Spectroscopic evolution of dusty starburst galaxies

By using a one-zone chemical and spectrophotometric evolution model of a disk galaxy undergoing a dusty starburst, we investigate, numerically, the optical spectroscopic properties in order to explore galaxy evolution in distant clusters. We adopt an assumption that the degree of dust extinction (represented by $A_V$) depends on the ages of starburst populations in such a way that younger stars have larger $A_V$ (originally referred to as selective dust extinction by Poggianti & Wu 2000). In particular, we investigate how the time evolution of the equivalent widths of [OII]$\lambda$3727 and H$\delta$ is controlled by the adopted age dependence. This leads to three main results: (1) If a young stellar population (with the age of $\sim$ $10^6$ yr) is more heavily obscured by dust than an old one ($>$ $10^8$ yr), the galaxy can show an ``e(a)'' spectrum characterized by strong H$\delta$ absorption and relatively modest [OII] emission. (2) A dusty starburst galaxy with an e(a) spectrum can evolve into a poststarburst galaxy with an a+k (or k+a) spectrum 0.2 Gyr after the starburst and then into a passive one with a k-type spectrum 1 Gyr after the starburst. This result clearly demonstrates an evolutionary link between galaxies with different spectral classes (i.e., e(b), e(a), a+k, k+a, and k). (3) A dusty starburst galaxy can show an a+k or k+a spectrum even in the dusty starburst phase if the age-dependence of dust extinction is rather weak, i.e., if young starburst populations with different ages ($\le$ $10^7$ yr) are uniformly obscured by dust.