The innermost region of AGN tori: implications from the HST/NICMOS type 1 point sources and near-IR reverberation

By M. Kishimoto, S. Hoenig, T. Beckert, G. Weigelt. 2007

161 citations recorded by the source corpus.

Spatially resolving the innermost region of the putative torus-like structure in an active galactic nucleus (AGN) is one of the main goals of high-spatial-resolution studies. This could be done in the near-IR observations of type 1 AGNs where we directly see the hottest dust grains in the torus. We discuss two critical issues in such studies. One is the possible contribution from the central putative accretion disk (the near-IR part of the big blue bump emission), which should be taken into account for the torus measurements. The other is the expected size of the inner boundary of the torus, essential for the feasibility of spatially resolving the region.
We examine the nuclear near-IR point sources in the HST/NICMOS images of nearby type 1 AGNs to evaluate the accretion disk contribution. After the subtraction of the host bulge flux through two-dimensional decompositions, we show that near-IR colors of the point sources appear quite interpretable simply as a composite of a black-body-like spectrum and a relatively blue distinct component, as expected for a torus and an accretion disk in the near-IR. The near-IR colors of our radiative transfer models for clumpy tori also support this simple two-component interpretation. The observed near-IR colors for the available sample suggest a fractional accretion disk contribution of ~25% or less at 2.2  μ m.
We also show that the innermost torus radii as indicated by recent near-IR reverberation measurements are systematically smaller by a factor of ~3 than the predicted dust sublimation radius with a reasonable assumption for graphite grains of a sublimation temperature 1500 K and size 0.05  μ m in radius. The discrepancy might indicate a much higher sublimation temperature or a typical grain size much larger in the innermost tori, though the former case appears to be disfavored by the observed colors of the HST point sources studied above. Alternatively, the central engine radiation might be significantly anisotropic. The near-IR interferometry with a baseline of ~100 m should be able to provide important, independent size measurements for the innermost torus region, based on the low fractional contribution from the accretion disk obtained above.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/c88363b679eda6da75f966215b2ccf7e9b6d9809

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In Astronomy and Astrophysics

1. D. M. Alexander
2. H. Netzer
3. Mara Salvato
4. P. Hopkins
5. W. N. Brandt
6. L. Hernquist
7. A. Koekemoer
8. R. Hickox

Citations in the corpus

• The incidence of obscuration in active galactic nuclei (2013, methodology): “…treatment of dust sublimation was presented in Barvainis (1987) and Fritz, Franceschini & Hatziminaoglou (2006), and subsequently applied to sophisticated clumpy torus models (Nenkova et al. 2008) or to interferometric observations of galactic nuclei in the near-IR (NIR; Kishimoto et al. 2007).”
• A diversity of dusty AGN tori - Data release for the VLTI/MIDI AGN Large Program and first results for 23 galaxies (2013, methodology): “More recently, Kishimoto et al. (2007) studied the innermost regions of eight nearby (z < 0.2) type 1 AGNs using NICMOS data from the HST archive.”
• The dust covering factor in active galactic nuclei (2016, background): “Hence, rather than a well defined Rin determined by the sublimation radius, we expect to have a sublimation zone, from larger to smaller grains and from graphite to silicate (see e.g., Kishimoto et al. 2007; Mor & Netzer 2012).”
• TORUS AND ACTIVE GALACTIC NUCLEUS PROPERTIES OF NEARBY SEYFERT GALAXIES: RESULTS FROM FITTING INFRARED SPECTRAL ENERGY DISTRIBUTIONS AND SPECTROSCOPY (2011, background): “Kishimoto et al. (2007) demonstrated for type 1 Seyferts that most of the unresolved emission at 2.2µm is produced by hot dust emission from the inner walls of the torus, and that there is a very small contribution from the big blue bump emission (i.e., AGN emission) at this wavelength.”
• DUST IN THE POLAR REGION AS A MAJOR CONTRIBUTOR TO THE INFRARED EMISSION OF ACTIVE GALACTIC NUCLEI (2013, background): “Observational evidence strongly suggest that large graphite grains dominate this hot sublimation zone (e.g. Kishimoto et al. 2007, 2009b; Mor et al. 2009; Kishimoto et al. 2011a; Mor & Netzer 2012).”
• The dusty heart of nearby active galaxies. II. From clumpy torus models to physical properties of dust around active galactic nuclei (2009, background): “In our definition of the accretion disk spectrum, the reverberation radius from Kishimoto et al. (2007) for Lbol = 1046 erg/s would berτK = 0.38 pc, with approximately 25% scatter. massive black hole (Beckert & Duschl 2004; Hönig & Beckert 2007).”
• Revisiting the Unified Model of Active Galactic Nuclei (2015, methodology): “Other experiments were designed to measure the time-dependent dust emission in the NIR, mainly in the K-band, in response to the variable continuum (“dust RM”, see e.g. Glass 1992, Suganuma et al. 2006, Kishimoto et al. 2007, Koshida et al. 2014, Pozo Nuñez et al. 2014).”
• Hot graphite dust and the infrared spectral energy distribution of active galactic nuclei (2011, background): “There have been several studieshat fitted the NIR to MIR SEDs of AGN using a black-body spectrum to represent emission from such hot dust (e.g., Edelson & Malkan 1986; Barvainis 1987; Kishimoto et al. 2007; Riffel et al. 2009; M09; Deo et al. 2011).”
• REVERBERATION MEASUREMENTS OF THE INNER RADIUS OF THE DUST TORUS IN 17 SEYFERT GALAXIES (2014, methodology): “Although the previously observed luminosity dependency of the radius of the innermost dust torus is consistent with the theoretical expectation, Kishimoto et al. (2007) found that the innermost torus radii based on dust reverberation were systematically smaller than those based on the theoretical…”
• Resolving the mid-infrared cores of local Seyferts (2009, methodology): “This relation is approximately correct for a sublimation temperature of 1500 K of graphite grains with an average radial size of 0.05µm (e.g., Barvainis 1987, Kishimoto et al. 2007; we have also assumed that the bulk of the bolometric contribution is due to ultraviolet photons).”