The EMPIRE survey: systematic variations in the dense gas fraction and star formation efficiency from full-disk mapping of M51

Frank Bigiel (1) , Adam Leroy (2) , Maria Jiménez-Donaire (1) , Jerome Pety (3) , Antonio Usero (4) , Diane Cormier (1) , Alberto Bolatto (5) , Santiago García-Burillo (4) , Dario Colombo (6) , Manuel González-García (7) , Annie Hughes (8, 9) , Amanda Kepley (10) , Carsten Kramer (11) , Karin Sandstrom (12) , Eva Schinnerer (13) , Andreas Schruba (14) , Karl Schuster (3) , Neven Tomicic (15) , Laura Zschaechner (15)

1 Institut für Theoretische Astrophysik, Zentrum für Astronomie, Universität Heidelberg
2 The Ohio State University Press
3 IRAM - Institut de RadioAstronomie Millimétrique
4 Observatorio Astronomico Nacional, Madrid
5 Department of Astronomy [College Park]
6 University of Alberta
7 IAA - Instituto de Astrofísica de Andalucía
8 IRAP - Institut de recherche en astrophysique et planétologie
9 UT - Université de Toulouse
10 NRAO - National Radio Astronomy Observatory [Charlottesville]
11 Instituto de Radio Astronomía Milimétrica
12 UC San Diego - University of California [San Diego]
13 MPIA - Max-Planck-Institut für Astronomie
14 MPE - Max-Planck-Institut für Extraterrestrische Physik
15 Max-Planck-Institut fur Astronomie, Konigstuhl 17, D-69117 Heidelberg, Germany

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, fdense traced by HCN/CO, and the rate at which dense gas forms stars, SFEdense 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

Domaines: Physique [physics]; Astrophysique [astro-ph]

Frank Bigiel, Adam Leroy, Maria Jiménez-Donaire, Jerome Pety, Antonio Usero, et al.. The EMPIRE survey: systematic variations in the dense gas fraction and star formation efficiency from full-disk mapping of M51. The Astrophysical journal letters, 2016, 822 (2), pp.L26. ⟨10.3847/2041-8205/822/2/L26⟩. ⟨hal-02318264⟩