Self-consistent modelling of aromatic dust species and extinction curves
  in galaxy evolution
  We formulate and calculate the evolution of dust in a galaxy focusing on the
distinction among various dust components -- silicate, aromatic carbon, and
non-aromatic carbon. We treat the galaxy as a one-zone object and adopt the
evolution model of grain size distribution developed in our previous work. We
further include aromatization and aliphatization (inverse reaction of
aromatization). We regard small aromatic grains in a radius range of 3--50 \AA\
as polycyclic aromatic hydrocarbons (PAHs). We also calculate extinction curves
in a consistent manner with the abundances of silicate and aromatic and
non-aromatic carbonaceous dust. Our model nicely explains the PAH abundance as
a function of metallicity in nearby galaxies. The extinction curve become
similar to the Milky Way curve at age $\sim$ 10 Gyr, in terms of the carbon
bump strength and the far-ultraviolet slope. We also apply our model to
starburst galaxies by shortening the star formation time-scale (0.5 Gyr) and
increasing the dense-gas fraction (0.9), finding that the extinction curve
maintains bumpless shapes (because of low aromatic fractions), which are
similar to the extinction curves observed in the Small Magellanic Cloud and
high-redshift quasars. Thus, our model successfully explains the variety in
extinction curve shapes at low and high redshifts.
