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A&A 367, L9-L13 (2001)
DOI: 10.1051/0004-6361:20010065

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

M. Haas1, U. Klaas1, S. A. H. Müller2, R. Chini2 and I. Coulson3

1  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

(Received 11 December 2000 / Accepted 11 January 2001)

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 $\mu$m feature and the sub-mm continuum at 850 $\mu$m. While the sub-mm radiation is optically thin and serves for normalization, any high MIR extinction reduces the observed PAH 7.7 $\mu$m strength. Since the emitters (PAH carriers) and the absorbers (large dust grains reemitting at 850 $\mu$m) 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 $\mu$m/850 $\mu$m flux ratio ($4 \pm 2$). Their MIR extinction may be moderate. In contrast, two ULIRGs show an exceptionally low PAH 7.7 $\mu$m/850 $\mu$m ratio indicative of high extinction: UGC 5101 has $A_V{\rm (mixed)} \approx 50$ consistent with former spectroscopic estimates. Arp 220 has a huge extinction of at least $A_V{\rm (mixed)} \approx 110$, exceeding former estimates of $A_V{\rm (screen)} > 45$ based on the [SIII] 18.7 $\mu$m/33.5 $\mu$m 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

Offprint request: M. Haas, haas@mpia-hd.mpg.de

SIMBAD Objects in preparation



© ESO 2001