The EMPIRE Survey: Systematic Variations in the Dense Gas Fraction and Star Formation Efficiency from Full-Disk Mapping of M51

TL;DR

The EMPIRE survey maps dense molecular gas across galaxy disks, revealing how dense gas fraction and star formation efficiency vary with environment, supporting models where pressure influences dense gas formation and star formation.

Contribution

First full-disk maps of dense gas tracers in multiple galaxies showing environmental dependence of dense gas fraction and star formation efficiency.

Findings

Dense gas correlates with recent star formation.

High surface density regions have higher dense gas fractions.

Dense gas star formation efficiency is lower in high-density regions.

Abstract

We present the first results from the EMPIRE survey, an IRAM large program that is mapping tracers of high density molecular gas across the disks of nine nearby star-forming galaxies. Here, we present new maps of the 3-mm transitions of HCN, HCO+, and HNC across the whole disk of our pilot target, M51. As expected, dense gas correlates with tracers of recent star formation, filling the "luminosity gap" between Galactic cores and whole galaxies. In detail, we show that both the fraction of gas that is dense, f_dense traced by HCN/CO, and the rate at which dense gas forms stars, SFE_dense traced by IR/HCN, depend on environment in the galaxy. The sense of the dependence is that high surface density, high molecular gas fraction regions of the galaxy show high dense gas fractions and low dense gas star formation efficiencies. This agrees with recent results for individual pointings by Usero et al. 2015 but using unbiased whole-galaxy maps. It also agrees qualitatively with the behavior observed contrasting our own Solar Neighborhood with the central regions of the Milky Way. The sense of the trends can be explained if the dense gas fraction tracks interstellar pressure but star formation occurs only in regions of high density contrast.