TRACING COLD H I GAS IN NEARBY, LOW-MASS GALAXIES

Abstract

We analyze line-of-sight atomic hydrogen (HI) line profiles of 31 nearby,\nlow-mass galaxies selected from the Very Large Array - ACS Nearby Galaxy Survey\nTreasury (VLA-ANGST) and The HI Nearby Galaxy Survey (THINGS) to trace regions\ncontaining cold (T $\\lesssim$ 1400 K) HI from observations with a uniform\nlinear scale of 200 pc/beam. Our galaxy sample spans four orders of magnitude\nin total HI mass and nine magnitudes in M_B. We fit single and multiple\ncomponent functions to each spectrum to isolate the cold, neutral medium given\nby a low dispersion (<6 km/s) component of the spectrum. Most HI spectra are\nadequately fit by a single Gaussian with a dispersion of 8-12 km/s. Cold HI is\nfound in 23 of 27 (~85%) galaxies after a reduction of the sample size due to\nquality control cuts. The cold HI contributes ~20% of the total line-of-sight\nflux when found with warm HI. Spectra best fit by a single Gaussian, but\ndominated by cold HI emission (i.e., have velocity dispersions <6 km/s) are\nfound primarily beyond the optical radius of the host galaxy. The cold HI is\ntypically found in localized regions and is generally not coincident with the\nvery highest surface density peaks of the global HI distribution (which are\nusually areas of recent star formation). We find a lower limit for the mass\nfraction of cold-to-total HI gas of only a few percent in each galaxy.\n