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Register a new userAn HI census of Loose Groups of Galaxies
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D.J. Pisano
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We present results from our Parkes Multibeam HI survey of 3 loose groups of galaxies that are analogous to the Local Group. This is a survey of groups containing only spiral galaxies with mean separations of a few hundred kpc, and total areas of approximately 1 sq. Mpc; groups similar to our own Local Group. We present a census of the HI-rich objects in these groups down to an M(HI), 1-sigma sensitivity ~7x10^5 M(sun), as well as the detailed properties of these detections from follow-up Compact Array observations. We found 7 new HI-rich members in the 3 groups, all of which have stellar counterparts and are, therefore, typical dwarf galaxies. The ratio of low-mass to high-mass gas-rich galaxies in these groups is less than in the Local Group meaning that the ``missing satellite problem is not unique. No high-velocity cloud analogs were found in any of the groups. If HVCs in these groups are the same as in the Local Group, this implies that HVCs must be located within ~300-400 kpc of the Milky Way.
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To investigate galaxy properties as a function of their total stellar mass, we obtained 21cm HI line observations at the 100-m class Nanc{c}ay Radio Telescope of 2839 galaxies from the Sloan Digital Sky Survey (SDSS) in the Local Volume (900<cz<12,000 km/s), dubbed the Nanc{c}ay Interstellar Baryons Legacy Extragalactic Survey (NIBLES) sample. They were selected evenly over their entire range of absolute SDSS z-band magnitudes (-13.5 to -24 mag), which were used as a proxy for their stellar masses. Here, a first, global presentation of the observations and basic results is given, their further analysis will be presented in other papers in this series. The galaxies were selected based on their properties, as listed in SDSS DR5. Comparing this photometry to their total HI masses, we noted that, for a few percent, the SDSS magnitudes appeared severely misunderestimated, as confirmed by our re-measurements for selected objects. Although using the later DR9 results eliminated this problem in most cases, 384 still required manual photometric source selection. Usable HI spectra were obtained for 2600 galaxies, of which 1733 (67%) were clearly detected and 174 (7%) marginally. The spectra for 241 other observed galaxies could not be used for further analysis owing to problems with either the HI or the SDSS data. We reached the target number of about 150 sources per half-magnitude bin over the Mz range -16.5 to -23 mag. Down to -21 mag the overall detection rate is rather constant at the ~75% level but it starts to decline steadily towards the 30% level at -23 mag. Making regression fits by comparing total HI and stellar masses for our sample, including our conservatively estimated HI upper limits for non-detections, we find the relationship log(M_HI/M*) = -0.59 log(M*) + 5.05, which lies significantly below the relationship found in the M_HI/M* - M* plane when only using HI detections.
We present and explore the resolved atomic hydrogen (HI) content of 13 HI-rich and late-type dominated groups denoted `Choirs. We quantify the HI content of the Choir galaxies with respect to the median of the HI-mass fraction ($f_{textrm{HI}}$) of their grandparent HIPASS sample. We find that the HI mass fraction of the Choir galaxies is dispersed around the HIPASS median in the range $-1.4 leq Delta f_{textrm{HI}}textrm{[dex]}leq 0.7$, from HI-excess to HI-deficient galaxy regime. The HI-excess/HI-deficient galaxies contain more/less than 2.5 times their expected HI content with respect to the HIPASS median. We show and discuss that the environmental processing in Choirs occurs via tidal stripping and galaxy mergers. Our analysis suggests that tidal stripping contributes to the loss of the HI, while galaxy mergers contribute to the enhancement of the HI. Exploring the mid-infrared properties of Choir galaxies we find possible environmental processing in only nine Choir galaxies, which indicates that environmental processing is more perceptible in the HI content than the mid-infrared properties. Moreover, we find that environmental processing occurs in Choir groups regardless of their global environment, whether they are in isolation or in proximity to the denser structures, such as cosmic web filaments. We explore possible scenarios of the Choirs evolution, taking into account their HI content, velocity dispersion, crossing time and their global environment. We conclude that the most likely evolution for the majority of Choir groups is that they will become more compact as their members undergo multiple HI-rich mergers.
We analyze the three catalogs of nearby loose groups identified by Garcia (1993). She used a percolation and a hierarchical method, and proposed a third group catalog defined as a combination of the two. Each catalog contains almost 500 groups. In agreement with previous works on earlier catalogs, we find that groups can be described as collapsing systems. Their sampled size is much larger than their expected virialized region. We compute the virial masses and correct them by taking into account the young dynamical status. We estimate group masses, M, for two cosmological models, a flat one with Omega_0=1 and an open one with Omega_0=0.2. For each of the three catalogs we calculate the mass function, MF. The number density of groups with M>9x10^{12}msun, which is the adopted limit of sample completeness, ranges within 1.3-1.9x10^{-3}h^3/Mpc^3 for Omega_0=1, and it is about a factor of 15% lower for Omega_0=0.2. The MFs of the hierarchical and combined catalogs have essentially the same shape, while the MF of the percolation catalog shows a flattening towards large masses. However, the difference decreases if we do not consider the most massive groups, for which reliable results come from galaxy cluster studies. After having estimated the mass contained within the central, presumably virialized, regions of groups by adopting a reduction in mass of 30-40%, we do a comparison with the results coming from the virial analysis of nearby rich clusters (Girardi et al. 1998). All three group MFs turn out to be a smooth extrapolation of the cluster MF at M<4x10^{14}msun, which is the completeness limit of the cluster sample. The resulting optical virial MF of galaxy systems, which extends over two orders of magnitude, is fitted to a Schechter expression with a slope of about -1.5 and a characteristic mass of about 3x10^{14}msun.
We present a recalibration of the luminosity-metallicity relation for gas-rich, star-forming dwarfs to magnitudes as faint as M$_R$ ~ -13. We use the Dopita et al. (2013) metallicity calibrations to calibrate the relation for all of the data in this analysis. In metallicity-luminosity space we find two sub-populations within a sample of high-confidence SDSS DR8 star-forming galaxies; 52% are metal-rich giants and 48% are metal-medium galaxies. Metal-rich dwarfs classified as tidal dwarf galaxy (TDG) candidates in the literature are typically of metallicity 12 + log(O/H) = 8.70 $pm$ 0.05, while SDSS dwarfs fainter than M$_R$ = -16 have a mean metallicity of 12 + log(O/H) = 8.28 $pm$ 0.10, regardless of their luminosity, indicating that there is an approximate floor to the metallicity of low luminosity galaxies. Our hydrodynamical simulations predict that TDGs should have metallicities elevated above the normal luminosity-metallicity relation. Metallicity can therefore be a useful diagnostic for identifying TDG candidate populations in the absence of tidal tails. At magnitudes brighter than M$_R$ ~ -16 our sample of 53 star-forming galaxies in 9 HI gas-rich groups is consistent with the normal relation defined by the SDSS sample. At fainter magnitudes there is an increase in dispersion in metallicity of our sample, suggestive of a wide range of HI content and environment. In our sample we identify three (16% of dwarfs) strong TDG candidates (12 + log(O/H) > 8.6), and four (21%) very metal poor dwarfs (12 + log(O/H) < 8.0), which are likely gas-rich dwarfs with recently ignited star formation.
Using data taken as part of the Bluedisk project we study the connection between neutral hydrogen (HI) in the environment of spiral galaxies and that in the galaxies themselves. We measure the total HI mass present in the environment in a statistical way by studying the distribution of noise peaks in the HI data cubes obtained for 40 galaxies observed with WSRT. We find that galaxies whose HI mass fraction is high relative to standard scaling relations have an excess HI mass in the surrounding environment as well. Gas in the environment consists of gas clumps which are individually below the detection limit of our HI data. These clumps may be hosted by small satellite galaxies andor be the high-density peaks of a more diffuse gas distribution in the inter-galactic medium. We interpret this result as an indication for a picture in which the HI-rich central galaxies accrete gas from an extended gas reservoir present in their environment.
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