Tracing cold HI gas in nearby, low-mass galaxies

Abstract

We analyze line-of-sight atomic hydrogen (H I) line profiles of 31 nearby, low-mass galaxies selected from the Very Large Array - ACS Nearby Galaxy Survey Treasury (VLA-ANGST) and The H I Nearby Galaxy Survey (THINGS) to trace regions containing cold (T ≲ 1400K) H I from observations with a uniform linear scale of 200pcbeam-1. Our galaxy sample spans four orders of magnitude in total H I mass and nine magnitudes in MB . We fit single and multiple component functions to each spectrum to isolate the cold, neutral medium given by a low-dispersion (<6km s-1) component of the spectrum. Most H I spectra are adequately fit by a single Gaussian with a dispersion of 8-12kms-1. Cold H I is found in 23 of 27 (85%) galaxies after a reduction of the sample size due to quality-control cuts. The cold H I contributes 20% of the total line-of-sight flux when found with warm H I. Spectra best fit by a single Gaussian, but dominated by cold H I emission (i.e., have velocity dispersions of <6kms-1), are found primarily beyond the optical radius of the host galaxy. The cold H I is typically found in localized regions and is generally not coincident with the very highest surface density peaks of the global H I distribution (which are usually areas of recent star formation). We find a lower limit for the mass fraction of cold-to-total H I gas of only a few percent in each galaxy. © 2012 The American Astronomical Society. All rights reserved.