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

Details 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 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

Feb 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001a%26a...367l...9h&link_type=abstract

Astronomy and Astrophysics, v.367, p.L9-L13 (2001)

Astronomy and Astrophysics

Astrophysics

33

Ism: Dust Extinction, Galaxies: Arp 220, Ism, Quasars, Starburst, Infrared: Galaxies

Scientific paper

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 +/- 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(mixed) ~ 50 consistent with former spectroscopic estimates. Arp 220 has a huge extinction of at least A_V(mixed) ~ 110, exceeding former estimates of A_V(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. 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.

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