2.5-11 micron spectroscopy and imaging of AGNs - Implication for unification schemes

We present low resolution spectrophotometric and imaging ISO observations o f a sample of 57 AGNs and one non-active SE galaxy over the 2.5-11 mu m ran ge. The sample is about equally divided into type I (less than or equal to 1.5; 28 sources) and type II(> 1.5; 29 sources) objects. The mid-IR (MIR) s pectra of type I (Sf1) and type II (Sf2) objects are statistically differen t: Sf1 spectra are characterized by a strong continuum well approximated by a power-law of average index [alpha] = -0.84 +/- 0.24 with only weak emiss ion features from Polycyclic Aromatic Hydrocarbon (PAH) bands at 3.3, 6.2, 7.7 and 8.6 mu m. In sharp contrast to Sf1s, most Sf2s display a weak conti nuum but very strong PAH emission bands, with equivalent widths (EW) up to 7.2 mu m. On the other hand, Sf1s and Sf2s do not have statistically differ ent PAH luminosities while the 7 mu m continuum is on the average a factor similar to 8 less luminous in Sf2s than in Sf1s. Because the PAH emission i s unrelated to the nuclear activity and arises in the interstellar medium o f the underlying galactic bulge, its EW is a sensitive nuclear redenning in dicator. These results are consistent with unification schemes and imply th at the MIR nuclear continuum source of Sf2s is, on the average, extinguishe d by 92 +/- 37 visual magnitudes whereas it is directly visible in Sf1s. Th e dispersion in SB's PAH EW is consistent with the expected spread in viewi ng angles. Those Sf2s with EW(PAH) > 5 mu m 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 i ntercepts the full extent of the molecular torus. Conversely, about a third of the Sf2s have PAH EW less than or equal to 2 mu m, in the range of Sf1s . Among them, those which have been observed in spectropolarimetry and/or i n IR spectroscopy invariably display "hidden" broad lines. As proposed by H eisler et al. (1997), such Sf2s are most likely seen at grazing incidence s uch that one has a direct view of both the "reflecting screen" and the toru s inner wall responsible for the near and mid-IR continuum. Our observation s therefore constrain the screen and the toms inner wall to be spatially co -located. Finally, the 9.7 mu m Silicate feature appears weakly in emission in Sf1s, implying that the torus vertical optical thickness cannot signifi cantly exceed 10(24) cm(-2).