Cross correlation of Lyman-alpha absorbers with gas-rich galaxies

The HI Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated with known low redshift, low column density (N_HI <10^15 cm^-2) Lyman-alpha absorbers from the literature. The redshift-space correlation is found to be similar in strength to HIPASS galaxy self-clustering (correlation length s_0,ag=6+/-4 and s_0,gg=3.1+/-0.5 h^-1 Mpc respectively). In real-space the cross-correlation is stronger than the galaxy auto-correlation (correlation length r_0,ag=7.2+/-1.4 and r_0,gg=3.5+/-0.7 h^-1 Mpc respectively) on scales from 1-10 h^-1 Mpc, ruling out the mini-halo model for the confinement Lyman-alpha absorbers at the 99 percent confidence level. Provided that the cause of the strong cross-correlation is purely gravitational, the ratio of correlation lengths suggest that absorbers are embedded in dark matter haloes with masses log(M/Msun)=14.2 h^-1, similar to those of galaxy groups. The flattening of the cross-correlation at separations less than ~600 h^-1 kpc could correspond to the thickness of filaments in which absorbers are embedded. This work provides indirect statistical evidence for the notion that galaxy groups and large-scale filaments, particularly those that comprise gas-rich galaxies, are the dominant environments of low column density Lyman-alpha absorbers at z=0.Comment: 10 pages, 10 figures, accepted for publication in MNRAS, minor typos fixed and references update

Correlation of Low z Lyman-alpha Absorbers with HI-selected Galaxies

In this work, observational evidence for the connection between low column density Lyman-alpha absorbers and large-scale structure traced by gas-rich galaxies is investigated. The HI Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated with known low redshift, low column density Lyman-alpha absorbers from the literature. The absorber-galaxy cross-correlation function shows that on scales from 1 to 10 h^-1 Mpc, absorbers are imbedded in halos with masses similar to that of galaxy groups. This statistical evidence suggests that galaxy groups could be the dominant environment of low column density Lyman-alpha absorbers at z=0.Comment: 3 pages, 1 figure, to appear in IAU 199 conf. proc.: "Probing Galaxies through Quasar Absorption Lines," eds. Williams, Shu, Menar

Local Column Density Distribution Function from HI selected galaxies

The cross-section of sky occupied by a particular neutral hydrogen column density provides insight into the nature of Lyman-alpha absorption systems. We have measured this column density distribution at z=0 using 21-cm HI emission from a blind survey. A subsample of HI Parkes All Sky Survey (HIPASS) galaxies have been imaged with the Australia Telescope Compact Array (ATCA). The contribution of low HI mass galaxies 10^7.5 to 10^8 M_solar is compared to that of M_star (10^10 to 10^10.5 M_solar) galaxies. We find that the column density distribution function is dominated by low HI mass galaxies with column densities in the range 3x10^18 to 2x10^20 cm^-2. This result is not intuitively obvious. M_star galaxies may contain the bulk of the HI gas, but the cross-section presented by low HI mass galaxies 10^7.5 to 10^8 M_solar is greater at moderate column densities. This result implies that moderate column density Lyman-alpha absorption systems may be caused by a range of galaxy types and not just large spiral galaxies as originally thought.Comment: 5 pages, including 1 figure. To appear in "Extragalactic Gas at Low Redshift" (ASP Conf. Series, Weymann Conf.

The Column Density Distribution Function at z=0 from HI Selected Galaxies

We have measured the column density distribution function, f(N), at z=0 using 21-cm HI emission from galaxies selected from a blind HI survey. f(N) is found to be smaller and flatter at z=0 than indicated by high-redshift measurements of Damped Lyman-alpha (DLA) systems, consistent with the predictions of hierarchical galaxy formation. The derived DLA number density per unit redshift, dn/dz =0.058, is in moderate agreement with values calculated from low-redshift QSO absorption line studies. We use two different methods to determine the types of galaxies which contribute most to the DLA cross-section: comparing the power law slope of f(N) to theoretical predictions and analysing contributions to dn/dz. We find that comparison of the power law slope cannot rule out spiral discs as the dominant galaxy type responsible for DLA systems. Analysis of dn/dz however, is much more discriminating. We find that galaxies with log M_HI < 9.0 make up 34% of dn/dz; Irregular and Magellanic types contribute 25%; galaxies with surface brightness > 24 mag arcsec^{-2} account for 22% and sub-L* galaxies contribute 45% to dn/dz. We conclude that a large range of galaxy types give rise to DLA systems, not just large spiral galaxies as previously speculated.Comment: 13 pages, low resolution figures in the appendix, MNRAS accepte

Evolution of damped Lyman alpha kinematics and the effect of spatial resolution on 21-cm measurements

We have investigated the effect of spatial resolution on determining pencil-beam like velocity widths and column densities in galaxies. Three 21-cm datasets are used, the HIPASS galaxy catalogue, a subset of HIPASS galaxies with ATCA maps and a high-resolution image of the LMC. Velocity widths measured from 21-cm emission in local galaxies are compared with those measured in intermediate redshift Damped Lyman alpha (DLA) absorbers. We conclude that spatial resolution has a severe effect on measuring pencil-beam like velocity widths in galaxies. Spatial smoothing by a factor of 240 is shown to increase the median velocity width by a factor of two. Thus any difference between velocity widths measured from global profiles or low spatial resolution 21-cm maps at z=0 and DLAs at z>1 cannot unambiguously be attributed to galaxy evolution. The effect on column density measurements is less severe and the values of dN/dz from local low-resolution 21-cm measurements are expected to be overestimated by only ~10 per cent.Comment: 5 pages, 6 figures, accepted for publication in MNRAS letter

Theoretical study of an LAE-CIV absorption pair at z = 5.7

We present a theoretical model to predict the properties of an observed $z =$ 5.72 Lyman $\alpha$ emitter galaxy - CIV absorption pair separated by 1384 comoving kpc/h. We use the separation of the pair and an outflow velocity/time travelling argument to demonstrate that the observed galaxy cannot be the source of metals for the CIV absorber. We find a plausible explanation for the metal enrichment in the context of our simulations: a dwarf galaxy with $M_{\star} =$ 1.87 $\times$ 10$^{9} M_{\odot}$ located 119 comoving kpc/h away with a wind velocity of $\sim$ 100 km/s launched at $z \sim$ 7. Such a dwarf ($M_{\text{UV}} =$ - 20.5) is fainter than the detection limit of the observed example. In a general analysis of galaxy - CIV absorbers, we find galaxies with -20.5 $< M_{\text{UV}} <$ - 18.8 are responsible for the observed metal signatures. In addition, we find no correlation between the mass of the closest galaxy to the absorber and the distance between them, but a weak anti-correlation between the strength of the absorption and the separation of galaxy - absorber pairs.Comment: 5 pages, 4 figures. Accepted April 10, 2017 in MNRA