Diffuse dust in elliptical galaxies

Abstract

It is well established that the broadband colors of elliptical galaxies become redder near their centres, which provides an important constraint on models of elliptical galaxy formation. The observed radial color gradients are usually explained by a gradient in stellar metallicity, but if a diffuse dust component were present, it could also contribute to the reddening. Dust clouds are commonly observed near the centres of elliptical galaxies, but the dust mass implied from optical extinction is generally 5-10 times less than values derived from far infrared (FIR) emission measured by IRAS photometry. One explanation for this discrepancy is that the majority of the dust is diffusely distributed, and not concentrated in dense clouds, but previous FIR experiments have lacked the spatial resolution to determine the location of the existing dust. In an effort to determine the importance of a diffuse dust component, 23 elliptical galaxies have been observed with the SCUBA instrument on the JCMT. The observations, combined with published fluxes, are modeled using a three dimensional Monte Carlo radiation transfer code to establish the mass, temperature, and distribution of diffuse dust in elliptical galaxies. The implied dust masses are often an order of magnitude larger than IRAS estimates, and the impact on the observed colour gradients is found to be negligible.