Thick gas discs in faint dwarf galaxies

Abstract

We determine the intrinsic axial ratio distribution of the gas discs of extremely faint M_B < −14.5 dwarf irregular galaxies. We start with the measured (beam corrected) distribution of apparent axial ratios in the HI 21-cm images of dwarf irregular galaxies observed as part of the Faint Irregular Galaxy GMRT Survey (FIGGS). Assuming that the discs can be approximated as oblate spheroids, the intrinsic axial ratio distribution can be obtained from the observed apparent axial ratio distribution. We use a variety of methods to do this, and our final results are based on using Lucy's deconvolution algorithm. This method is constrained to produce physically plausible distributions, and also has the added advantage of allowing for observational errors to be accounted for. While one might a priori expect that gas discs would be thin (because collisions between gas clouds would cause them to quickly settle down to a thin disc), we find that the HI discs of faint dwarf irregulars are quite thick, with mean axial ratio ‹q›^~ 0.6. While this is substantially larger than the typical value of ^~0.2 for the stellar discs of large spiral galaxies, it is consistent with the much larger ratio of velocity dispersion to rotational velocity (σ/v_c) in dwarf galaxy HI discs as compared to that in spiral galaxies. Our findings have implications for studies of the mass distribution and the Tully-Fisher relation for faint dwarf irregular galaxies, where it is often assumed that the gas is in a thin disc.