Dust in active galactic nuclei

Mid-infrared T-ReCS/Gemini spectra using the new RedCan pipeline^⋆

¹ Instituto de Astrofísica de Canarias (IAC), C/Vía Láctea, s/n, 38205 La Laguna, Spain
e-mail: omairagm@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Spain
³ Spitzer Science Center, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁴ Dept. of Physics & Astronomy, University of Texas San Antonio, San Antonio, TX. 78249, USA
⁵ Instituto de Física de Cantabria, CSIC-Universidad de Cantabria, 39005 Santander, Spain
⁶ Departamento de Física Moderna, Universidad de Cantabria, Avda. de Los Castros s/n, 39005 Santander, Spain
⁷ Centro de Astrobiología, INTA-CSIC, 28850 Madrid, Spain
⁸ Gemini Observatory, Northern Operations Center, 670 N. A’ohoku Place, Hilo, HI 96720, USA
⁹ Department of Astronomy, University of Florida, Gainesville, Florida 32611, USA

Received: 13 September 2012
Accepted: 21 December 2012

Abstract

Context. The unified model of active galactic nuclei (AGN) claims that the properties of AGN depend on the viewing angle of the observer with respect to a toroidal distribution of dust surrounding the nucleus. Both the mid-infrared (MIR) attenuation and continuum luminosity are expected to be related to dust associated with the torus. Therefore, isolating the nuclear component is essential for studying the MIR emission of AGN.

Aims. This work is aimed at studying the MIR emission of AGN with the highest spatial resolution available to date, isolating its contribution from extended emission. We would like to address three fundamental questions: (1) how important is the AGN contribution to the MIR spectrum; (2) where dust attenuation arises; and (3) how does spatial resolution affect these issues.

Methods. We compiled all the T-ReCS spectra (Gemini Observatory) available in the N-band for 22 AGN: 5 Type-1 and 17 Type-2 AGN. The high angular resolution of the T-ReCS spectra allowed us to probe physical regions of 57 pc (median). We used a novel pipeline called RedCan capable of producing flux- and wavelength-calibrated spectra for the CanariCam (GTC) and T-ReCS (Gemini) instruments. We measured the fine-structure [S IV] at 10.5 μm and the PAH at 11.3 μm line strengths, together with the silicate absorption/emission features. We also compiled Spitzer/IRS spectra to understand how spatial resolution influences the results. We complemented our sample with the results of 19 VISIR/VLT spectra (Paranal Observatory) and 20 nearby, highly obscured AGN (N_H > 1.5 × 10²⁴ cm^-2) Spitzer spectra.

Results. The 11.3 μm PAH feature is only clearly detected in the nuclear spectra of two AGN, while it is more common in the Spitzer data. For those two objects, the AGN emission in NGC 7130 accounts for more than 80% of the MIR continuum at 12 μm, while in the case of NGC 1808 the AGN does not dominate the MIR emission. This is confirmed by the correlation between the MIR and X-ray continuum luminosities. The [S IV] emission line at 10.5 μm, which is believed to originate in the narrow line region, is detected in most AGN. We have found an enhancement of the optical depth at 9.7 μm (τ_9.7) in the high-angular resolution data for higher values of N_H. Clumpy torus models reproduce the observed values only if the host-galaxy properties are taken into account.