A complete census of silicate features in the mid-infrared spectra of active galaxies

Journal Article · Mon Apr 20 04:00:00 EDT 2015 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/803/2/110· OSTI ID:22883270

Hatziminaoglou, E. ^[1]; Hernán-Caballero, A. ^[2]; Feltre, A. ^[3]; Ferrer, N. Piñol ^[4]

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1. ESO, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München (Germany)
2. Instituto de Física de Cantabria, CSIC-UC, Avenida de los Castros s/n, E-39005, Santander (Spain)
3. UPMC-CNRS, UMR7095, Institut d’Astrophysique de Paris, 98 bis boulevard Arago, F-75014 Paris (France)
4. Department of Astronomy, Stockholm University, AlbaNova Center, SE-106 91 Stockholm (Sweden)

We present a comprehensive study of the silicate features at 9.7 and 18 μm of a sample of almost 800 active galactic nuclei (AGNs) with available spectra from the Spitzer InfraRed Spectrograph (IRS). We measure the strength of the silicate feature at 9.7 μm, S {sub 9.7}, before and after subtracting the host galaxy emission from the IRS spectra. The number of type 1 and 2 AGNs with the feature in emission increases by 20 and 50%, respectively, once the host galaxy is removed, while 35% of objects with this feature originally in absorption exhibit it in even deeper absorption. The peak of S {sub 9.7}, λ{sub peak}, has a bimodal distribution when the feature is in emission, with about 65% of the cases showing λ{sub peak}>10.2 μm. Silicates can appear in emission in objects with mid-infrared (MIR) luminosity spanning over six orders of magnitude. The derived distributions of the strength of the silicate features at 9.7 and 18 μm provide a solid test bed for modeling the dust distribution in AGNs. Clumpiness is needed in order to produce absorption features in unobscured AGNs and can also cause the silicates to be in absorption at 9.7 μm and in emission at 18 μm in type 1 sources. We find the “cosmic” silicates of Ossenkopf et al. to be more consistent with the observations than Draine's “astronomical” silicates. Finally, we discuss the possibility of a foreground absorber to explain the deep silicate absorption features in the MIR spectra of some type 2 AGNs.

• OSTI ID: 22883270
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 803; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United Kingdom
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
COMPUTERIZED SIMULATION
COSMIC DUST
DISTRIBUTION
EMISSION
GALAXIES
GALAXY NUCLEI
INTERMEDIATE INFRARED RADIATION
LUMINOSITY
SILICATES
SOLIDS