The H i column density distribution function in faint dwarf galaxies

• Published in: Monthly notices of the Royal Astronomical Society 2013-02, Vol.429 (2), p.1596-1601
• Main Authors: Patra, Narendra Nath, Chengalur, Jayaram N., Begum, Ayesha
• Format: Article
• Language: English
• Subjects: Astrophysics; Density; Dwarf stars; Red shift; Star & galaxy formation; Universe
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sts440

We present the H i column density distribution function, f(N H i), as measured from dwarf galaxies observed as part of the Faint Irregular Galaxy Giant Metrewave Radio Telescope (GMRT) Survey (FIGGS). We find that the shape of the dwarf galaxy f(N H i) is significantly different from the f(N H i) for high-redshift damped Lyman α absorbers (DLAs) or the f(N H i) for a representative sample of z = 0 gas-rich galaxies. The dwarf f(N H i) falls much more steeply at high H i column densities as compared to the other determinations. While ∼10 per cent of the cross-section above N H i = 1020.3 atoms cm-2 at z = 0 is provided by dwarf galaxies, the fraction falls to 1 per cent by N H i ∼ 1021.5 atoms cm-2. In the local universe, the contribution to the high N H i end of the f(N H i) distribution comes predominantly from the inclined discs of large galaxies. Dwarf galaxies, both because of their smaller scalelengths and their larger intrinsic axial ratios, do not produce large H i column densities even when viewed edge-on. If high-column-density DLAs/Gamma Ray Burst (GRB) hosts correspond to galaxies like the local dwarfs, this would require that either (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.