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