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

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.

13CO/C18O Gradients across the Disks of Nearby Spiral Galaxies

“…This includes O data now exist for the nine galaxies listed in Table 1. Bigiel et al (2016) described the basic EMPIRE observation and reduction strategy. In brief, we observed each galaxy in onthe-fly mode using the 3mm band of the dual-polarization EMIR receiver (Carter et al 2012) and the Fourier transform spectrometers.…”

“…EMPIRE (Bigiel et al 2016) is mapping a suite of molecular lines across the whole area of nine nearby galaxy disks using the IRAM 30-m telescope. This includes O data now exist for the nine galaxies listed in Table 1.…”

“…These are targets of the IRAM large program EMPIRE (EMIR Multiline Probe of the ISM Regulating Galaxy Evolution; Bigiel et al (2016)) and a related ALMA program (M. Gallagher et al 2017, in preparation;Jiménez-Donaire et al 2017). …”

High Dense Gas Fraction in a Gas-rich Star-forming Galaxy at z = 1.2∗

“…Concurrent observations of other dense gas tracers are also important, to confirm the utility of HCN as a dense gas tracer, and are well within the reach of ALMA and NOEMA's wide bandwidth receivers. Finally, spatially resolved studies of dense gas star formation in the local Universe, such as the ongoing EMPIRE survey (Bigiel et al 2016), will help to clarify the role of turbulenceregulated or density threshold models in star formation.…”

“…Spatially resolved studies of the star-forming regions in local galaxies show that L FIR /L HCN is strongly dependent on environment, and anti-correlated with L HCN /L CO (e.g., Usero et al 2015;Bigiel et al 2016). L FIR /L HCN is lower in galaxy nuclear regions, which typically show the highest L HCN /L CO , and increases in regions of low L HCN /L CO ; the overall L CO /L FIR remains nearly constant.…”