Far‐InfraredSpitzerObservations of Elliptical Galaxies: Evidence for Extended Diffuse Dust

Far-infrared Spitzer observations of elliptical galaxies are inconsistentwith simple steady state models of dust creation in red giant stars anddestruction by grain sputtering in the hot interstellar gas at T ~ 10^7 K. Theflux at 24 microns correlates with optical fluxes, suggesting that thisrelatively hot dust is largely circumstellar. But fluxes at 70 and 160 micronsdo not correlate with optical fluxes. Elliptical galaxies with similar L_B haveluminosities at 70 and 160 microns (L_70 and L_160) that vary over a factor ~100, implying an additional source of dust unrelated to that produced byongoing local stellar mass loss. Neither L_70/L_B nor L_160/L_B correlate withthe stellar age or metallicity. Optical line fluxes from warm gas at T ~ 10^4 Kcorrelate weakly with L_70 and L_160, suggesting that the dust may beresponsible for cooling this gas. Many normal elliptical galaxies have emissionat 70 microns that is extended to 5-10 kpc. Extended far-infrared emission withsputtering lifetimes of ~10^8 yrs is difficult to maintain by mergers withgas-rich galaxies. Instead, we propose that this cold dust is buoyantlytransported from reservoirs of dust in the galactic cores which is supplied bymass loss from stars in the core. Intermittent energy outbursts from AGNs candrive the buoyant outflow.Comment: 26 pages; Accepted by Ap