2000A&A...357..839C

We present low resolution spectrophotometric and imaging ISO observations of a sample of 57 AGNs and one non-active SB galaxy over the 2.5-11µm range. The sample is about equally divided into type I (≤1.5; 28 sources) and type II (>1.5; 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 <α≥-0.84±0.24 with only weak emission features from Polycyclic Aromatic Hydrocarbon (PAH) bands at 3.3, 6.2, 7.7 and 8.6µm. In sharp contrast to Sf1s, most Sf2s display a weak continuum but very strong PAH emission bands, with equivalent widths (EW) up to 7.2µm. On the other hand, Sf1s and Sf2s do not have statistically different PAH luminosities while the 7µm continuum is on the average a factor ∼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 92±37 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 EW(PAH)>5µm suffer from an extinction A_v>125 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 ≤2µm, 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. (1997Natur.385..700H), 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 10²⁴cm^–2.