Resolved observations in the Milky Way and integrated measurements in nearby galaxies have shown that the presence of molecular gas, in particular dense molecular gas, is linked to the presence of star formation. This trend, however, has a large amount of scatter suggesting that additional physical parameters beyond the amount of dense gas play a role in setting this relationship. Disentangling these parameters requires resolved measurements of dense molecular gas in environments with a much wider range of conditions than found in the Milky Way. A obvious population to do this experiment in is nearby (<20Mpc) galaxies where one can obtain sensitive, relatively high resolution (<500pc) observations. These observations were extremely difficult to obtain with the previous generation of millimeter telescopes because the brightest dense molecular gas tracers (HCN and HCO+) are fainter than CO by a factor of 10-30. Fortunately, the significantly increased sensitivity of new instruments like ALMA, the IRAM 30m, and the GBT 4mm system are enabling resolved surveys of dense molecular gas in galaxies. In this talk, I will highlight what we are learning from these surveys, focusing on early results from Dense Extragalactic GBT+Argus Survey (DEGAS). This survey uses Argus, a 16 pixel focal plane array, and the Green Bank Telescope to map the dense molecular gas in the centers of 36 nearby galaxies. When complete it will represent the largest dense molecular gas survey to date. I also will highlight future directions for studying dense gas in nearby galaxies including combination of lower HCN/HCO+ data with high resolution CO data from surveys like PHANGS, as well as the science that could be enabled by potential future instrumentation like a 100 pixel 4mm focal plane array on the GBT and the next generation Very Large Array.
Next Steps Towards Understanding Star Formation: Mapping Dense Gas in Nearby Galaxies