The Shapes of the H I Velocity Profiles of the THINGS Galaxies

We analyze the shapes of the H I velocity profiles of The H I Nearby Galaxy Survey to study the phase structure of the neutral interstellar medium and its relation to global galaxy properties. We use a method analogous to the stacking method sometimes used in high-redshift H I observations to construct high-signal-to-noise (S/N) profiles. We call these high-S/N profiles super profiles . We analyze and discuss possible systematics that may change the observed shapes of the super profiles. After quantifying these effects and selecting a subsample of unaffected galaxies, we find that the super profiles are best described by a narrow and a broad Gaussian component, which are evidence of the presence of the cold neutral medium and the warm neutral medium. The velocity dispersion of the narrow component ranges from ~3.4 to ~8.6 km s –1 with an average of 6.5 ± 1.5 km s –1 , whereas that of the broad component ranges from ~10.1 to ~24.3 km s –1 with an average of 16.8 ± 4.3 km s –1 . We find that the super profile parameters correlate with star formation indicators such as metallicity, far-UV–near-UV colors, and Hα luminosities. The flux ratio between the narrow and broad components tends to be highest for high-metallicity, high-star-formation-rate galaxies. We show that the narrow component identified in the super profiles is associated with the presence of star formation, and possibly with molecular hydrogen.