Global Law for Dust-to-gas Ratio of Spiral Galaxies
  Using a one-zone model for the evolution of dust in spiral galaxies and applying the instantaneous recycling approximation to the model equations, we investigate dust-to-gas ratio of spiral galaxies. Four processes are considered; dust formation from heavy elements ejected by stellar mass loss, dust destruction in supernova remnants, dust destruction in star-forming regions, and accretion of heavy elements onto preexisting dust grains. The equations describe a simplified relation between dust-to-gas ratio and metallicity. The relation is independent of star formation rate. By comparing the theoretical relation with the observational data of nearby spiral galaxies, we show (i) that the accretion process onto the preexisting dust particles must be taken into account for the spiral galaxies; (ii) that the efficiency of dust production from heavy elements ejected by stars can be constrained by the spiral galaxies with low metallicity; (iii) that the Salpeter and Scalo initial mass functions are both consistent with the data of the spiral galaxies. 
H. Hirashita (Department of Astronomy, Kyoto University)