HI and CO Velocity Dispersions in Nearby Galaxies

TL;DR

This study measures and compares the velocity dispersions of HI and CO gas in nearby galaxies, revealing a consistent ratio and evidence for both dense GMCs and diffuse gas components in galactic disks.

Contribution

It provides new high-resolution measurements of HI and CO velocity dispersions and analyzes their variation with flux and stacking methods, highlighting the presence of multiple gas components.

Findings

CO dispersion: 7.3 ± 1.7 km/s

HI dispersion: 11.7 ± 2.3 km/s

Dispersion ratio: 1.4 ± 0.2

Abstract

We analyze the velocity dispersions of individual HI and CO profiles in a number of nearby galaxies from the high-resolution HERACLES CO and THINGS HI surveys. Focusing on regions with bright CO emission, we find a CO dispersion value: 7.3 $\pm$ 1.7 km/s. The corresponding HI dispersion is 11.7 $\pm$ 2.3 km/s, yielding a mean HI/CO dispersion ratio of 1.4 $\pm$ 0.2, independent of radius. We find that the CO velocity dispersion increases towards lower peak fluxes. This is consistent with previous work where we showed that when using spectra averaged ("stacked") over large areas, larger values for the CO dispersion are found, and a lower dispersion ratio: 1.0 $\pm$ 0.2. The stacking method is more sensitive to low-level diffuse emission, whereas individual profiles trace narrow-line, GMC-dominated, bright emission. These results provide further evidence that disk galaxies contain not only a thin, low velocity dispersion, high density CO disk that is dominated by GMCs, but also a fainter, higher dispersion, diffuse disk component.