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*
Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
2 Astronomisches Institut, Ruhr-Universität Bochum, 44780 Bochum, Germany
3 Joint Astronomy Centre, 660 N. Aohuku Place, University Park, Hilo 96720, Hawaii, USA
Corresponding author: M. Haas, firstname.lastname@example.org
Accepted: 11 January 2001
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.
Key words: ISM: dust extinction / galaxies: Arp 220 / ISM / quasars / starburst / infrared: galaxies
Based on observations with the James Clerk Maxwell Telescope JCMT, the Swedish ESO Submillimetre Telescope SEST and the Infrared Space Observatory ISO, an ESA project funded by Member States (especially France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA. The development and operation of ISOPHOT and the Postoperation Phase are supported by MPIA and funds from Deutsches Zentrum für Luft- und Raumfahrt.
© ESO, 2001