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Astron. Astrophys. 357, 839-849 (2000)

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2.5-11 micron spectroscopy and imaging of AGNs * **

Implication for unification schemes

J. Clavel 1, B. Schulz 2, B. Altieri 1, P. Barr 3, P. Claes 3, A. Heras 3, K. Leech 2, L. Metcalfe 2 and A. Salama 2

1 XMM Science Operations, Astrophysics Division, ESA Space Science Department, P.O. Box 50727, 28080 Madrid, Spain (jclavel@xmm.vilspa.esa.es)
2 ISO Data Centre, Astrophysics Division, ESA Space Science Department, P.O. Box 50727, 28080 Madrid, Spain
3 Astrophysics Division, ESA Space Science Department, ESTEC Postbus 299, 2200 AG-Noordwijk, The Netherlands

Received 24 August 1999 / Accepted 14 March 2000

Abstract

We present low resolution spectrophotometric and imaging ISO observations of a sample of 57 AGNs and one non-active SB galaxy over the 2.5-11 µm range. The sample is about equally divided into type I ([FORMULA]; 28 sources) and type II ([FORMULA]; 29 sources) objects. The mid-IR (MIR) spectra of type I (Sf1) and type II (Sf2) objects are statistically different : Sf1 spectra are characterized by a strong continuum well approximated by a power-law of average index [FORMULA] with only weak emission features from Polycyclic Aromatic Hydrocarbon (PAH) bands at 3.3, 6.2, 7.7 and 8.6 µm. In sharp contrast to Sf1s, most Sf2s display a weak continuum but very strong PAH emission bands, with equivalent widths (EW) up to 7.2 µm. On the other hand, Sf1s and Sf2s do not have statistically different PAH luminosities while the 7 µm continuum is on the average a factor [FORMULA] 8 less luminous in Sf2s than in Sf1s. Because the PAH emission is unrelated to the nuclear activity and arises in the interstellar medium of the underlying galactic bulge, its EW is a sensitive nuclear redenning indicator. These results are consistent with unification schemes and imply that the MIR nuclear continuum source of Sf2s is, on the average, extinguished by [FORMULA] visual magnitudes whereas it is directly visible in Sf1s. The dispersion in Sf2's PAH EW is consistent with the expected spread in viewing angles. Those Sf2s with [FORMULA] suffer from an extinction [FORMULA] magnitudes and are invariably extremely weak X-ray sources. Such Sf2s presumably represent the highly inclined objects where our line of sight intercepts the full extent of the molecular torus. Conversely, about a third of the Sf2s have PAH EW [FORMULA]m, in the range of Sf1s. Among them, those which have been observed in spectropolarimetry and/or in IR spectroscopy invariably display "hidden" broad lines. As proposed by Heisler et al. (1997), such Sf2s are most likely seen at grazing incidence such that one has a direct view of both the "reflecting screen" and the torus inner wall responsible for the near and mid-IR continuum. Our observations therefore constrain the screen and the torus inner wall to be spatially co-located. Finally, the 9.7 µm Silicate feature appears weakly in emission in Sf1s, implying that the torus vertical optical thickness cannot significantly exceed [FORMULA].

Key words: galaxies: active – galaxies: Seyfert – infrared: galaxies

* Based on observations with ISO, an ESA projects with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA
** Tables 2, 3, 4, 5 are only available in electronic form via anonymous ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/357/839 or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/357/839

Send offprint requests to: J. Clavel

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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