The EMPIRE survey of dense gas in nearby galaxies - star formation law, star formation efficiency and line modeling

Johannes Puschnig
2019 Zenodo  
My recent research focus is dense gas physics in external galaxies, based on observations of the EMPIRE survey. EMPIRE is a ~600hr IRAM-30m large program to observe high-critical density molecular lines (e.g. HCN, HCO+, HNC) across entire star forming disks for a sample of nine nearby spiral galaxies. I will present key results from this survey, focusing on relations of dense gas fractions (HCN/CO) and dense gas star formation efficiencies (IR/HCN) with local conditions across the sample
more » ... s the sample galaxies. One of our key findings is a variable dense gas fraction and efficiency: while the dense gas fraction increases as expected with hydrostatic pressure, stellar and molecular gas surface density, star formation seems less efficient in such environments. I will discuss these results, which are at odds with a whole class of models triggered by Milky Way work. In addition, I will present dense gas tracer line ratios across galaxies and constraints on their optical depths using optically thin isotopologues. To first order, EMPIRE demonstrates that the conditions in a galaxy disk set the gas density distribution and that the dense gas traced by HCN shows an environment-dependent relation to star formation. Motivated by that finding, we have developed a comprehensive RADEX-based radiative transfer modeling approach to constrain physical quantities (e.g. density and temperature) from dense gas observations, taking into account that at ~kpc scale emission lines of CO and HCN emerge from a mixture of gas densities rather than from a single-density medium.
doi:10.5281/zenodo.3476231 fatcat:ffirbbi52fcmdjmrylisaehcbm