Dust Masses in Early-Type AGN and Inactive Galaxies

It has been known for some time that elliptical galaxies have some form of dust structure at their centers (Kormendy & Djorgovski 1989; van Dokkum & Franx 1995). Nevertheless the exact amount and the origin of the dust is still unknown. In a previous paper, we have shown that all early-type galaxies hosting an AGN have dust at their centers as opposed to only a quarter of the inactive galaxies in a pair-matched sample (Simões Lopes et al. 2007). Here we use Spitzer (IRAC and MIPS) observations of the same sample — 32 active and 32 inactive early-type galaxies — and state-of-the-art spectral modelling from Draine & Li (2007) to estimate the dust mass present in these galaxies. We have also created PAH images by estimating and subtracting the stellar-only contribution to the flux at the 7.9 μm image from the 3.6 μm image.