Molecular Gas Velocity Dispersion in Nearby Galaxies

Abstract

Despite the fact that molecular gas in galaxies is the most essential ingredient for the star formation process, its thorough characterization has not yet been accomplished. A common assumption is that molecular gas emission (mostly traced by CO) arises from molecular clouds with observed velocity dispersions of 2-5 km/s. In this thesis, I present the results obtained from investigating the velocity dispersions measured in the molecular gas disks of nearby galaxies. On 0.5 kpc scales (the average spatial resolution), the measured CO velocity dispersions have a mean value of ~12 km/s (1sigma dispersion of 3.9 km/s). These values are higher than previously expected, and are comparable to those measured for neutral atomic gas. To investigate the origin of these large dispersions, a comparison between interferometric and single-dish line width measurements for NGC 4736 and NGC 5055 (at ~0.5 kpc resolution) and for the neighboring Andromeda galaxy, M 31, (at ~100 pc resolution) is presented. Despite the different scales studied, the single-dish line widths are ~50% greater than the corresponding interferometric ones. Additionally, the interferometer recovers only a fraction (50 – 90%) of the flux that is measured by the single-dish. After stacking the high-sensitivity M 31 data, an analysis of the resulting spectral profiles from the two distinct instruments is performed in detail. The results are that single-dish spectra are better described by two components, one narrow (FWHMN !7.5 +- 0.4 km/s) and one broad (FWHMB ~ 14.4+-1.5 km/s); while for the interferometric data, one component suffices (FWHM ~7.1+-0.4 km/s). The overall implication is that molecular gas is present in two distinct phases: one that is clumpy and organized as molecular clouds, and another one that is more diffuse and has larger velocity dispersions.