Interstellar matter in Shapley-Ames elliptical galaxies. IV. A diffusely distributed component of dust and its effect on colour gradients

Abstract

The presence of dust in elliptical galaxies has recently been shown to be quite common. Deep optical multi-colour CCD imaging has revealed the presence of dust lanes and patches, and the technique of co-adding IRAS survey scans has led to many detections of elliptical galaxies. The optical and far-infrared surveys reported similar detection rates of dust, which may indicate that dust in elliptical galaxies is generally distributed in the optically detected lanes or patches. However, we show here that the amount of dust as derived from the optical extinction values is typically an order of magnitude smaller than that derived from the IRAS flux densities, in strong contrast with the situation in spiral galaxies. To unriddle this dilemma, we postulate that the majority of the dust in elliptical galaxies exists as a diffusely distributed component which is undetectable using optical methods. Employing a multiple scattering model for the dust, we show that the presence of this newly postulated dust component implies significant radial colour gradients which were essentially thought to arise from stellar population gradients only. The energetics of the diffusely distributed dust component is shown to be consistent with the IRAS data. A comparison of the detectable amount of dust in elliptical galaxies embedded in hot, X-ray-emitting gas with the amount expected in case the production and destruction rates of dust are equal indicates that most of the dust in elliptical galaxies has been accreted from other galaxies.