Tracing Cold HI Gas in Nearby, Low-Mass Galaxies

TL;DR

This study investigates cold atomic hydrogen in 31 nearby low-mass galaxies, revealing that cold HI is prevalent, localized, and constitutes a small fraction of total HI, with implications for understanding galaxy gas phases.

Contribution

It provides the first systematic analysis of cold HI in low-mass galaxies using uniform high-resolution observations, identifying its distribution and relative mass fraction.

Findings

Cold HI detected in 85% of galaxies.

Cold HI contributes about 20% of line-of-sight flux.

Cold HI is often located beyond the optical radius.

Abstract

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