The CO-to-H2 Conversion Factor and Dust-to-gas Ratio on Kiloparsec Scales in Nearby Galaxies

Sandstrom, K. M., Leroy, A. K., Walter, F., Bolatto, A. D., Croxall, K. V., Draine, B. T., Wilson, C. D., Wolfire, M., Calzetti, D., Kennicutt, R. C., Aniano, G., Donovan Meyer, J., Usero, A., Bigiel, F., Brinks, E., de Blok, W. J. G., Crocker, A., Dale, D., Engelbracht, C. W., Galametz, M., Groves, B., Hunt, L. K., Koda, J., Kreckel, K., Linz, H., Meidt, S., Pellegrini, E., Rix, H.-W., Roussel, H., Schinnerer, E., Schruba, A., Schuster, K.-F., Skibba, R., van der Laan, T., Appleton, P., Armus, L., Brandl, B., Gordon, K., Hinz, J., Krause, O., Montiel, E., Sauvage, M., Schmiedeke, A., Smith, J. D. T., & Vigroux, L.
2013, The Astrophysical Journal, 777, 5

We present ~kiloparsec spatial resolution maps of the CO-to-H2 conversion factor (ЮБCO) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for ЮБCO 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 ЮБCO 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 ЮБCO 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 ЮБCO and the DGR. On average, ЮБCO = 3.1 M т?? 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 ЮБCO as a function of galactocentric radius. However, most galaxies exhibit a lower ЮБCO value in the central kiloparsecт??a factor of ~2 below the galaxy mean, on average. In some cases, the central ЮБCO value can be factors of 5-10 below the standard Milky Way (MW) value of ЮБCO, MW = 4.4 M т?? pc-2 (K km s-1)-1. While for ЮБCO 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 ЮБCO for studies of nearby galaxies.

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Copyright © 2012 Karl D. Gordon All Rights Reserved