2001A&A...367L...9H

A new method is presented to reveal high mid-infrared (MIR) extinction in ultraluminous infrared galaxies (ULIRGs): the ratio between the PAH 7.7µm feature and the sub-mm continuum at 850µm. While the sub-mm radiation is optically thin and serves for normalization, any high MIR extinction reduces the observed PAH 7.7µm strength. Since the emitters (PAH carriers) and the absorbers (large dust grains reemitting at 850µm) are typically mixed along the line of sight, the new method probes the absorption along the entire dust column, and not only the properties of a shallow surface. 13 out of a sample of 15 ULIRGs as well as 20 normal galaxies of a comparison sample populate the same well confined range of the PAH 7.7µm/850µm flux ratio (4±2). Their MIR extinction may be moderate. In contrast, two ULIRGs show an exceptionally low PAH 7.7µm/850 µm ratio indicative of high extinction: UGC 5101 has A_V(mixed)≃50 consistent with former spectroscopic estimates. Arp 220 has a huge extinction of at least A_V(mixed)≃110, exceeding former estimates of A_V(screen) > 45 based on the [SIII] 18.7µm/33.5µm ratio. As an application of the new diagnostics, after dereddening of the central MIR continuum and with the assumption of a disk-like dust distribution seen under a tilted angle, we find increasing evidence for a hidden quasar in Arp 220.