Speaker
Anahi Caldu Primo
(MPIA)
Description
We present evidence for the presence of a high-dispersion molecular gas component in nearby galaxies as traced through the CO(2-1) emission. The measured dispersion is higher than predicted if it was just due to the cold dense molecular gas phase in a galaxy's disk. Our analysis is based on a comprehensive study of the velocity dispersions of the atomic and molecular gas components in the disks (R < R25) of a sample of 12 nearby galaxies. To obtain reliable measurements of the velocity dispersion, we stack regions after accounting for intrinsic velocity shifts due to rotation and large-scale galactic motions. We stack using various parameters: galacto-centric distance, star formation rate surface density, HI surface density, H2 surface density, and total gas surface density. We fit single-Gaussian components and measure average velocity dispersions for the atomic gas of 11.9 +- 3.1 km/s and for the molecular gas of 12.0 +- 3.9 km/s. The CO velocity dispersions are thus similar to the corresponding ones of HI, witha an average ratio of Sigma(HI)/Sigma(CO)= 1.0 +- 0.2, irrespective of the stacking parameter. The measured CO velocity dispersions are significantly higher (by at least a factor of ~2) than expected from just the cold molecular gas phase that is thought to be associated with star formation in the disk. Such a high dispersion implies an additional thick molecular gas disk (possibly as thick as the HI disk).
Author
Anahi Caldu Primo
(MPIA)