The radial variation of HI velocity dispersions in dwarfs and spirals

TL;DR

This study investigates how HI velocity dispersions vary with radius in dwarf and spiral galaxies, revealing differences in gas dynamics and phases that influence star formation activity.

Contribution

It provides the first detailed comparison of HI velocity dispersion profiles and phases across galaxy types, highlighting the role of energy sources in outer disks.

Findings

Velocity dispersions decline exponentially in spirals but are flatter in dwarfs.

Outer disk velocity dispersions do not decrease as fast as star formation rates.

Dwarfs have a lower cold-to-warm HI ratio, possibly explaining their low star formation activity.

Abstract

Gas velocity dispersions provide important diagnostics of the forces counteracting gravity to prevent collapse of the gas. We use the 21 cm line of neutral atomic hydrogen (HI) to study HI velocity dispersion and HI phases as a function of galaxy morphology in 22 galaxies from The HI Nearby Galaxy Survey (THINGS). We stack individual HI velocity profiles and decompose them into broad and narrow Gaussian components. We study the HI velocity dispersion and the HI surface density, as a function of radius. For spirals, the velocity dispersions of the narrow and broad components decline with radius and their radial profiles are well described by an exponential function. For dwarfs, however, the profiles are much flatter. The single Gaussian dispersion profiles are, in general, flatter than those of the narrow and broad components. In most cases, the dispersion profiles in the outer disks do not drop as fast as the star formation profiles, derived in the literature. This indicates the importance of other energy sources in driving HI velocity dispersion in the outer disks. The radial surface density profiles of spirals and dwarfs are similar. The surface density profiles of the narrow component decline more steeply than those of the broad component, but not as steep as what was found previously for the molecular component. As a consequence, the surface density ratio between the narrow and broad components, an estimate of the mass ratio between cold HI and warm HI, tends to decrease with radius. On average, this ratio is lower in dwarfs than in spirals. This lack of a narrow, cold HI component in dwarfs may explain their low star formation activity.