%0 Generic
%A Jimenez Donaire, Maria Jesus
%D 2017
%F heidok:23788
%R 10.11588/heidok.00023788
%T Dense Gas and the Insterstellar Medium in Nearby Galaxies: the interplay between dense gas and the galactic environment
%U https://archiv.ub.uni-heidelberg.de/volltextserver/23788/
%X Galaxies act like engines converting gas into stars, which in turn produce the matter around us, laying the foundations of life. Studying the composition of this interstellar gas informs our understanding of how star formation proceeds, while also providing insight into the structure and evolution of our own Galaxy and the Universe. Research has found a strong connection between star formation and the molecular gas within galaxies.  However, the most easily accessed molecular observables, such as carbon monoxide (CO) emission, only probe low-density gas in extragalactic systems, allowing us to scratch the surface of the star-forming structures.     Molecules which trace denser gas, such as hydrogen cyanide (HCN), are more challenging to observe but probe the immediate sites of star formation. In this thesis I analyze data from the first survey mapping the entire star forming disk of a sample of nearby galaxies in a suite of dense gas tracers (EMPIRE), to understand the interplay between dense gas and a wide range of galactic environments, distinct from the Milky Way. By studying the content of this dense gas in nearby galaxies I find surprising results: systematic variations of the dense gas fraction and its efficiency to form stars within and among galaxies. While more dense gas is located in regions of high interstellar pressure, this dense gas is less efficient to form stars.    To characterize the dense gas, I also explore how changes in the optical depth can affect the effective gas densities where the dense gas tracers emit, a new measurement in the disks normal star-forming galaxies. To better understand the bulk, lower-density molecular gas out of which the dense gas eventually forms, I also analyze CO line ratios  to constrain carbon isotope abundances. I find the abundance to vary systematically within galaxy disks, likely due to strong fractionation effects. The results obtained in this thesis support a model where star formation depends strongly on host galaxy and the local galactic environment in the disk.