Internal extinction and gas column density in spiral galaxies

The method for the extinction correction based on the gas column density is tested. A positive correlation is found between the total gas column density and the extinction estimated using amodel based on energy conservation (‘frequencyconverter model’). Assuming a Solar Neighbourhood opticaldepth–to–gas ratio, extinction is predicted from the gas column density using a radiative transfer model which adopts a ‘Sandwich’ configuration for the stellar and dust distributions, and which takes also into account the effect of scattering. This prediction agreeswellwith the extinction estimated from the energy conservation consideration. This indicates that (1) the method for the extinction correction based on the gas column density is a robust one; (2) in the inner part of galaxy disks where most of the extinction occurs, the dust-to-gas ratio is on average about the same as the Solar Neighbourhood value. Our results also suggest that dust grains associatedwith both gas phases (HI and H2) participate in the extinction, with a relative importance depending on the abundance of the gas phase in the inner disk. For most of the galaxies in our sample the extinction is mainly due to the dust associated with the molecular gas because they have high molecular to atomic gas column density ratios. On the other hand, for galaxies whose gas column density is dominated by the atomic gas, the extinction seems to be mainly caused by the dust associated with atomic gas.