The gas-to-extinction ratio and the gas distribution in the Galaxy

Abstract

We investigate the relation between the optical extinction (AV) and the hydrogen column density (NH) determined from X-ray observations of a large sample of Galactic sightlines towards 35 supernova remnants, 6 planetary nebulae and 70 X-ray binaries for which NH was determined in the literature with solar abundances. We derive an average ratio of 〈NH/AV〉 = (2.08 ± 0.02) × 1021 Hcm-2 mag-1 for the whole Galaxy. We find no correlation between 〈NH/AV〉 and the number density of hydrogen, the distance away from the Galactic Centre and the distance above or below the Galactic plane. The 〈NH/AV〉 ratio is generally invariant across the Galaxy, with 〈NH/AV〉 = (2.04 ± 0.05) × 1021 Hcm-2 mag-1 for the first and fourth Galactic quadrants and 〈NH/AV〉 = (2.09±0.03)×1021H cm-2 mag-1 for the second and third Galactic quadrants. We also explore the distribution of hydrogen in the Galaxy by enlarging our sample with additional 74 supernova remnants for which both NH and distances are known.We find that, between the Galactic radius of 2 and 10 kpc, the vertical distribution of hydrogen can be roughly described by a Gaussian function with a scaleheight of h = 75.5 ± 12.4 pc and a mid-plane density of nH(0) = 1.11 ± 0.15 cm-3, corresponding to a total gas surface density of Σgas ~ 7.0M⊙ pc-2.We also compile NH from 19 supernova remnants and 29 X-ray binaries for which NH was determined with subsolar abundances. We obtain 〈NH/AV〉 = (2.47 ± 0.04) × 1021 Hcm-2 mag-1 which exceeds that derived with solar abundances by ~20 per cent. We suggest that in future studies one may simply scale NH derived from subsolar abundances by a factor of ~1.2 when converting to NH of solar abundances.