THE CO-TO-H2CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES

Abstract

We present ∼kiloparsec spatial resolution maps of the CO-to-H 2 conversion factor (α CO ) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for α 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 α 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, 12 CO 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 α CO results on the more typically used 12 CO 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 α CO and the DGR. On average, α CO = 3.1 M ☉ 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 α CO as a function of galactocentric radius. However, most galaxies exhibit a lower α CO value in the central kiloparsec—a factor of ∼2 below the galaxy mean, on average. In some cases, the central α CO value can be factors of 5–10 below the standard Milky Way (MW) value of α CO, MW = 4.4 M ☉ pc −2 (K km s −1 ) −1 . While for α 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 α CO for studies of nearby galaxies.