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

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 ∼< 1400 K) H I from observations with a uniform linear scale of 200 pc beam–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 (<6 km s–1) component of the spectrum. Most H I spectra are adequately fit by a single Gaussian with a dispersion of 8-12 km s–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 <6 km s–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.