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