The PAH 7.7 <i>μ</i>m/850 <i>μ</i>m ratio as new diagnostics for high extinction in ULIRGs -increasing evidence for a hidden quasar in Arp 220

Abstract

A new method is presented to reveal high mid-infrared (MIR) extinction in ultraluminous infrared galaxies (ULIRGs): the ratio between the PAH 7.7 μm feature and the sub-mm continuum at 850 μm. While the sub-mm radiation is optically thin and serves for normalization, any high MIR extinction reduces the observed PAH 7.7 μm strength. Since the emitters (PAH carriers) and the absorbers (large dust grains reemitting at 850 μm) 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 7.7 μm/850 μm flux ratio (). Their MIR extinction may be moderate. In contrast, two ULIRGs show an exceptionally low PAH 7.7 μm/850 μm ratio indicative of high extinction: UGC 5101 has consistent with former spectroscopic estimates. Arp 220 has a huge extinction of at least , exceeding former estimates of based on the [SIII] 18.7 μm/33.5 μm 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.