The PAH 7.7 mu m/850 mu m 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 u ltraluminous infrared galaxies (ULIRGs): the ratio between the PAH 7.7 mum feature and the sub-mm continuum at 850 mum. While tile sub-mm radiation is optically thin and serves for normalization, any high MIR extinction reduc es tile observed PAH 7.7 mum strength. Since the emitters (PAH carriers) an d the absorbers (large dust grains reemitting: at 850 mum) are typically mi xed 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 o ut of a sample of 15 ULIRGs as well as 20 normal galaxies of a comparison s ample 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 UL IRGs 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 for mer 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 as sumption of a disk-like dust distribution seen under a tilted angle, we fin d increasing evidence for a hidden quasar in Arp 220.