INTERNAL EXTINCTION AND GAS COLUMN DENSITY IN SPIRAL GALAXIES

The method for the extinction correction based on the gas column densi ty is tested. A positive correlation is found between the total gas co lumn density and the extinction estimated using a model based on energ y conservation ('frequency-converter model'). Assuming a Solar Neighbo urhood optical-depth-to-gas ratio, extinction is predicted from the ga s column density using a radiative transfer model which adopts a 'Sand wich' configuration for the stellar and dust distributions, and which takes also into account the effect of scattering. This prediction agre es well with the extinction estimated from the energy conservation con sideration. This indicates that (1) the method for the extinction corr ection based on the gas column density is a robust one; (2) in the inn er 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 v alue. Our results also suggest that dust grains associated with both g as phases (HI and H-2) participate in the extinction, with a relative importance depending on the abundance of the gas phase in the inner di sk. For most of the galaxies in our sample the extinction is mainly du e 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 atom ic gas.