The HI column density distribution function in faint dwarf galaxies

TL;DR

This study measures the HI column density distribution in faint dwarf galaxies, revealing a steeper decline at high densities compared to larger galaxies and implications for DLA observations.

Contribution

First measurement of the HI column density distribution function in dwarf galaxies, highlighting differences from larger galaxies and implications for high-redshift DLA interpretations.

Findings

Dwarf nhalls more steeply at high h han larger galaxies.

Dwarf galaxies contribute ~10% to high h\fnhut less at very high densities.

High h egions in DLAs likely arise from merging systems or biased sightlines.

Abstract

We present the HI column density distribution function,\fnh, as measured from dwarf galaxies observed as part of the Faint Irregular Galaxy GMRT (FIGGS) survey. We find that the shape of the dwarf galaxy \fnh\ is significantly different from the \fnh\ for high redshift Damped \lya\ absorbers (DLAs) or the \fnh\ for a representative sample of $z = 0$ gas rich galaxies. The dwarf \fnh\ falls much more steeply at high HI column densities as compared to the other determinations. While $\sim 10%$ of the cross section above $\nh = 10^{20.3} \acc$ at $z = 0$ is provided by dwarf galaxies, the fraction falls to $\lesssim 1%$ by $\nh \sim 10^{21.5} \acc.$ In the local universe, the contribution to the high \nh\ end of the \fnh\ distribution comes predominantly from the inclined disks of large galaxies. Dwarf galaxies, both because of their smaller scale lengths, and their larger intrinsic axial ratios do not produce large HI column densities even when viewed edge-on. If high column density DLAs/GRB hosts correspond to galaxies like the local dwarfs, this would require either that (i) the absorption arises from merging and not isolated systems or (ii) the observed lines of sight are strongly biased towards high column density regions.