Spectral energy distributions of dusty galaxies

Abstract

We present an SED model of dusty galaxies, in which the equations of radiative transfer were solved by assuming spherical symmetry. The temperature fluctuation of very small dust particles was calculated consistently with the radiative transfer. The adopted dust model consisted of graphite and silicate grains and PAHs, whose relative fractions were determined for each MW, LMC, and SMC-type extinction curve. This model allowed us to derive the intrinsic SEDs of stellar populations embedded in dusty ISM, which are very important indicators for the age of stellar populations. Therefore, the evolutionary phase of starburst galaxies which frequently have very dusty ISM can be investigated with this SED model. We showed that the SED of both Arp 220 and M 82 can be explained by the same single stellar population, despite significant differences in the SED and the infrared luminosity. The apparent difference between their SEDs is mainly caused by the difference in the optical depth. In contrast, the SED of prototypical star-forming ERO, HR 10, indicates that this galaxy is relatively old compared with Arp 220 and M 82. It has been found that, in the case of an optically-thin limit like elliptical galaxies, the optical depth cannot be inferred only from the SED, because of a degeneracy between the optical depth, galactic size, and the spatial distribution of dust; the latter two are important for estimating the average temperature of dust grains in elliptical galaxies. SEDs can be used to constrain the spatial distribution of dust in elliptical galaxies, when the observed size of elliptical galaxies is taken into account.