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The Shapes of the HI Velocity Profiles of the THINGS Galaxies

We analyze the shapes of the HI velocity profiles of The HI Nearby Galaxy Survey (THINGS) to study the phase structure of the neutral interstellar medium (ISM) and its relation to global galaxy properties. We use a method analogous to the stacking method sometimes used in high redshift HI 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 sub-sample 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 (CNM) and the Warm Neutral Medium (WNM). The velocity dispersion of the narrow component range from ~3.4 to ~8.6 km/s with an average of 6.5+/-1.5 km/s, whereas that of the broad component range from ~10.1 to ~24.3 km/s with an average of 16.8+/-4.3 km/s. We find that the super profile parameters correlate with star formation indicators such as metallicity, FUV-NUV colors and H_alpha luminosities. The flux ratio between the narrow and broad components tends to be highest for high metallicity, high star formation rate (SFR) 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.