The PAH 7.7 μm/850 μm ratio as new diagnostics for high extinction in ULIRGs -: increasing evidence for a hidden quasar in Arp 220

A new method is presented to reveal high mid-infrared (MIR) extinction in ultraluminous infrared galaxies (ULIRGs): the ratio between the PAH 7.7 mum feature and the sub-mm continuum at 850 mum. While the sub-mm radiation is optically thin and serves for normalization, any high MIR extinction reduces the observed PAH 7.7 mum strength. Since the emitters (PAH carriers) and the absorbers (large dust grains reemitting: at 850 mum) 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 mum/850 mum flux ratio (4 +/- 2). Their MIR extinction may be moderate. In contrast, two ULIRGs show an exceptionally loa PAH 7.7 mum/850 mum ratio indicative of high extinction: UGC 5101 has A(v)(mixed) approximate to 50 consistent with former spectroscopic estimates. Arp 220 has a huge extinction of at least A(v)(mixed) approximate to 110, exceeding former estimates of A(v)(screen) > 45 based on the [SIII] 18.7 mum/33.5 mum 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.