I-rich Galaxies Research Articles

The circular velocity and halo mass functions of galaxies in the nearby Universe

ABSTRACT The circular velocity function (CVF) of galaxies is a fundamental test of the Lambda cold dark matter ($\Lambda$CDM) paradigm as it traces the variation of galaxy number densities with circular velocity ($v_{\rm {circ}}$), a proxy for dynamical mass. Previous observational studies of the CVF have either been based on H i-rich galaxies, or encompassed low-number statistics and probed narrow ranges in $v_{\rm {circ}}$. We present a benchmark computation of the CVF between $100\,{\text{and}}\,350\ \rm {km\ s^{-1}}$ using a sample of 3527 nearby Universe galaxies, representative for stellar masses between $10^{9.2}\,{\text{and}}\,10^{11.9} \rm {{\rm M}_{\odot }}$. We find significantly larger number densities above 150 $\rm {km\ s^{-1}}$ compared to results from H i surveys, pertaining to the morphological diversity of our sample. Leveraging the fact that circular velocities are tracing the gravitational potential of haloes, we compute the halo mass function (HMF), covering $\sim$1 dex of previously unprobed halo masses ($10^{11.7}{\!-\!}10^{12.7} \rm {{\rm M}_{\odot }}$). The HMF for our sample, representative of the galaxy population with $M_{200}\geqslant 10^{11.35} \rm {{\rm M}_{\odot }}$, shows that spiral morphologies contribute 67 per cent of the matter density in the nearby Universe, while early types account for the rest. We combine our HMF data with literature measurements based on H i kinematics and group/cluster velocity dispersions. We constrain the functional form of the HMF between $10^{10.5}-10^{15.5} \rm {{\rm M}_{\odot }}$, finding a good agreement with $\Lambda$CDM predictions. The halo mass range probed encompasses 72$\substack{+5 -6}$ per cent ($\Omega _{\rm {M,10.5-15.5}} = 0.227 \pm 0.018$) of the matter density in the nearby Universe; 31$\substack{+5 -6}$ per cent is accounted for by haloes below $10^{12.7}\rm {{\rm M}_{\odot }}$ occupied by a single galaxy.

Finding Passive Galaxies in H i-MaNGA: The Impact of Star Formation Rate Indicator

H i-rich galaxies typically have high star formation rates (SFRs), but there exist interesting H i-rich and low star-forming galaxies. Previous work on a sample of these galaxies identified from H i-Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) (H i follow-up to the MaNGA survey) using an infrared indicator of specific-SFR (sSFR; namely W2 − W3 < 2) could find no single physical process to explain their unusual behavior. The method by which galaxies are identified as low sSFR may be important in this conclusion. In this Research Note, we explore how an H i-rich, low sSFR sample of H i-MaNGA galaxies differs using Hα, single stellar population, and ultraviolet estimators of SFR. We find that samples are statistically similar to each other so long as W2 − W3 < 2 is interpreted as corresponding to sSFR < 10−11.15 yr−1.

A serendipitous discovery of H i-rich galaxy groups with MeerKAT

ABSTRACT We report on the serendipitous discovery of 49 H i-rich galaxies in a 2.3 h Open Time observation with MeerKAT. We present their properties including their H i masses, intensity and velocity maps, and spectra. We determine that at least three H i-rich galaxy groups have been detected, potentially as part of a supergroup. Some members of these galaxy groups show clear interaction with each other in their H i emission. We cross-match the detections with PanSTARRS, Wide-field Infrared Survey Explorer, and Galaxy Evolution Explorer, and obtain stellar masses and star formation rates. One source is found to be a potential OH megamaser, but further follow-up is required to confidently determine this. For six sources with sufficient spatial resolution in H i, we produce rotation curves with BBarolo, generate mass models, and derive a dark matter halo mass. While the number of galaxies detected in this relatively short pointing appears to be at the high end of expectations compared to other MeerKAT observations and group H i mass function studies, this finding highlights the capability of MeerKAT for other serendipitous discoveries, and the potential for many more H i-rich galaxies to be revealed within both existing and upcoming Open Time data sets.

On the Existence, Rareness, and Uniqueness of Quenched H i-rich Galaxies in the Local Universe

Using data from ALFALFA, xGASS, H i-MaNGA, and the Sloan Digital Sky Survey (SDSS), we identify a sample of 47 “red but H i-rich” (RR) galaxies with near-UV (NUV) − r > 5 and unusually high H i-to-stellar mass ratios. We compare the optical properties and local environments between the RR galaxies and a control sample of “red and H i-normal” (RN) galaxies that are matched in stellar mass and color. The two samples are similar in the optical properties typical of massive red (quenched) galaxies in the local Universe. The RR sample tends to be associated with slightly lower-density environments and has lower clustering amplitudes and smaller neighbor counts at scales from several hundred kiloparsecs to a few megaparsecs. The results are consistent with the RR galaxies being preferentially located at the center of low-mass halos, with a median halo mass ∼1012 h −1 M ⊙ compared to ∼1012.5 h −1 M ⊙ for the RN sample. This result is confirmed by the SDSS group catalog, which reveals a central fraction of 89% for the RR sample, compared to ∼60% for the RN sample. If assumed to follow the H i size–mass relation of normal galaxies, the RR galaxies have an average H i-to-optical radius ratio of R HI/R 90 ∼ 4, four times the average ratio for the RN sample. We compare our RR sample with similar samples in previous studies, and quantify the population of RR galaxies using the SDSS complete sample. We conclude that the RR galaxies form a unique but rare population, accounting for only a small fraction of the massive quiescent galaxy population. We discuss the formation scenarios of the RR galaxies.

FAST-ASKAP Synergy: Quantifying Coexistent Tidal and Ram Pressure Strippings in the NGC 4636 Group

Combining new H i data from a synergetic survey of Australian Square Kilometre Array Pathfinder (ASKAP) Widefield ASKAP L-band Legacy All-sky Blind surveY and Five-hundred-meter Aperture Spherical radio Telescope with the Arecibo Legacy Fast ALFA data, we study the effect of ram pressure and tidal interactions in the NGC 4636 group. We develop two parameters to quantify and disentangle these two effects on gas stripping in H i-bearing galaxies: the strength of external forces at the optical-disk edge, and the outside-in extents of H i-disk stripping. We find that gas stripping is widespread in this group, affecting 80% of H i-detected nonmerging galaxies, and that 41% are experiencing both types of stripping. Among the galaxies experiencing both effects, the two types of strengths are independent, while two H i-stripping extents moderately anticorrelate with each other. Both strengths are correlated with H i-disk shrinkage. The tidal strength is related to a rather uniform reddening of low-mass galaxies (M * < 109 M ☉) when tidal stripping is the dominating effect. In contrast, ram pressure is not clearly linked to the color-changing patterns of galaxies in the group. Combining these two stripping extents, we estimate the total stripping extent, and put forward an empirical model that can describe the decrease of H i richness as galaxies fall toward the group center. The stripping timescale we derived decreases with distance to the center, from ∼1 Gyr beyond R 200 to ≲10 Myr near the center. Gas depletion happens ∼3 Gyr since crossing 2R 200 for H i-rich galaxies, but much quicker for H i-poor ones. Our results quantify in a physically motivated way the details and processes of environmental-effects-driven galaxy evolution, and might assist in analyzing hydrodynamic simulations in an observational way.

H i-rich but low star formation galaxies in MaNGA: physical properties and comparison to control samples

ABSTRACT Gas-rich galaxies are typically star forming. We make use of H i-MaNGA, a programme of H i follow-up for the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey of the Sloan Digital Sky Surveys, to construct a sample of unusual neutral hydrogen (H i, 21 cm)-rich galaxies that have low star formation rates (SFRs), using infrared colour from the Wide-field Infrared Survey Explorer as a proxy for specific SFR. Out of a set of 1575 MaNGA galaxies with H i-MaNGA detections, we find that 83 (5 per cent) meet our selection criteria to be H i rich with low SFR. We construct two stellar mass-matched control samples: H i-rich galaxies with typical SFR (high SF control) and H i-poor galaxies with low SFR (low H i control). We investigate the properties of each of these samples, comparing physical parameters such as ionization state maps, stellar and ionized gas velocity and dispersion, environment measures, metallicity, and morphology to search for the reasons why these unusual H i-rich galaxies are not forming stars. We find evidence for recent external accretion of gas in some galaxies (via high counter-rotating fractions), along with some evidence for active galactic nucleus (AGN) feedback (from a high central low-ionization emission-line region and/or red geyser fraction), and bar quenching (via an enhanced strong bar fraction). Some galaxies in the sample are consistent with simply having their H i in a high angular momentum, large-radius, low-density disc. We conclude that no single physical process can explain all H i-rich, low-SFR galaxies.

WALLABY pre-pilot survey: two dark clouds in the vicinity of NGC 1395

ABSTRACT We present the Australian Square Kilometre Array Pathfinder (ASKAP) WALLABY pre-pilot observations of two ‘dark’ H i sources (with H i masses of a few times 108 $\rm {M}_\odot$ and no known stellar counterpart) that reside within 363 kpc of NGC 1395, the most massive early-type galaxy in the Eridanus group of galaxies. We investigate whether these ‘dark’ H i sources have resulted from past tidal interactions or whether they are an extreme class of low surface brightness galaxies. Our results suggest that both scenarios are possible, and not mutually exclusive. The two ‘dark’ H i sources are compact, reside in relative isolation, and are more than 159 kpc away from their nearest H i-rich galaxy neighbour. Regardless of origin, the H i sizes and masses of both ‘dark’ H i sources are consistent with the H i size–mass relationship that is found in nearby low-mass galaxies, supporting the possibility that these H i sources are an extreme class of low surface brightness galaxies. We identified three analogues of candidate primordial ‘dark’ H i galaxies within the TNG100 cosmological, hydrodynamic simulation. All three model analogues are dark matter dominated, have assembled most of their mass 12–13 Gyr ago, and have not experienced much evolution until cluster infall 1–2 Gyr ago. Our WALLABY pre-pilot science results suggest that the upcoming large-area H i surveys will have a significant impact on our understanding of low surface brightness galaxies and the physical processes that shape them.

The BLUEDISK Survey: Thickness of H i Layers in Gas-rich Spiral Galaxies

Abstract We use an empirical relation to measure the H i scale height of relatively H i-rich galaxies using 21 cm observations. The galaxies were selected from the BLUEDISK, THINGS, and VIVA surveys. We aim to compare the thickness of the H i layer of unusually H i-rich galaxies with that of normal spiral galaxies and find any correlation between the H i scale height and other galaxies’ properties. We found that on average the unusually H i-rich galaxies have H i disk thickness similar to that of the control sample and the galaxies selected from the THINGS and VIVA surveys within their uncertainties. Our results also show that the average thickness of the neutral hydrogen inside the optical disk is correlated with the atomic gas fraction inside the optical disk with a scatter of ∼0.22 dex. A correlation is also found between the H i scale height with the atomic-to-molecular hydrogen ratio, which indicates a link between star formation and the vertical distribution of H i, which is consistent with previous studies. This new scaling relation between the H i scale height and atomic gas fraction will allow us to predict the H i scale heights of a large number of galaxies, but a larger sample is needed to decrease the scatter.

The baryonic Tully–Fisher relation in the simba simulation

ABSTRACT We investigate the Baryonic Tully–Fisher relation (BTFR) in the $(100\, h^{-1}{\rm Mpc})^3$simba hydrodynamical galaxy formation simulation together with a higher resolution $(25\, h^{-1}{\rm Mpc})^3$simba run, for over 10 000 disc-dominated, H i-rich galaxies. We generate simulated galaxy rotation curves from the mass distribution, which we show yields similar results to using the gas rotational velocities. From this, we measure the galaxy rotation velocity Vcirc using four metrics: $V_{\rm max}, V_{\rm flat}, V_{2R_e},$ and Vpolyex. We compare the predicted BTFR to the SPARC observational sample and find broad agreement. In detail, however, simba is biased towards higher Vcirc by up to 0.1 dex. We find evidence for the flattening of the BTFR in Vcirc &amp;gt; 300 km s−1 galaxies, in agreement with recent observational findings. simba’s rotation curves are more peaked for lower mass galaxies, in contrast with observations, suggesting overly bulge-dominated dwarf galaxies in our sample. We investigate for residuals around the BTFR versus H i mass, stellar mass, gas fraction, and specific star formation rate, which provide testable predictions for upcoming BTFR surveys. simba’s BTFR shows sub-optimal resolution mass convergence, with the higher resolution run lowering V in better agreement with data.

Direct Measurement of the H i-halo Mass Relation through Stacking

Abstract We present accurate measurements of the total H i mass in dark matter halos of different masses at z ∼ 0, by stacking the H i spectra of entire groups from the Arecibo Fast Legacy ALFA Survey. The halos are selected from the optical galaxy group catalog constructed from the Sloan Digital Sky Survey DR7 Main Galaxy sample, with reliable measurements of halo mass and halo membership. We find that the H i-halo mass relation is not a simple monotonic function, as assumed in several theoretical models. In addition to the dependence of halo mass, the total H i gas mass shows a strong dependence on the halo richness, with larger H i masses in groups with more members at fixed halo masses. Moreover, halos with at least three member galaxies in the group catalog have a sharp decrease of the H i mass, potentially caused by the virial halo shock-heating and the active galactic nucleus (AGN) feedback. The dominant contribution of the H i gas comes from the central galaxies for halos of , while the satellite galaxies dominate over more massive halos. Our measurements are consistent with a three-phase formation scenario of the H i-rich galaxies. The smooth cold gas accretion is driving the H i mass growth in halos of , with late-forming halos having more H i accreted. The virial halo shock-heating and AGN feedback will take effect to reduce the H i supply in halos of . The H i mass in halos more massive than generally grows by mergers, with the dependence on halo richness becoming much weaker.

Cold gas and dust: Hunting spiral-like structures in early-type galaxies

Context. Observations of neutral hydrogen (H I) and molecular gas show that 50% of all nearby early-type galaxies contain some cold gas. Molecular gas is always found in small gas discs in the central region of the galaxy, while neutral hydrogen is often distributed in a low-column density disc or ring typically extending well beyond the stellar body. Dust is frequently found in early-type galaxies as well. Aims. The goal of our study is to understand the link between dust and cold gas in nearby early-type galaxies as a function of H I content. Methods. We analyse deep optical g − r images obtained with the MegaCam camera at the Canada-France-Hawaii Telescope for a sample of 21 H I-rich and 41 H I-poor early-type galaxies. Results. We find that all H I-rich galaxies contain dust seen as absorption. Moreover, in 57% of these H I-rich galaxies, the dust is distributed in a large-scale spiral pattern. Although the dust detection rate is relatively high in the H I-poor galaxies (∼59%), most of these systems exhibit simpler dust morphologies without any evidence of spiral structures. We find that the H I-rich galaxies possess more complex dust morphology extending to almost two times larger radii than H I-poor objects. We measured the dust content of the galaxies from the optical colour excess and find that H I-rich galaxies contain six times more dust (in mass) than H I-poor ones. In order to maintain the dust structures in the galaxies, continuous gas accretion is needed, and the substantial H I gas reservoirs in the outer regions of early-type galaxies can satisfy this need for a long time. We find that there is a good correspondence between the observed masses of the gas and dust, and it is also clear that dust is present in regions further than 3 Reff. Conclusions. Our findings indicate an essential relation between the presence of cold gas and dust in early-type galaxies and offer a way to study the interstellar medium in more detail than what is possible with H I observations.

XGASS: passive discs do not host unexpectedly large reservoirs of cold atomic hydrogen

ABSTRACT We use the extended GALEX Arecibo SDSS Survey (xGASS) to quantify the relationship between atomic hydrogen (H i) reservoir and current star formation rate (SFR) for central disc galaxies. This is primarily motivated by recent claims for the existence, in this sample, of a large population of passive discs harbouring H i reservoirs as large as those observed in main-sequence galaxies. Across the stellar mass range 109 &amp;lt; M*/M⊙ &amp;lt; 1011, we practically find no passive (≳2σ below the star forming main sequence) disc galaxies with H i reservoirs comparable to those typical of star-forming systems. Even including H i non-detections at their upper limits, passive discs typically have ≥0.5 dex less H i than their active counterparts. We show that previous claims are due to the use of aperture-corrected SFR estimates from the MPA/JHU SDSS DR7 catalogue, which do not provide a fair representation of the global SFR of H i-rich galaxies with extended star-forming discs. Our findings confirm that the bulk of the passive disc population in the local Universe is H i-poor. These also imply that the reduction of star formation, even in central disc galaxies, has to be accompanied by a reduction in their H i reservoir.

The growth of bulges and discs in relatively H i-rich galaxies: indication from H i scaling relations

ABSTRACT We study the relation between the properties of the bulge/disc components and the H i mass fraction of galaxies. We find that at fixed stellar mass, disc colours are correlated with the H i mass fraction, while bulge colours are not. The lack of a correlation between the bulge colour and the H i mass fraction is regardless whether the bulges are pseudo, or whether the galaxies host bars or are interacting with a neighbour. There is no strong correlation between the colours of the discs and bulges either. These results suggest that the current total amount of H i is closely related to the formation of discs, but does not necessarily fuel the formation of (pseudo) bulges in an efficient way. We do not find evidence for the star formation in the discs to be quenched by the bulges.

Ring galaxies in the EAGLE hydrodynamical simulations

We study the formation and evolution of ring galaxies in the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations. We use the largest reference model Ref-L100N1504, a cubic cosmological volume of 100 cMpc on a side, to identify and characterize these systems through cosmic time. The number density of ring galaxies in EAGLE is in broad agreement with the observations. The vast majority of ring galaxies identified in EAGLE (⁠|$83\,$|per cent) have an interaction origin, i.e. form when one or more companion galaxies drop-through a disc galaxy. The remainder (⁠|$17\,$|per cent) have very long-lived ring morphologies (>2 Gyr) and host strong bars. Ring galaxies are H i-rich galaxies, yet display inefficient star formation activity and tend to reside in the green valley particularly at |$z\, \gtrsim \, 0.5$|⁠. This inefficiency is mainly due to the low pressure and metallicity of their interstellar medium (ISM) compared with the ISM of similar star-forming galaxies. We find that the interaction(s) is responsible for decreasing the ISM pressure by causing the ISM gas to flow from the inner regions to the outer disc, where the ring feature forms. At a fixed radius, the star formation efficiency of ring galaxies is indistinguishable from their star-forming counterparts, and thus the main reason for their integrated lower efficiency is the different gas surface density profiles. Since galaxy morphologies are not used to tune the parameters in hydrodynamical simulations, the experiment performed here demonstrates the success of the current numerical models in EAGLE.

Constraining the H i–Halo Mass Relation from Galaxy Clustering

Abstract We study the dependence of galaxy clustering on H i mass using ∼16,000 galaxies with redshift in the range of 0.0025 &lt; z &lt; 0.05 and H i mass of M H I &gt; 10 8 M ☉ , drawn from the 70% complete sample of the Arecibo Legacy Fast ALFA survey. We construct subsamples of galaxies with M H I above different thresholds and make volume-limited clustering measurements in terms of three statistics: the projected two-point correlation function, the projected cross-correlation function with respect to a reference sample, and the redshift-space monopole moment. In contrast to previous studies, which found no/weak H i mass dependence, we find both the clustering amplitudes on scales above a few megaparsecs and the bias factors to increase significantly with increasing H i mass for M H I &gt; 10 9 M ☉ . For H i mass thresholds below ∼ 10 9 M ☉ , the inferred galaxy bias factors are systematically lower than the minimum halo bias from mass-selected halo samples. We extend the simple halo model, in which the galaxy content is only determined by halo mass, by including the halo formation time as an additional parameter. A model that puts H i-rich galaxies into halos that formed late can reproduce the clustering measurements reasonably well. We present the implications of our best-fitting model on the correlation of H i mass with halo mass and formation time, as well as the halo occupation distributions and H i mass functions for central and satellite galaxies. These results are compared with the predictions from semianalytic galaxy formation models and hydrodynamic galaxy formation simulations.

(Almost) Dark Galaxies in the ALFALFA Survey: Isolated H i-bearing Ultra-diffuse Galaxies

Abstract We present a sample of 115 very low optical surface brightness, highly extended, H i-rich galaxies carefully selected from the ALFALFA survey that have similar optical absolute magnitudes, surface brightnesses, and radii to recently discovered “ultra-diffuse” galaxies (UDGs). However, these systems are bluer and have more irregular morphologies than other UDGs, are isolated, and contain significant reservoirs of H i. We find that while these sources have normal star formation rates for H i-selected galaxies of similar stellar mass, they have very low star formation efficiencies. We further present deep optical and H i-synthesis follow-up imaging of three of these H i-bearing ultra-diffuse sources. We measure H i diameters extending to ∼40 kpc, but note that while all three sources have large H i diameters for their stellar mass, they are consistent with the H i mass–H i radius relation. We further analyze the H i velocity widths and rotation velocities for the unresolved and resolved sources, respectively, and find that the sources appear to inhabit halos of dwarf galaxies. We estimate spin parameters, and suggest that these sources may exist in high spin parameter halos, and as such may be potential H i-rich progenitors to the ultra-diffuse galaxies observed in cluster environments.

The outer stellar populations and environments of unusually H i-rich galaxies

We investigate the nature of HI-rich galaxies from the ALFALFA and GASS surveys, which are defined as galaxies in the top 10th percentile in atomic gas fraction at a given stellar mass. We analyze outer (R>1.5 Re) stellar populations for a subset of face-on systems using optical g-r versus r-z colour/colour diagrams. The results are compared with those from control samples that are defined without regard to atomic gas content, but are matched in redshift, stellar mass and structural parameters. HI-rich early-type (C>2.6) and late-type (C<2.6) galaxies are studied separately. When compared to the control sample, the outer stellar populations of the majority of HI-rich early-type galaxies are shifted in the colour/colour plane along a locus consistent with younger stellar ages, but similar metallicities. The outer colours of HI-rich late-type galaxies are much bluer in r-z than the HI-rich early types, and we infer that they have outer disks which are both younger and more metal-poor. We then proceed to analyze the galaxy environments of HI-rich galaxies on scales of 500 kpc. HI-rich early-type galaxies with low (log M* < 10.5) stellar masses differ significantly from the control sample in that they are more likely to be central rather than satellite systems. Their satellites are also less massive and have younger stellar populations. Similar, but weaker effects are found for low mass HI-rich late-type galaxies. In addition, we find that the satellites of HI-rich late-types exhibit a greater tendency to align along the major axis of the primary. No environmental differences are found for massive (log M* > 10.5) HI-rich galaxies, regardless of type.

The Bluedisks project, a study of unusually H i-rich galaxies – I. H i sizes and morphology

We introduce the "Bluedisk" project, a large program at the Westerbork Synthesis Radio Telescope (WSRT) that has mapped the HI in a sample of 23 nearby galaxies with unusually high HI mass fractions, along with a similar-sized sample of control galaxies matched in stellar mass, size, inclination and redshift. This paper presents the sample selection, observational set-up, data reduction strategy, and a first analysis of the sizes and structural properties of the HI disks. We find that the HI-rich galaxies lie on the same HI mass versus HI size relation as normal spiral galaxies, extending it to total HI masses of $2 \times 10^{10} M_{\odot}$ and radii R1 of $\sim 100$ kpc (where R1 is defined as the radius where the HI column density reaches 1 $M_{\odot}$ pc$^{-2}$). HI-rich galaxies have significantly larger values of HI-to-optical size ratio at fixed stellar mass, concentration index, stellar and star formation rate surface density compared to the control sample. The disks of HI-rich galaxies are also significantly more clumpy (i.e. have higher HI Gini and $\Delta$Area coefficient) than those of normal spirals. There is no evidence that the disks of HI-rich galaxies are more disturbed: HI-rich galaxies exhibit no difference with respect to control samples in their distributions of HI asymmetry indices or optical/HI disk position angle differences. In fact, the center of the HI distribution corresponds more closely with the center of the optical light in the HI-rich galaxies than in the controls. All these results argue against a scenario in which new gas has been brought in by mergers. It is possible that they may be more consistent with cooling from a surrounding quasi-static halo of warm/hot gas.

The H i environment of counter-rotating gas hosts: gas accretion from cold gas blobs

We probe the H i properties and the gas environments of three early-type barred galaxies harbouring counter-rotating ionized gas: NGC 128, NGC 3203 and NGC 7332. Each system has one or more optically identified galaxy at a similar or as yet unknown redshift within a 50-kpc projected radius. Using H i synthesis imaging data, we investigate the hypothesis that the counter-rotating gas in these galaxies has been accreted from their neighbours. In NGC 128 and NGC 3203, we find 9.6 × 107 and 2.3 × 108 M⊙ of H i, respectively, covering almost the entire stellar bodies of dwarf companions that appear physically connected. Both the H i morphology and kinematics are suggestive of tidal interactions. In NGC 7332, we do not find any directly associated H i. Instead, NGC 7339, a neighbour of a comparable size at about 10 kpc, is found with 8.9 × 108 M⊙ of H i gas. More recently in a single dish observation, however, another group discovered a large H i structure which seems to be an extension of NGC 7339’s H i disc and also covers NGC 7332. All these observations thus suggest that H i gas is being accreted in these three galaxies from their companions, which is likely responsible for the kinematically decoupled gas component present in their central region. In particular, the dynamical friction time-scales of the nearest neighbours with H i gas of NGC 128 and NGC 3203 are comparable to their orbital time-scales around the counter-rotators, several ∼108 yr, implying that those neighbours will likely soon merge with the primary galaxies, fuelling them with gas. NGC 7332 also appears to be in the merging process with its neighbour through the common H i envelope. Besides, we find some other potential gas donors around NGC 128 and NGC 7332: two H i-rich galaxies with and 2.5 × 109 M⊙ at a distance of ≈67 kpc from NGC 128 and two dwarf systems with MHI= 3.9 × 107 and 7.4 × 107 M⊙ at ≲100 kpc from NGC 7332. Among the seven H i features identified in this study, three of them are associated with dwarf galaxies, two of which have only been recently identified in a blind survey, while the third one is still not catalogued at optical wavelengths. Considering the incompleteness of existing studies of the faint dwarf galaxy population both in the optical and in H i, accretion from cold gas blobs, presumably gas-rich dwarfs, is expected to occur even more frequently than what is inferred from such cases that have been observed to date.

The Weak Clustering of Gas‐rich Galaxies

We examine the clustering properties of H I-selected galaxies through an analysis of the H I Parkes All-Sky Survey Catalogue (HICAT) two-point correlation function. Various subsamples are extracted from this catalog to study the overall clustering of H I-rich galaxies and its dependence on luminosity, H I gas mass, and rotational velocity. These samples cover the entire southern sky δ < 0°, containing up to 4174 galaxies over the radial velocity range 300-12,700 km s-1. A scale length of r0 = 3.45 ± 0.25 h-1 Mpc and slope of γ = 1.47 ± 0.08 is obtained for the H I-rich galaxy real-space correlation function, making gas-rich galaxies among the most weakly clustered objects known. H I-selected galaxies also exhibit weaker clustering than optically selected galaxies of comparable luminosities. Good agreement is found between our results and those of synthetic H I-rich galaxy catalogs generated from the Millennium Run cold dark matter (CDM) simulation. Bisecting HICAT using different parameter cuts, clustering is found to depend most strongly on rotational velocity and luminosity, while the dependency on H I mass is marginal. Splitting the sample around vrot = 108 km s-1, a scale length of r0 = 2.86 ± 0.46 h-1 Mpc is found for galaxies with low rotational velocities compared to r0 = 3.96 ± 0.33 h-1 Mpc for the high rotational velocity sample.