The IRAM-30m EMPIRE Nearby Galaxy Dense Gas Survey

Abstract

This work presents an overview of the main results from the EMIR Multi-Line Probe of the ISM Regulating Galaxy Evolution (EMPIRE) survey, an IRAM-30m Large Program (∼ 500 h) which observed tracers of high density molecular gas across the disks of nine nearby, star-forming galaxies. EMPIRE is the first comprehensive and systematic study mapping high-density tracers including HCN, HCO+ and HNC, as well as optically thin J = 1−0 transitions of 13CO and C18O. Such a combination of spectroscopic tracers offers the best way to study cold, immediately star-forming gas to address how dense gas fractions and star formation efficiencies vary across and among galaxies. The extensive and sensitive data collected from EMPIRE has allowed us to relate the fraction of star-forming gas and its ability to form stars to local interstellar medium (ISM) and dynamical conditions, such as stellar surface densities, ISM dynamical pressure or molecular gas surface densities. The main results from EMPIRE show that the star formation efficiency in the dense gas varies systematically in all galactic disks. Therefore, this provides support for a context-dependent role of gas density, where dense gas fractions follow interstellar pressure, but star formation only takes place in local over-densities. The EMPIRE survey has successfully turned into a stepping stone for on-going and future projects, aiming to link the large-scale EMPIRE extragalactic results to high-resolution measurements, accessible from our Milky Way.