Gas-rich Companion Research Articles

An ALMA CO(1-0) survey of the 2Jy sample: large and massive molecular discs in radio AGN host galaxies

ABSTRACT 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 ($P_{1.4\,{\rm GHz}} \gt 10^{25}$ W Hz$^{-1}$ and $0.05 \lt z \lt 0.3$) with an angular resolution of about 2–3 arcsec (corresponding to 2–8 kpc). We detect molecular gas with masses in the range $10^{8.9} \lt M_{{\rm H}_2} \lt 10^{10.2}$ M$_\odot$ 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 – $34\pm 9$ per cent – is higher than in the general population of non-active early-type galaxies (ETGs: $\lt $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.

ALMA Reveals Spatially Resolved Properties of Molecular Gas in the Host Galaxy of FRB 20191001A at z = 0.2340

We report the detection of the CO(2–1) emission line with a spatial resolution of 0.″9 (3.5 kpc) from the host galaxy of the fast radio burst (FRB), FRB 20191001A at z = 0.2340, using the Atacama Large Millimeter/submillimeter Array. This is the first detection of spatially resolved CO emission from the host galaxy of an FRB at a cosmological distance. The inferred molecular gas mass of the host galaxy is (2.3 ± 0.4) × 1010 M ⊙, indicating that it is gas-rich, as evidenced by the measured molecular gas fraction μ gas = 0.50 ± 0.22. This molecular gas mass and the star formation rate of the host, SFR = (8.06 ± 2.42) M ⊙ yr−1, differ from those observed in the other FRB host galaxies with the average M gas = 9.6 × 108 M ⊙ and SFR = 0.90M ⊙ yr−1. This lends further credibility to the hypothesis that FRBs may originate from single or multiple progenitors across a diverse range of galaxy environments. Based on the observed velocity field modeling, we find that the molecular gas disk is dominated by an ordered circular rotation, despite the fact that the host galaxy has a gas-rich companion galaxy with a projected separation of ∼25 kpc. The formation of the FRB’s progenitor might not have been triggered by this interaction. We derive the 3σ upper limit of the molecular gas column density at the FRB detection site to be <2.1 × 1021 cm−2 with a 3σ upper limit.

Uncovering the formation of the counter-rotating stellar discs in SDSS J074834.64+444117.8

ABSTRACT Using the integral field spectroscopic data from Mapping Nearby Galaxies at Apache Point Observatory survey, we study the kinematics and stellar population properties of the two counter-rotating stellar discs in a nearby galaxy SDSS J074834.64+444117.8. We disentangle the two stellar discs by three methods, including Ca ii λ8542 double Gaussian fit, penalized pixel fitting (pPXF) spectral decomposition, and orbit-based dynamical model. These three different methods give consistent stellar kinematics. The pPXF spectral decomposition provides the spectra of two stellar discs, with one being more luminous across the whole galaxy named primary disc, and the other named secondary disc. The primary disc is counter-rotating with ionized gas, while the secondary disc is co-rotating with ionized gas. The secondary disc has younger stellar population and poorer stellar metallicity than the primary disc. We estimate the stellar mass ratio between the primary and secondary discs to be ∼5.2. The DESI g, r, z colour image does not show any merger remnant feature in this galaxy. These findings support a scenario that the counter-rotating stellar discs in SDSS J074834.64+444117.8 formed through gas accretion from the cosmic web or a gas-rich companion.

J0107a: A Barred Spiral Dusty Star-forming Galaxy at z = 2.467

Dusty star-forming galaxies (DSFGs) are among the most massive and active star-forming galaxies during the cosmic noon. Theoretical studies have proposed various formation mechanisms of DSFGs, including major merger-driven starbursts and secular star-forming disks. Here, we report J0107a, a bright (∼8 mJy at observed-frame 888 μm) DSFG at z = 2.467 that appears to be a gas-rich massive disk and might be an extreme case of the secular disk scenario. J0107a has a stellar mass M ⋆ ∼ 5 × 1011 M ⊙, molecular gas mass M mol ≳ 1011 M ⊙, and a star formation rate of ∼500M ⊙ yr−1. J0107a does not have a gas-rich companion. The rest-frame 1.28 μm JWST NIRCam image of J0107a shows a grand-design spiral with a prominent stellar bar extending ∼15 kpc. The Atacama Large Millimeter/submillimeter Array Band 7 continuum map reveals that the dust emission originates from both the central starburst and the stellar bar. 3D disk modeling of the CO(4–3) emission line indicates a dynamically cold disk with rotation-to-dispersion ratio . The results suggest a bright DSFG may have a nonmerger origin, and its vigorous star formation may be triggered by the bar and/or rapid gas inflow.

Inverted metallicity gradients in two Virgo cluster star-forming dwarf galaxies: evidence of recent merging?

ABSTRACT We present integral field spectroscopy observations of two star-forming dwarf galaxies in the Virgo cluster (VCC 135 and VCC 324) obtained with Potsdam Multi-Aperture Spectrograph/PPak at the Calar Alto 3.5 m telescope. We derive metallicity maps using the N2 empirical calibrator. The galaxies show positive gas metallicity gradients, contrarily to what is usually found in other dwarfs or spiral galaxies. We measure gradient slopes of 0.20 ± 0.06 and 0.15 ± 0.03 dex/Re for VCC 135 and VCC 324, respectively. Such a trend has been only observed in few, very isolated galaxies or at higher redshifts (z &amp;gt; 1). It is thought to be associated with the accretion of metal-poor gas from the intergalactic medium, a mechanism that would be less likely to occur in a high-density environment like Virgo. We combine emission-line observations with deep optical images to investigate the origin of the peculiar metallicity gradient. The presence of weak underlying substructures in both galaxies and the analysis of morphological diagnostics and ionized gas kinematics suggest that the inflow of metal-poor gas to the central regions of the dwarfs may be related to a recent merging event with a gas-rich companion.

Environmental processing in cluster core galaxies at z = 1.7

Today, the brightest cluster galaxies (BCGs) are passive and very massive galaxies at the center of their clusters, and they still accrete mass through swallowing companions and gas from cooling flows. However their formation history is not well known. We report CO(4→3) and continuum map observations of the SpARCS1049+56 BCG at z = 1.709, one of the most distant known BCGs. Our observations yield MH2 &lt; 1.1 × 1010 M⊙ for the BCG; while in CO(4→3), we detect two gas-rich companions at the northeast and southeast of the BCG, within 20 kpc, with LCO(4→3)′ = (5.8±0.6) × 109 K km s−1 pc2 and (7.4 ± 0.7)×109 K km s−1 pc2, respectively. The northern companion is associated with a pair of merging cluster galaxies, while the southern one shows a southern tail in CO(4→3), which was also detected in continuum, and we suggest it to be the most distant jellyfish galaxy for which ram pressure stripping is effectively able to strip off its dense molecular gas. This study probes the presence of rare gas-rich systems in the very central region of a distant cluster core, which will potentially merge into the BCG itself. Currently, we may thus be seeing the reversal of the star formation versus density relation at play in the distant universe. This is the first time the assembly of high-z progenitors of our local BCGs can be studied in such great detail.

Heterogeneity of inverted calcium ii H:K ratio cluster galaxies

ABSTRACT The ratio of calcium ii H plus Hϵ to calcium ii K inverts as a galaxy stellar population moves from being dominated by older stars to possessing more A and B class stars. This ratio – the H:K ratio – can serve as an indicator of stellar populations younger than 200 Myr. In this work, we provide a new method to determine H:K, and apply it to spectra taken of cluster galaxies in Abell 3888. Although H:K is on average systematically lower for the cluster than for a wider field sample, we show that H:K does not have a simple relationship with other indices such as the equivalent widths of Hδ and [O ii] beyond having a high value for strong [O ii] emission. Moreover, strongly inverted galaxies with H:K &amp;gt; 1.1 have no preferred location within the cluster and are only slightly lower in their velocity dispersions around the cluster compared to strongly emitting [O ii] galaxies. Our results indicate that selecting galaxies on H:K inversion results in a heterogeneous sample formed via a mixture of pathways that likely includes, but may not be limited to, merging spiral galaxies, and quiescent galaxies accreting lower mass, gas-rich companions. In concert with other selection criteria, H:K can provide a means to select a more ‘pure’ passive sample or to aid in the selection of highly star-forming galaxies, especially where other spectral line indicators such as H α may not have been observed.

Faraday rotation study of NGC 612 (PKS 0131−36): a hybrid radio source and its magnetized circumgalactic environment

We present a polarization and Faraday rotation study of the hybrid morphology radio galaxy NGC 612 (PKS 0131-36), using Australian Telescope Compact Array observations from 1 to 3 GHz. In general, the results are consistent with an external Faraday screen close to the radio source. In the eastern FRII lobe, the RM of the hotspot increases in magnitude towards the leading edge, as well as changing sign (compared to the rest of the lobe). The Faraday depolarization is also ~3 times larger at the hotspot than elsewhere. A plausible explanation for this is significant compression of ambient magnetised gas by the bow shock produced by the advancing hotspot. The western FRI lobe also exhibits some evidence of interaction with local magnetised gas, as a transverse band of high RM coincides with a distinct bend in the lobe. Previous observations of NGC 612 revealed an HI bridge of tidal debris along the direction of the eastern lobe towards the gas-rich companion NGC 619. We find no clear evidence that ionised gas associated with this bridge is either mixing with or lies in the foreground of the radio source. This is consistent with the absence of HI absorption against the hotspot, and indicates that the tidal debris must lie mostly behind the eastern lobe.

An ALMA survey of CO in submillimetre galaxies: companions, triggering, and the environment in blended sources

We present ALMA observations of the mid-J 12CO emission from six single-dish selected 870-micron sources in the Extended Chandra Deep Field-South (ECDFS) and UKIDSS Ultra-Deep Survey (UDS) fields. These six single-dish submillimetre sources were selected based on previous ALMA continuum observations, which showed that each comprised a blend of emission from two or more individual submillimetre galaxies (SMGs), separated on 5--10 arcsec scales. The six single-dish submillimetre sources targeted correspond to a total of 14 individual SMGs, of which seven have previously-measured robust optical/near-infrared spectroscopic redshifts, which were used to tune our ALMA observations. We detect CO(3-2) or CO(4-3) at z=2.3--3.7 in seven of the 14 SMGs, and in addition serendipitously detect line emission from three gas-rich companion galaxies, as well as identify four new 3.3-mm selected continuum sources in the six fields. Joint analysis of our CO spectroscopy and existing data suggests that 64 \pm 18% of the SMGs in blended submillimetre sources are unlikely to be physically associated. However, three of the SMG fields (50%) contain new, serendipitously-detected CO-emitting (but submillimetre-faint) sources at similar redshifts to the 870-micron selected SMGs we targeted. These data suggest that the SMGs inhabit overdense regions, but that these are not sufficiently overdense on ~100 kpc scales to influence the source blending given the short lifetimes of SMGs. We find that 21 \pm 12% of SMGs have spatially-distinct and kinematically-close companion galaxies (~8--150 kpc and <~300 km/s), which may have enhanced their star-formation via gravitational interactions.

The formation of S0 galaxies with counter-rotating neutral and molecular hydrogen

The observation of counter rotation in galaxies (i.e. gas that rotates in the opposite direction to the stellar component or two co-spatial stellar populations with opposite rotation) is becoming more commonplace with modern integral field spectroscopic surveys. In this paper we explore the emergence of counter-rotation (both stellar and gaseous) in S0 galaxies from smoothed-particle hydrodynamics simulations of 1/10 mass ratio minor mergers between a 10^10.8 solar mass disk galaxy with a bulge-to-total ration of 0.17 and a gas rich companion (gas-to-stellar mass fraction of 5.0). These simulations include a self-consistent treatment of gas dynamics, star formation, the production/destruction of H2 and dust, and the time evolution of the interstellar radiation field. We explore the effect of retrograde versus prograde obits, gas and bulge mass fractions of the primary galaxy, and orbital parameters of the companion. The key requirement for producing counter rotation in stars or gas in a merger remnant is a retrograde primary, while the relative spin of the companion affects only the radial extent of the accreted gas. We also find that including a significant amount of gas in the primary can prevent the emergence of counter-rotating gas, although accreted stars retain counter-rotation. Bulge mass and orbit have a secondary effect, generally influencing the final distribution of accreted stars and gas within the framework outlined above. In addition to our primary focus of counter-rotating components in galaxies, we also make some predictions regarding the SFRs, H2 distributions, and dust in minor merger remnants.

On the frequency of star-forming galaxies in the vicinity of powerful AGNs: The case of SMM J04135+10277

(Abridged) In the last decade several massive molecular gas reservoirs were found <100 kpc distance from active galactic nuclei (AGNs), residing in gas-rich companion galaxies. The study of AGN-gas-rich companion systems opens the opportunity to determine whether the stellar mass of massive local galaxies was formed in their host after a merger event or outside of their host galaxy in a close starbursting companion and later incorporated via mergers. We study the quasar-companion galaxy system of SMM J04135+10277 (z=2.84) and investigate the expected frequency of quasar-starburst galaxy pairs at high redshift using a cosmological galaxy formation model. We use archive data and new APEX ArTeMiS data to construct and model the spectral energy distribution of SMM J04135. We also carry out a comprehensive analysis of the cosmological galaxy formation model GALFORM with the aim of characterising how typical the system of SMM J04135 is and whether quasar-star-forming galaxy pairs may constitute an important stage in galaxy evolution. The companion galaxy of SMM J04135 is a heavily dust-obscured starburst galaxy with a median star formation rate (SFR) of $700\,\rm{M_{\odot}\,yr^{-1}}$, median dust mass of $5.1\times 10^9\,\rm{M_{\odot}}$ and median dust luminosity of $\textrm 9.3 \times 10^{12}\,\rm{L_{\odot}}$. Our simulations, performed at z=2.8, suggest that SMM J04135 is not unique. In fact, at a distance of <100 kpc, 22% of our simulated quasar sample have at least one companion galaxy of a stellar mass $>10^8\, \rm{M_{\odot}}$, and 0.3% have at least one highly star-forming companion ($\rm{SFR}>100\,\rm{M_{\odot}\,yr^{-1}}$). Our results suggest that quasar-gas-rich companion galaxy systems are common phenomena in the early Universe and the high incidence of companions makes the study of such systems crucial to understand the growth and hierarchical build-up of galaxies and black holes.

From galaxy-scale fueling to nuclear-scale feedback

Powerful radio galaxies are often associated with gas-rich galaxy mergers. These mergers may provide the fuel to trigger starburst and active galactic nuclear (AGN) activity. In this Research Note, we study the host galaxies of three seemingly young or re-started radio sources that drive fast outflows of cool neutral hydrogen (HI) gas, namely 3C 293, 3C 305 and 4C 12.50 (PKS 1345+12). Our aim is to link the feedback processes in the central kpc-scale region with new information on the distribution of stars and gas at scales of the galaxy. For this, we use deep optical V-band imaging of the host galaxies, complemented with HI emission-line observations to study their gaseous environments. We find prominent optical tidal features in all three radio galaxies, which confirm previous claims that 3C 293, 3C 305 and 4C 12.50 have been involved in a recent galaxy merger or interaction. Our data show the complex morphology of the host galaxies, and identify the companion galaxies that are likely involved in the merger or interaction. The radio sources appear to be (re-)triggered at a different stage of the merger; 4C 12.50 is a pre-coalescent and possibly multiple merger, 3C 293 is a post-coalescent merger that is undergoing a minor interaction with a close satellite galaxy, while 3C 305 appears to be shaped by an interaction with a gas-rich companion. For 3C 293 and 3C 305, we do not detect HI beyond the inner ~30-45 kpc region, which shows that the bulk of the cold gas is concentrated within the host-galaxy, rather than along the widespread tidal features.

Galactic interaction as the trigger for the young radio galaxy MRC B1221−423★

Mergers between a massive galaxy and a small gas-rich companion (minor mergers) have been proposed as a viable mechanism for triggering radio emission in an active galaxy. Until now the problem has been catching this sequence of events as they occur. With MRC B1221$-$423 we have an active radio galaxy that has only recently been triggered, and a companion galaxy that provides the "smoking gun". Using spectroscopic data taken with the VIMOS Integral Field Unit detector on the European Southern Observatory's Very Large Telescope, we have examined the distribution, ionization state and kinematics of ionized gas in this interacting system. We have also modelled the stellar continuum with synthesised spectra of stellar populations of different ages. From our study of the ionized gas, we have derived preliminary models for the geometry of the interaction, analysed the kinematic behaviour of the ionized gas, and examined the ionization mechanisms at work throughout the system. Our modelling of the stellar continuum allowed us to identify and date distinct stellar populations within the galaxy pair. We find evidence of multiple episodes of widespread starburst activity, and by dating these populations, we provide tentative insight into the history of the interaction.

Gas-rich mergers and feedback are ubiquitous amongst starbursting radio galaxies, as revealed by the VLA, IRAM PdBI and Herschel

We report new, sensitive observations of two z ∼ 3–3.5 far-infrared-luminous radio galaxies, 6C 1909+72 and B3 J2330+3927, in the 12CO J = 1−0 transition with the Karl Jansky Very Large Array and at 100–500 m using Herschel, alongside new and archival 12CO J = 4−3 observations from the Plateau de Bure Interferometer. We introduce a new colour–colour diagnostic plot to constrain the redshifts of several distant, dusty galaxies in our target fields. A bright SMG near 6C 1909+72 likely shares the same node or filament as the signpost active galactic nuclei (AGN), but it is not detected in 12CO despite ∼20 000 km s−1 of velocity coverage. Also in the 6C 1909+72 field, a large, red dust feature spanning ≈500 kpc is aligned with the radio jet. We suggest several processes by which metal-rich material may have been transported, favouring a collimated outflow reminiscent of the jet-oriented metal enrichment seen in local cluster environments. Our interferometric imaging reveals a gas-rich companion to B3 J2330+3927; indeed, all bar one of the eight z ≳ 2 radio galaxies (or companions) detected in 12CO provide some evidence that starburst activity in radio-loud AGN at high redshift is driven by the interaction of two or more gas-rich systems in which a significant mass of stars has already formed, rather than via steady accretion of cold gas from the cosmic web. We find that the 12CO brightness temperature ratios in radio-loud AGN host galaxies are significantly higher than those seen in similarly intense starbursts where AGN activity is less pronounced. Our most extreme example, where , provides evidence that significant energy is being deposited rapidly into the molecular gas via X-rays and/or mechanical (‘quasar-mode’) feedback from the AGN, leading to a high degree of turbulence globally and a low optical depth in 12CO – feedback that may lead to the cessation of star formation on a time-scale commensurate with that of the jet activity, ≲10 Myr.

UNVEILING THE MASK ON THE ULIRG-TO-QSO TRANSITION OBJECT [H89]1821+643 ATz= 0.3: A GAS-POOR/GAS-RICH GALAXY MERGER AND THE IMPLICATIONS FOR CO-BASED DYNAMICAL MASS ESTIMATES

We report the detection of the CO J=1-0 emission line in [H89]1821+643, one of the most optically luminous QSOs in the local Universe and a template ULIRG-to-QSO transition object, located in a rich cool-core cluster at z=0.297. The CO emission is likely to be extended, highly asymmetric with respect to the center of the host elliptical where the QSO resides, and corresponds to a molecular gas mass of ~ 8.0 x 10^{9} M_sun. The dynamical mass enclosed by the CO line emission region could amount up to ~ 1.7x10^{12} M_sun (80% of the total mass of the elliptical host). The bulk of the CO emission is located at ~9 kpc south-east from the nuclei position, and close to a faint optical structure, suggesting that the CO emission could either represent a gas-rich companion galaxy merging with the elliptical host or a tail-like structure reminiscent of a previous interaction. We argue that the first scenario is more likely given the large masses implied for the CO source, which would imply a highly asymmetrical elliptical host. The close alignment between the CO emission major axis and the radio-plume suggests a possible role of the latter excitation of the ambient gas reservoir. The stacking technique was used to search for CO emission and 3mm continuum emission from galaxies in the surrounding cluster, however no detection was found either toward individual galaxies or the stacked ensemble of galaxies, with a 3 sigma limit of < 1.1x10^9 M_sun for the molecular gas.

Warps and Accretion

AbstractWarps are a basic feature of disk galaxies. Usually they occur at radii where the optical disk fades and become most pronounced in the outermost gaseous disks.As such, warps present a massive reservoir to replenish star forming material in the inner, star forming disks. Furthermore, some possible excitation mechanisms for warps connect their formation to the accretion of extragalctic material. Interactions or mergers with gas-rich companions or the direct accretion of the ambient intergalactic medium might lead to the formation of warps, at the same time supplementing fuel to maintain star formation in galactic disks.Employing a number of H i studies of warped galaxies, including ultra-deep observations of the prototype warped galaxies NGC 5907 and NGC 4013, I discuss whether the observed kinematics may show evidence for a connection of warps and accretion from the ambient medium.

Neutral Hydrogen in the Polar Ring Galaxy UGC 9796

We have used the Very Large Array to observe the polar ring galaxy UGC 9796 (II Zw 73; PRC A-06) in the 21 cm line of neutral hydrogen. All the neutral gas in this galaxy is associated with the polar ring. Our observations show that 5 × 109 M⊙ of H I rotates in a plane projected about 25° from perpendicular to the equatorial plane of the early-type host galaxy, consistent with earlier observations in the Hα line. The outermost gas appears to bend away from the minor axis of the host system, in the same sense as the stellar ring. UGC 9796 is in a very gas-rich environment: our 21 cm images show five gas-rich companion galaxies in the same field within 100 km s-1, with a total of 2 × 1010 M⊙ between them. However, we see no H I streamers connecting the systems or any other evidence for an ongoing interaction between UGC 9796 and its companions.

A Second Shell in the Fornax dSph Galaxy

AbstractIn the search for tidal structure in Galactic satellite systems, we have conducted a photometric survey over a 10 deg2 area centred on the Fornax dSph galaxy. The survey was made in two colours, and the resulting colour–magnitude data were used as a mask to select candidate Fornax RGB stars, thereby increasing the contrast of Fornax stars to background sources in the outer regions. Previously, we reported the presence of a shell (age 2 Gyr) located towards the centre of Fornax. In this contribution we reveal a second shell, significantly larger than the first, located 1.3° north-west from the centre of Fornax, outside the nominal tidal radius. Moreover, the distribution of Fornax RGB stars reveals two lobes extending to the spatial limit of our survey, and aligned with the minor axis and with the two shells. These results support the hypothesis of a merger between Fornax and a gas-rich companion approximately 2 Gyr ago.

The WSRT wide-field H I survey

We have used the Westerbork array to carry out an unbiased wide-field survey for H i emission features, achieving an rms sensitivity of about 18 mJy/Beam at a velocity resolution of 17 km s-1 over 1800 deg2 and between km s-1. The primary data consists of auto-correlation spectra with an effective angular resolution of 49′ FWHM, although cross-correlation data were also acquired. The survey region is centered approximately on the position of Messier 31 and is Nyquist-sampled over in RA Dec. More than 100 distinct features are detected at high significance in each of the two velocity regimes (negative and positive LGSR velocities). In this paper we present the results for our H i detections of external galaxies at positive LGSR velocity. We detect 155 external galaxies in excess of 8σ in integrated H i flux density. Plausible optical associations are found within a 30′ search radius for all but one of our H i detections in DSS images, although several are not previously cataloged or do not have published red-shift determinations. Our detection without a DSS association is at low galactic latitude. Twenty-three of our objects are detected in H i for the first time. We classify almost half of our detections as “confused”, since one or more companions is cataloged within a radius of 30′ and a velocity interval of 400 km s-1. We identify a handful of instances of significant positional offsets exceeding 10 kpc of unconfused optical galaxies with the associated H i centroid, possibly indicative of severe tidal distortions or uncataloged gas-rich companions. A possible trend is found for an excess of detected H i flux in unconfused galaxies within our large survey beam relative to that detected previously in smaller telescope beams, both as function of increasing distance and increasing gas mass. This may be an indication for a diffuse gaseous component on 100 kpc scales in the environment of massive galaxies or a population of uncataloged low mass companions. We use our galaxy sample to estimate the H i mass function from our survey volume. Good agreement is found with the HIPASS BGC results, but only after explicit correction for galaxy density variations with distance.

The State of Galaxy Formation in the Local Universe

We present the analysis of the H I survey of isolated galaxies described in the first two papers in this series. This survey found 13 gas-rich companions around 10 of the 41 galaxies observed. In this paper, we present numerous pieces of evidence that the number of companions detected is less than expected by cold dark matter models of galaxy formation or H I observations of the population of galaxies in the field. We discuss the implications of these data for galaxy formation and the evolution of the properties of these companions. A statistical analysis of the observed distribution of the ensemble of companions with respect to the primary galaxies implies that, in general, these companions are on circular orbits and not eccentric orbits. While individual companions may be on eccentric orbits, the majority of the orbits are circular and, therefore, fairly stable. In fact, on the basis of the observed separations, both spatial and in velocity space, only those companions currently interacting with the galaxy they orbit should be accreted in the next few Gyr, and only half of the companies will be accreted in less than a Hubble time. Of the close companions, only UGC 260A and UGC 11152A appear to be significantly affecting the morphology of the primary galaxy. The remaining companions have too little mass and are too far away from the main galaxies to have any current effect on their morphology. None of these companions are massive enough to destroy the disk of the isolated galaxy when they are accreted, but instead will represent a minor merger that will supply a large amount of H I to fuel future star formation in these galaxies and produce small effects on the morphology. All of which implies that we are seeing nothing more than the tail end of the galaxy formation process. We have also compared the cumulative velocity distribution function of our companions with the predictions of cold dark matter and other galaxy formation models. Our data are only consistent with cold dark matter models when generous, but not unreasonable, correction factors are applied for nondetections of gas-poor galaxies. Models that suppress observable satellite galaxies, such as warm dark matter models or those that prevent neutral gas from accumulating in dark matter halos, would better explain our observed population of companions. Finally, we compare the H I mass function (MF) for companions to isolated galaxies with the H I MF from the work of others. Our H I MF is consistent with a variety of faint-end slopes up to α ~ -1.3. Such a slope argues against the Blitz et al. model for high-velocity clouds.