THE CO-TO-H-2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES

Abstract

We present similar to kiloparsec spatial resolution maps of the CO-to-H-2 conversion factor (alpha(CO)) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for alpha(CO) and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H I column density to solve for both alpha(CO) and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, (CO)-C-12 J = (2-1) maps from the IRAM 30 m large program HERACLES, and H I 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our alpha(CO) results on the more typically used (CO)-C-12 J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for alpha(CO) and the DGR. On average, alpha(CO) = 3.1M(circle dot) pc(-2) (K km s(-1))(-1) for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of alpha(CO) as a function of galactocentric radius. However, most galaxies exhibit a lower alpha(CO) value in the central kiloparsec-a factor of similar to 2 below the galaxy mean, on average. In some cases, the central alpha(CO) value can be factors of 5-10 below the standard Milky Way (MW) value of alpha(CO,MW) = 4.4 M-circle dot pc(-2) (K km s(-1))(-1). While for alpha(CO) we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate alpha(CO) for studies of nearby galaxies.