Interstellar medium properties and star formation in nearby galaxies

Doctoral Thesis


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University of Cape Town

We study the properties of the interstellar medium (ISM) of nearby galaxies from The HI Nearby Galaxy Survey (THINGS) by analyzing the shapes of their HI emission velocity profiles. We apply a stacking method to increase the signal-to-noise (S/N) of the profiles and obtain what we call a super profile. We quantify all the relevant systematic effects that could confuse the interpretation of the shapes of the super profiles. We identify a sample of 22 galaxies from THINGS that are mostly free from these effects and analyze their super profile shapes. We derive super profiles from the entire HI disks of galaxies, inside and outside the optical radius r25, as a function of radius, column density, and star formation rate surface density. The super profiles can be described as the sum of a narrow and a broad Gaussian components. We associate the narrow component with the cold HI phase of the ISM and the broad component with the warm HI phase of the ISM. We find that the shapes of the super profiles correlate with star formation indicators such as metallicity, HK and far-UV near-UV colors. We also find that the mass fraction of the narrow component tends to be higher inside the optical radius r25. In addition, the velocity dispersions of the narrow and broad components decline exponentially with radius for virtually all the analyzed galaxies. Moreover, the flux ratio or mass ratio of the narrow and broad components, which serves as an estimate of the fraction of HI gas in the cold phase, tends to decrease with radius. Our results also show that regions having higher narrow component fractions usually corresponds to regions of higher HI or total (HI+H2) gas column density. Furthermore, the velocity dispersions of the broad and narrow components increase with increasing column density and star formation rate surface density. We have also investigated the physical mechanisms that can explain the observed width of the super profiles. These are supernova explosions (SNe), thermal effects from ultra-violet (UV) photons and magnetorotational instability (MRI). We find that SNe can explain the observed width of the super profile within the star forming disk (r25) and our data implies a supernova efficiency between 0.01 and 0.1. In the outer disk, the observed width of the super profiles can be attributed to thermal effects from extragalactic background UV photons. Finally, in most cases, MRI is not sufficient to explain the width of the super profile.

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