THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES.

We present ∼kiloparsec spatial resolution maps of the CO-to-H₂ 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, ¹²CO 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 ¹²CO 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. [ABSTRACT FROM AUTHOR]