HI Power Spectra and the Turbulent ISM of Dwarf Irregular Galaxies

TL;DR

This study analyzes the spatial power spectra of HI in 24 dwarf irregular galaxies, revealing how turbulence, thermal effects, and galaxy properties influence the interstellar medium's structure.

Contribution

It provides the first detailed analysis of HI power spectra across a sample of dwarf irregular galaxies, linking turbulence characteristics to galaxy luminosity and star formation activity.

Findings

Inner disks have more cooler HI than outer disks.

Lower luminosity galaxies tend to have steeper power spectra.

No correlation found between turbulence spectral indices and star formation rate.

Abstract

HI spatial power spectra (PS) were determined for a sample of 24 nearby dwarf irregular galaxies selected from the LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes - The HI Nearby Galaxy Survey) sample. The two-dimensional (2D) power spectral indices asymptotically become a constant for each galaxy when a significant part of the line profile is integrated. For narrow channel maps, the PS become shallower as the channel width decreases, and this shallowing trend continues to our single channel maps. This implies that even the highest velocity resolution of 1.8 km/s is not smaller than the thermal dispersion of the coolest, widespread HI component. The one-dimensional PS of azimuthal profiles at different radii suggest that the shallower PS for narrower channel width is mainly contributed by the inner disks, which indicates that the inner disks have proportionally more cooler HI than the outer disks. Galaxies with lower luminosity (M_B > -14.5 mag) and star formation rate (SFR, log(SFR (M\odot/yr)) < -2.1) tend to have steeper PS, which implies that the HI line-of-sight depths can be comparable with the radial length scales in low mass galaxies. A lack of a correlation between the inertial-range spectral indices and SFR surface density implies that either non-stellar power sources are playing a fundamental role in driving the interstellar medium (ISM) turbulent structure, or the nonlinear development of turbulent structures has little to do with the driving sources.