[Paper Review] The dusty heart of nearby active galaxies. I. High-spatial resolution mid-IR spectro-photometry of Seyfert galaxies

S. Hoenig, Makoto Kishimoto+6 more|arXiv (Cornell University)|Mar 3, 2010

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy76 references90 citations

TL;DR

This study presents high-spatial-resolution mid-infrared spectro-photometry of 19 nearby Seyfert galaxies using VLT/VISIR, resolving the dusty torus emission at scales <100 pc. It demonstrates that the mid-IR spectral index α and silicate feature strength jointly constrain the radial dust distribution (power-law index a ≈ −1.0 ± 0.5) and average cloud number N₀ ≈ 5–8 along the line of sight, supporting the clumpy torus model and suggesting a possible luminosity-dependent change in torus structure.

ABSTRACT

We present 8-13 micron imaging and spectroscopy of 9 type 1 and 10 type 2 AGN obtained with the VLT/VISIR instrument at spatial resolution &lt;100 pc. The emission from the host galaxy sources is resolved out in most cases. The silicate absorption features are moderately deep and emission features are shallow. We compare the mid-IR luminosities to AGN luminosity tracers and found that the mid-IR radiation is emitted quite isotropically. In two cases, IC5063 and MCG-3-34-64, we find evidence for extended dust emission in the narrow-line region. We confirm the correlation between observed silicate feature strength and Hydrogen column density recently found in Spitzer data. In a further step, our 3D clumpy torus model has been used to interpret the data. We show that the strength of the silicate feature and the mid-IR spectral index can be used to get reasonable constraints on the dust distribution in the torus. The mid-IR spectral index, alpha, is almost exclusively determined by the radial dust distribution power-law index, a, and the silicate feature depth is mostly depending on the average number of clouds, N0, along an equatorial line-of-sight and the torus inclination. A comparison of model predictions to our type 1 and type 2 AGN reveals typical average parameters a=-1.0+/-0.5 and N0=5-8, which means that the radial dust distribution is rather shallow. As a proof-of-concept of this method, we compared the model parameters derived from alpha and the silicate feature to more detailed studies of IR SEDs and interferometry and found that the constraints on a and N0 are consistent. Finally, we might have found evidence that the radial structure of the torus changes from low to high AGN luminosities towards steeper dust distributions, and we discuss implications for the IR size-luminosity relation. (abridged)

Motivation & Objective

• To characterize the physical properties of the AGN dust torus using high-spatial-resolution mid-IR data.
• To test the validity of 3D clumpy torus models by comparing observed mid-IR spectral features with model predictions.
• To investigate whether the radial dust distribution and cloud geometry differ between type 1 and type 2 AGN, supporting or challenging the unification scheme.
• To explore potential luminosity-dependent changes in torus structure, particularly in the radial dust distribution power-law index a.
• To validate the use of mid-IR spectral index α and silicate feature strength as proxies for torus parameters using independent interferometric and SED data.

Proposed method

• High-spatial-resolution mid-IR imaging and low-resolution spectroscopy (8–13 μm) were obtained with VLT/VISIR for 9 type 1 and 10 type 2 Seyfert galaxies.
• The data resolve emission at scales <100 pc, minimizing contamination from host galaxy star formation.
• The mid-IR spectral index α was derived from the observed flux density slope, while silicate feature strength was measured via τ_silicate or F_Si/F_c.
• 3D clumpy torus models were used to simulate the observed mid-IR SEDs, varying parameters a (radial dust distribution), N₀ (mean number of clouds along line of sight), and inclination i.
• Model predictions were compared to observed α and silicate feature strength to constrain a and N₀ for individual objects and the sample as a whole.
• The method was validated using independent interferometric and SED data for NGC 1068 and NGC 3783, confirming consistency of derived parameters.

Experimental results

Research questions

• RQ1 Can the mid-IR spectral index α and silicate feature strength jointly constrain the radial dust distribution (a) and average cloud number (N₀) in clumpy torus models?
• RQ2 Is there a significant difference in torus parameters (a and N₀) between type 1 and type 2 Seyfert galaxies, consistent with the unification scheme?
• RQ3 Does the radial dust distribution in the torus change with AGN luminosity, particularly at low versus high luminosities?
• RQ4 Can the observed α and silicate feature strength be used as reliable proxies for torus parameters without full SED or interferometric data?
• RQ5 How do the derived torus parameters from mid-IR data compare to those obtained from full SED modeling and infrared interferometry?

Key findings

• The mid-IR spectral index α is primarily sensitive to the radial dust distribution power-law index a, with α being almost exclusively determined by a.
• The silicate feature strength is mainly dependent on the average number of clouds N₀ along the line of sight and the torus inclination i.
• The sample shows typical values of a = −1.0 ± 0.5 and N₀ = 5–8, with no significant difference between type 1 and type 2 AGN, supporting the unification scheme.
• A correlation is found between silicate feature strength and hydrogen column density, with τ_silicate ∝ 0.14 × log N_H.
• Evidence suggests that low-luminosity AGN may have shallower dust distributions (a ≈ −1.0) compared to high-luminosity AGN, which may have steeper distributions (a ≈ −2 to −3), though this requires further confirmation.
• The derived parameters from α and silicate feature strength are consistent with those from full SED and interferometric modeling of NGC 1068 and NGC 3783, validating the method as a reliable proxy for torus structure.