A Molecular Star Formation Law in the Atomic-Gas-Dominated Regime in Nearby Galaxies

Abstract

We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using 21 cm line atomic hydrogen (H i) data, mostly from THINGS, we predict the local mean CO velocity based on the mean H i velocity. By re-normalizing the CO velocity axis so that zero corresponds to the local mean H i velocity we are able to stack spectra coherently over large regions. This enables us to measure CO intensities with high significance as low as, an improvement of about one order of magnitude over previous studies. We detect CO out to galactocentric radii r gal ∼ r 25 and find the CO radial profile to follow a remarkably uniform exponential decline with a scale length of ∼0.2 r 25 . Here we focus on stacking as a function of radius, comparing our sensitive CO profiles to matched profiles of H i, Hα, far-UV (FUV), and Infrared (IR) emission at 24 μm and 70 μm. We observe a tight, roughly linear relationship between CO and IR intensity that does not show any notable break between regions that are dominated by molecular gas (Σ H 2 > Σ H i ) and those dominated by atomic gas (Σ H 2 < Σ H i ). We use combinations of FUV + 24 μm and Hα + 24 μm to estimate the recent star formation rate (SFR) surface density, Σ SFR , and find approximately linear relations between Σ SFR and Σ H 2 . We interpret this as evidence of stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H 2 ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relationships between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between atomic and molecular gas depends strongly on the total gas surface density and galactocentric radius. It must also depend on additional parameters. Our results reinforce and extend to lower surface densities, a picture in which star formation in galaxies can be separated into two processes: the assembly of star-forming molecular clouds and the formation of stars from H 2 . The interplay between these processes yields a total gas-SFR relation with a changing slope, which has previously been observed and identified as a star formation threshold.

Dusty star-forming galaxies at high redshift

“…These authors found that the SFR is principally correlated with H 2 gas (as traced by CO (J=2-1) in this case), and unassociated with HI in nearby galaxies. This result was extended to the atomic gas-dominated outskirts of nearby galaxies by Schruba et al (2011). A principle result from these groups is that the SFR is linearly related to the molecular gas surface density on resolved (a few hundred pc) scales, when assuming a constant line ratio from CO (J=2-1) to (J=1-0) (with Σ SFR ), as well as a constant X CO .…”

Cool Gas in High-Redshift Galaxies

“…The molecular gas phase is thought to immediately preceed star formation (e.g. , Schruba et al 2011) and this phase is thus most relevant to study a galaxy's potential to form new stars. The other phases of the ISM cannot form stars directly (but see, e.g., Glover & Clark 2012), unless they cool sufficiently to form cold and dense molecular gas.…”

The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function

“…Green pixels are the data of Bolatto et al (2011) for the SMC, but with each pixel representing a 12 pc aperture; green squares and circles are the same data set, but averaged over 200 pc and 1 kpc apertures instead. Red points are averages over azimuthal rings, with widths from 220 − 1800 pc depending on the distance of the target, in nearby spiral galaxies from Schruba et al (2011). The size of the symbol indicates whether Schruba et al classify the detection as strong or marginal.…”