ON THE EXTERNAL ORIGIN FOR DUST IN ELLIPTIC GALAXIES

We use 100-mu-m fluxes from IRAS co-added data to estimate the mass of dust present in ellipticals and discuss whether the data imply an external origin. The distribution function of the relative dust content from a large sample of ellipticals (Bender et al.) reveals that the dust is not coupled to the stellar content; this is in contrast to the distribution derived for spiral galaxies. We come to the same conclusion as Knapp, Turner & Cunniffe (who applied this method to the H I gas content of 152 ellipticals), that the cold phase of the ISM in many ellipticals has an external origin. Using a maximum-likelihood survival analysis technique we investigate the possibility that cooling flows are the source of the accreted material and conclude that cooling hot gas is not a significant source for dust or cold H I gas in ellipticals. We find that the majority of cooling flow ellipticals reveal independent evidence for a merger, or interaction. These galaxies also show boxy isophotes (which have been claimed to be associated with mergers or interactions). We have also examined elliptical galaxies from the literature which show features strongly indicative of a merger i.e., extensive dust lanes, infalling H I gas, kinematically peculiar cores and shells. In general, the dust and isophotal properties are consistent with the currently accepted merger model for these galaxies.