Dust in elliptical galaxies — how often, how much?

Abstract

We report results showing that dust-lanes are a common feature of nearby elliptical galaxies. About 40% of ellipticals have dust-lanes with diameters of a few kpc and dust masses of 10⁴–10⁶ M⊙. This result has some possible consequences for galaxy surface photometry, since dust-lanes too small to be seen on photographic plates may produce both “twisting” of inner isophotes and off-centre nuclei.

In this contribution, we are concerned with two specific questions about dust in elliptical galaxies. Firstly, is it common? Secondly, how much dust is likely to be present? As it turns out, the first question can be answered by a simple examination of sky atlas plates; while the second requires the use of calibrated surface photometry for which a CCD is very useful.

Many dust-lane ellipticals are well-known, Centaurus A (NGC 5128) being a famous example, and galaxies of this kind are interesting for a number of reasons. Dust-lanes are a useful probe of the true shape of ellipticals (van Albada, Kotanyi and Schwarzschild 1982; Tohline, Simonson and Caldwell 1982), the appearance of at least some suggests that they may be the result of a merger (Graham 1979, Schweizer 1980) and there is also a suggestive correlation of dust with both the presence and orientation of non-thermal radio sources (Kotanyi and Ekers 1979, Sadler 1982). It is therefore of interest to know whether we are dealing with something rare and exotic, or whether a dust-lane is a rather common feature of early-type galaxies.