Observing the Physical State of the Cold Gas in Nearby Galaxies
Thanks to new and revitalized facilities including ALMA, the IRAM telescopes, and the GBT, we are now able to access the physical conditions in the cold gas of other galaxies in ways previously reserved to studies of the Milky Way and Local Group galaxies. Using multi-line spectroscopy we are able to constrain the gas density distribution in each part of a galaxy, while using high resolution CO imaging we can measure the structure (density, turbulence, and self-gravity) of the interstellar medium on the scale of individual star-forming clouds across all types of galaxies. The ability to make such measurements over a large part of a star forming galaxy is new in the last few years, thanks to the IRAM programs EMPIRE and PAWS, the beginning of ALMA operations (and our PHANGS CO survey), and upgrades to the GBT. I will show how the amount of dense gas changes across the disk of galaxies, and illustrate a more nuanced role for gas density in star formation than commonly asserted. I will also show how the basic cloud-scale structure of the cold interstellar medium changes from galaxy-to-galaxy, reflecting the local environment, and I will show how these local structural conditions relate to gas density and star formation in the best studied nearby galaxies. I will highlight first results of work by the PHANGS team, which will use ALMA to image the cloud scale ISM across the local galaxy population over the next few years.