hydrogen gas in spiral galaxies

Context. The velocity dispersion ellipsoid of gas in galactic discs is usually assumed to be isotropic. Under this approximation, no projection effect occurs in the random motions of gas, as traced by the line-of-sight velocity dispersion. However, it has been recently shown that random motions of the neutral hydrogen gas of the Triangulum galaxy (M 33) exhibit a bisymmetric perturbation which is aligned with the minor axis of the galaxy, suggesting a projection effect. Aims. To investigate if perturbations in the velocity dispersion of nearby discs are comparable to those of M 33, the sample is extended to 32 galaxies from The H I Nearby Galaxy Survey (THINGS) and the Westerbork H I Survey of Spiral and Irregular Galaxies (WHISP). Methods. We studied velocity asymmetries in the disc planes by performing Fourier transforms of high-resolution H I velocity dispersion maps corrected for beam-smearing effects, and we measured the amplitudes and phase angles of the Fourier harmonics. Results. In all velocity dispersion maps, we find strong perturbations of first, second, and fourth orders. The strongest asymmetry is the bisymmetry, which is predominantly associated with the presence of spiral arms. The first order asymmetry is generally orientated close to the disc major axis, and the second and fourth order asymmetries are preferentially orientated along intermediate directions between the major and minor axes of the discs. These results are evidence that strong projection effects shape the H I velocity dispersion maps. The most likely source of systematic orientations is the anisotropy of velocities, through the projection of streaming motions that are stronger along one of the planar directions in the discs. Moreover, systematic phase angles of asymmetries in the H I velocity dispersion could arise from tilted velocity ellipsoids, that is when the velocities are correlated. We expect a larger incidence of correlation between the radial and tangential velocities of H I gas with |ρRθ|∼0.6, which could be tested against the kinematics of the youngest stellar populations of the Milky Way. Conclusions. H I velocity dispersions cannot be considered devoid of projection effects. The systematic orientations of asymmetries can be explained by the projection of unresolved streaming motions mainly arising from spiral arms. Our methodology is a powerful tool to constrain the dominant direction of streaming motions and thus the shape of the velocity ellipsoid of H I gas, which is de facto anisotropic at the angular scales probed by the observations. The next step is to study the shape of the velocity ellipsoids of molecular and ionised gas and their link with galaxy mass and/or morphology, in addition to extending the sample size.

The AGN fuelling/feedback cycle in nearby radio galaxies

We present Australia Compact Telescope Array (ATCA) 21 cm observations of the nearby low-excitation radio galaxy (LERG) NGC 3100. This is the brightest galaxy of a loose group and it hosts a young (∼2 Myr) radio source. The ATCA observations reveal for the first time the presence of neutral hydrogen (H I) gas in absorption in the centre of this radio galaxy, and in emission in two low-mass galaxies of the group and in a diffuse dark cloud in the proximity of NGC 3100. The sensitivity to low-column density gas (NHI ∼ 1019 cm−2) allows us to reveal asymmetries in the periphery of most of the H I-detected galaxies, suggesting that tidal interactions may be ongoing. The diffuse cloud does not show a stellar counterpart down to 27 mag arcsec−2 and could be the remnant of these interactions. The analysis of the H I absorption complex in NGC 3100 indicates that the atomic phase of the hydrogen is distributed in the same way as its molecular phase (observed at arcsecond resolution through several carbon monoxide emission lines). We suggest that the interactions occurring within the group are causing turbulent cold gas clouds in the intra-group medium to be slowly accreted towards the centre of NGC 3100. This caused the recent formation of the cold circum-nuclear disc, which is likely sustaining the young nuclear activity.

The MeerKAT Fornax Survey

The MeerKAT Fornax Survey maps the distribution and kinematics of atomic neutral hydrogen gas (H I) in the nearby Fornax galaxy cluster using the MeerKAT telescope. The 12 deg2 survey footprint covers the central region of the cluster out to ∼Rvir and stretches south-west out to ∼2Rvir to include the NGC 1316 galaxy group. The H I column density sensitivity (3σ over 25 km s−1) ranges from 5 × 1019 cm−2 at a resolution of ∼10″ (∼1 kpc at the 20 Mpc distance of Fornax) down to ∼1018 cm−2 at ∼1′ (∼6 kpc), and slightly below this level at the lowest resolution of ∼100″ (∼10 kpc). The H I mass sensitivity (3σ over 50 km s−1) is 6 × 105 M⊙. The H I velocity resolution is 1.4 km s−1. In this paper, we describe the survey design and H I data processing, and we present a sample of six galaxies with long, one-sided, starless H I tails (only one of which was previously known) radially oriented within the cluster and with measurable internal velocity gradients. We argue that the joint properties of the H I tails represent the first unambiguous evidence of ram pressure shaping the distribution of H I in the Fornax cluster. The disturbed optical morphology of all host galaxies supports the idea that the tails consist of H I that was initially pulled out of the galaxies’ stellar body by tidal forces. Ram pressure was then able to further displace the weakly bound H I and give the tails their current direction, length, and velocity gradient.

Influence of turbulence on Ly α scattering

ABSTRACT We develop a Monte Carlo radiative transfer code to study the effect of turbulence with a finite correlation length on scattering of Ly α photons propagating through neutral atomic hydrogen gas. We investigate how the effective mean free path, the emergent spectrum, and the average number of scatterings that Ly α photons experience change in the presence of turbulence. We find that the correlation length is an important and sensitive parameter that has an influence on physically relevant properties of Ly α radiative transfer. In particular, it can significantly, by orders of magnitude, reduce the number of scattering events that the average Ly α photon undergoes before it escapes the turbulent cloud.

A cosmic stream of atomic carbon gas connected to a massive radio galaxy at redshift 3.8.

The growth of galaxies in the early Universe is driven by accretion of circum- and intergalactic gas. Simulations have predicted that steady streams of cold gas penetrate the dark matter halos of galaxies and provide the raw material necessary to sustain star formation. We report a filamentary stream of gas that extends for 100 kiloparsecs and connects to the massive radio galaxy 4C 41.17. We detected the stream using submillimeter observations of the 3P1 to 3P0 emission from the [C i] line of atomic carbon, a tracer of neutral atomic or molecular hydrogen gas. The galaxy contains a central gas reservoir that is fueling a vigorous starburst. Our results show that the raw material for star formation can be present in cosmic streams outside galaxies.

The thesan project: Lyman-α emitter luminosity function calibration

ABSTRACT The observability of Lyα emitting galaxies (LAEs) during the Epoch of Reionization can provide a sensitive probe of the evolving neutral hydrogen gas distribution, thus setting valuable constraints to distinguish different reionization models. In this study, we utilize the new thesan suite of large-volume ($L_\text{box} = 95.5\, \text{cMpc}$) cosmological radiation-hydrodynamic simulations to directly model the Lyα emission from individual galaxies and the subsequent transmission through the intergalactic medium. thesan combines the arepo-rt radiation-hydrodynamic solver with the IllustrisTNG galaxy formation model and includes high- and medium-resolution simulations designed to investigate the impacts of halo-mass-dependent escape fractions, alternative dark matter models, and numerical convergence. We find important differences in the Lyα transmission based on reionization history, bubble morphology, frequency offset from line centre, and galaxy brightness. For a given global neutral fraction, Lyα transmission reduces when low-mass haloes dominate reionization over high-mass haloes. Furthermore, the variation across sightlines for a single galaxy is greater than the variation across all galaxies. This collectively affects the visibility of LAEs, directly impacting observed Lyα luminosity functions (LFs). We employ Gaussian Process Regression using SWIFTEmulator to rapidly constrain an empirical model for dust escape fractions and emergent spectral-line profiles to match observed LFs. We find that dust strongly impacts the Lyα transmission and covering fractions of MUV ≲ −19 galaxies in $M_\text{vir} \gtrsim 10^{11}\, \text{M}_{\bigodot }$ haloes, such that the dominant mode of removing Lyα photons in non-LAEs changes from low-IGM transmission to high dust absorption around z ∼ 7.

Molecular clouds at the eastern edge of radio nebula W 50

Abstract Microquasar SS 433 located at the geometric center of radio nebula W 50 is a suitable source for investigating the physical process of how galactic jets affect the surrounding interstellar medium (ISM). Previous studies have searched for evidence of the interaction between the SS 433 jet and ISM, such as neutral hydrogen gas and molecular clouds; however, it is still unclear which ISM interacts with the jet. We looked for new molecular clouds that possibly interact at the terminal of the SS 433 eastern jet using the Nobeyama 45 m telescope and the Atacama Submillimeter Telescope Experiment (ASTE). We identified two molecular clouds, comprising many small clumps, in the velocity range of 30.1–36.5 km s−1 for the first time. These clouds have complex velocity structures, and one of them has a density gradient toward SS 433. Although it is difficult to conclude the relation between the molecular clouds and the SS 433/W 50 system, there is a possibility that the eastern structure of W 50 constructed by the SS 433 jet swept up tiny molecular clumps drifting in the surroundings and formed the molecular clouds that we identified in this study.

WALLABY pilot survey: The diversity of HI structural parameters in nearby galaxies

Abstract We investigate the diversity in the sizes and average surface densities of the neutral atomic hydrogen (H i) gas discs in $\sim$ 280 nearby galaxies detected by the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY). We combine the uniformly observed, interferometric H i data from pilot observations of the Hydra cluster and NGC 4636 group fields with photometry measured from ultraviolet, optical, and near-infrared imaging surveys to investigate the interplay between stellar structure, star formation, and H i structural parameters. We quantify the H i structure by the size of the H i relative to the optical disc and the average H i surface density measured using effective and isodensity radii. For galaxies resolved by $&gt;$ $1.3$ beams, we find that galaxies with higher stellar masses and stellar surface densities tend to have less extended H i discs and lower H i surface densities: the isodensity H i structural parameters show a weak negative dependence on stellar mass and stellar mass surface density. These trends strengthen when we limit our sample to galaxies resolved by $&gt;$ 2 beams. We find that galaxies with higher H i surface densities and more extended H i discs tend to be more star forming: the isodensity H i structural parameters have stronger correlations with star formation. Normalising the H i disc size by the optical effective radius (instead of the isophotal radius) produces positive correlations with stellar masses and stellar surface densities and removes the correlations with star formation. This is due to the effective and isodensity H i radii increasing with mass at similar rates while, in the optical, the effective radius increases slower than the isophotal radius. Our results are in qualitative agreement with previous studies and demonstrate that with WALLABY we can begin to bridge the gap between small galaxy samples with high spatial resolution H i data and large, statistical studies using spatially unresolved, single-dish data.

SAMI-H i: the connection between global asymmetry in the ionized and neutral atomic hydrogen gas in galaxies

ABSTRACT Observations of the neutral atomic hydrogen (${\rm H\, {\small I}}$) gas in galaxies are predominantly spatially unresolved, in the form of a global ${\rm H\, {\small I}}$ spectral line. There has been substantial work on quantifying asymmetry in global ${\rm H\, {\small I}}$ spectra (‘global ${\rm H\, {\small I}}$ asymmetry’), but due to being spatially unresolved, it remains unknown what physical regions of galaxies the asymmetry traces, and whether the other gas phases are affected. Using optical integral field spectrograph (IFS) observations from the Sydney AAO Multi-object IFS (SAMI) survey for which global ${\rm H\, {\small I}}$ spectra are also available (SAMI-${\rm H\, {\small I}}$), we study the connection between asymmetry in galaxies’ ionized and neutral gas reservoirs to test if and how they can help us better understand the origin of global ${\rm H\, {\small I}}$ asymmetry. We reconstruct the global Hα spectral line from the IFS observations and find that while some global Hα asymmetries can arise from disturbed ionized gas kinematics, the majority of asymmetric cases are driven by the distribution of Hα-emitting gas. When compared to the ${\rm H\, {\small I}}$, we find no evidence for a relationship between the global Hα and ${\rm H\, {\small I}}$ asymmetry. Further, a visual inspection reveals that cases where galaxies have qualitatively similar Hα and ${\rm H\, {\small I}}$ spectral profiles can be spurious, with the similarity originating from an irregular 2D Hα flux distribution. Our results highlight that comparisons between global Hα and ${\rm H\, {\small I}}$ asymmetry are not straightforward, and that many global ${\rm H\, {\small I}}$ asymmetries trace disturbances that do not significantly impact the central regions of galaxies.