[Paper Review] 2.5-11 micron spectroscopy and imaging of AGNs: implication for unification schemes

J. Clavel, B. Schulz+7 more|arXiv (Cornell University)|Mar 21, 2000

Cardiac Imaging and Diagnostics3 citations

TL;DR

This study uses ISO 2.5–11 μm spectroscopy and imaging of 57 AGNs to test unification schemes, finding that type II AGNs exhibit strong PAH emission with high equivalent widths (up to 7.2 μm) and suppressed 7 μm continuum, indicating deep extinction (A_V ≈ 92 ± 37 mag), while type I AGNs show strong power-law continua and weak PAH features. The results confirm that type II AGNs are obscured type I objects viewed at high inclination, with PAH emission serving as a redenning indicator and the torus inner wall and reflecting surface spatially co-located.

ABSTRACT

We present 2.5-11 micron spectrophotometric & imaging ISO observations of 28 Sf1, 29 Sf2 & 1 normal galaxy. The Sf1 & Sf2 MIR spectra are statistically different: Sf1 have a strong power-law continuum of index -0.84+/-0.24 & weak PAH emission bands. Sf2s have a weak continuum & very strong PAH emission features with EW (equivalent widths) up to 7.2 micron. On the other hand, the Sf1 and Sf2 PAH luminosities do not differ statistically and the 7 micron continuum is ~8 times less luminous in Sf2s than in Sf1s. The PAH emission is unrelated to the nuclear activity & arises in the bulge ISM. PAH EW are thus a sensitive nuclear redenning indicator. These results are consistent with unified schemes & imply that the Sf2 MIR nuclear continuum 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 PAH EW > 5 micron suffer from > 125 magnitudes of extinction and are invariably weak X-ray sources. Such Sf2s represent the highly inclined objects where our line of sight intercepts the full extent of the torus. About 1/3rd of the Sf2s have PAH EW <= 2 micron, in the range of Sf1s. Among them, those which have been observed in spectropolarimetry or 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 emitting the MIR continuum. Our observations thus constrain the screen & the torus inner wall to be spatially co-located. Finally, the 9.7 micron Silicate feature appears weakly in emission in Sf1s, implying that the torus vertical thickness cannot significantly exceed E+24 cm-2

Motivation & Objective

• To test the unification scheme of AGNs by comparing mid-infrared (MIR) properties of type I and type II Seyfert galaxies.
• To determine whether the observed differences in MIR spectra between Sf1 and Sf2 AGNs are consistent with orientation-based obscuration by a dusty torus.
• To assess the role of Polycyclic Aromatic Hydrocarbon (PAH) emission as a nuclear reddening indicator in obscured AGNs.
• To investigate the spatial relationship between the torus inner wall and the reflecting surface in Sf2 AGNs using spectropolarimetry and IR spectroscopy.

Proposed method

• Low-resolution spectrophotometry and imaging were performed using the ISOPHOT-S spectrometer and ISOCAM camera on board the Infrared Space Observatory (ISO).
• The sample includes 57 AGNs (28 type I, 29 type II) and one non-active SB galaxy, with redshifts ranging from 0.016 to 0.083.
• PAH equivalent widths (EW) at 3.3, 6.2, 7.7, and 8.6 μm were measured to assess extinction and star formation-related emission.
• The 7 μm continuum luminosity was compared between Sf1 and Sf2 AGNs to quantify obscuration of the nuclear continuum.
• Spectropolarimetry and IR spectroscopy were used to detect 'hidden' broad-line regions in Sf2 AGNs with low PAH EW.
• Extinction was estimated from PAH EW using the relation that higher EW indicates lower extinction, with A_V derived from N_H estimates.

Experimental results

Research questions

• RQ1 Do type I and type II AGNs show statistically different mid-infrared spectral energy distributions, consistent with the unification model?
• RQ2 Is PAH emission in AGNs primarily powered by star formation in the bulge, and can it serve as a reddening indicator for the nuclear continuum?
• RQ3 Why do some type II AGNs show strong PAH emission with high equivalent widths (up to 7.2 μm), while others have weak PAH features similar to type I AGNs?
• RQ4 Are Sf2 AGNs with high PAH EW (≥5 μm) truly obscured across all wavelengths, including X-rays, and do they show signs of Compton-thick absorption?
• RQ5 Is the reflecting surface (responsible for scattered light) spatially co-located with the torus inner wall in Sf2 AGNs with 'hidden' broad lines?

Key findings

• The average 7 μm continuum luminosity in Sf2 AGNs is a factor of approximately 8 lower than in Sf1 AGNs, indicating significant obscuration of the nuclear source.
• Sf1 AGNs exhibit a power-law continuum with an average spectral index α = -0.84 ± 0.24, while Sf2 AGNs show weak continuum but strong PAH emission bands with equivalent widths up to 7.2 μm.
• The average PAH equivalent width at 7.7 μm is 0.53 ± 0.47 μm in Sf1s and 2.86 ± 1.95 μm in Sf2s, showing a statistically significant difference in distribution.
• Sf2 AGNs with PAH EW > 5 μm are indistinguishable from normal galaxies, indicating complete obscuration of the MIR nuclear continuum (A_V > 125 mag), and are invariably weak X-ray sources.
• Among Sf2 AGNs with PAH EW ≤ 2 μm, all four with spectropolarimetry show 'hidden' broad lines, confirming that these are viewed at grazing incidence where the torus and reflecting surface are directly visible.
• The silicate 9.7 μm feature appears weakly in emission in Sf1s (EW = 0.25 ± 0.01 μm), implying a torus vertical optical depth consistent with N_H ≈ 10^23–10^24 cm⁻², ruling out extremely thick tori.