Variations in the Star Formation Efficiency of the Dense Molecular Gas Across the Disks of Star-Forming Galaxies

Abstract

We present a new survey of HCN (1-0) emission, a tracer of dense molecular gas, focused on the littleexplored regime of normal star-forming galaxy disks. Combining HCN, CO, and infrared (IR) emission, we investigate the role of dense gas in star formation, finding systematic variations in both the apparent dense gas fraction (traced by the HCN-to-CO ratio) and the apparent star formation efficiency of dense gas (traced by the IR-to-HCN ratio). The latter may be unexpected, given the recent popularity of gas density threshold models to explain star formation scaling relations. Our survey used the IRAM 30-m telescope to observe HCN(1-0), CO (1-0), and several other emission lines across 29 nearby disk galaxies whose CO (2-1) emission has previously been mapped by the HERACLES survey. We detected HCN in 48 out of 62 observed positions. Because our observations achieve a typical resolution of ∼ 1.5 kpc and span a range of galaxies and galactocentric radii (56% lie at r gal > 1 kpc), we are able to investigate the properties of the dense gas as a function of local conditions in a galaxy disk. We focus on how the ratios IR-to-CO, HCN-to-CO, and IR-to-HCN (observational cognates of the star formation efficiency, dense gas fraction, and dense gas star formation efficiency) depend on the stellar surface density, Σ star , and the molecular-to-atomic gas ratio, Σ mol /Σ atom . The HCN-to-CO ratio is low, often ∼ 1/30, and correlates tightly with both the molecular-to-atomic ratio and the stellar mass surface density across a range of 2.1 dex (factor of ≈ 125) in both parameters. Thus for the assumption of fixed CO-to-H 2 and HCN-to-dense gas conversion factors, the dense gas fraction depends strongly on location in the disk, being higher in the high surface density, highly molecular parts of galaxies. At the same time, the IR-to-HCN ratio (closely related to the star formation efficiency of dense molecular gas) decreases systematically with these same parameters and is ∼ 6 − 8 times lower near galaxy centers than in the outer regions of the galaxy disks. For fixed conversion factors, these results are incompatible with a simple model in which star formation depends only on the amount of gas mass above some density threshold. Moreover, only a very specific set of environment-dependent conversion factors can render our observations compatible with such a model. Whole cloud models, such as the theory of turbulence regulated star formation, do a better job of matching our observations. We explore one such model in which variations in the Mach number driving many of the trends within galaxy disks while density contrasts drive the differences between disk and merging galaxies.

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

“…Usero et al (2015) found SFE dense to depend on environment, being lower at high stellar surface densities and high molecular fractions Chen et al (2015) observed qualitatively similar behavior in their analysis of M51, noting that the IR-to-HCN ratio drops with decreasing radius in the galaxy.…”

“…All data combined scatter about the ratio L TIR /L HCN ≈ 900 L ⊙ (K km s −1 pc 2 ) −1 (gray line; dashed lines show a factor of 3 scatter). (right) Zoom-in on data for the full-galaxy map of M51 and the pointings from Usero et al (2015). The typical uncertainty on the HCN luminosity for each line of sight is ∼ 10 5 K km s −1 pc 2 .…”

“…Red points show the binned trend for the M51 data; including upper limits. Blue points show results for the Usero et al (2015) sample. The disk pointings overlap results for our whole-galaxy map, though M51 has a somewhat lower IR-to-HCN ratio than other galaxies in the survey.…”

“…These surveys targeted whole bright galaxies. For individual pointings across 30 disk galaxies spanning a range of physical conditions, Usero et al (2015) showed that both f dense and SFE dense within their ∼ 1-2 kpc beam depended strongly on these conditions. Given the apparent dependence of f dense and SFE dense on local conditions, the natural next step must be complete, unbiased maps of a set of galaxies with a wide range of global and local conditions.…”

Origins of Scatter in the Relationship Between HCN 1-0 and Dense Gas Mass in the Galactic Center

“…In this case, HCN could systematically overestimate the total dense gas mass present. We thus recommend that observations of HCN 1-0 in extreme environments should be checked for systematically weak emission due to self-absorption (e.g., via comparison with tracers like HC 3 N or CH 3 CN) before seeking a physical explanation for a lack of dense gas such as variations in the gas depletion time (e.g., García-Burillo et al 2012;Juneau et al 2009;Leroy et al 2015;Usero et al 2015). Such studies will also be important for determining whether the Galactic center conditions responsible for variations in the HCN 1-0 to dense gas conversion factor are actually relevant for other galaxies, or whether deviations from the Gao-Solomon relation in other systems are due to entirely separate effects that are a result of environments that are not sampled in our own Galaxy.…”