On the anomalous silicate emission features of active galactic nuclei: a possible interpretation based on porous dust

Abstract

The recent Spitzer detections of the 9.7 μm Si-O silicate emission in type 1 active galactic nuclei (AGN) provide support for the AGN unification scheme. The properties of the silicate dust are of key importance to understand the physical, chemical and evolutionary properties of the obscuring dusty torus around the AGN. Compared to that of the Galactic interstellar medium (ISM), the 10 μm silicate emission profile of type 1 AGN is broadened and has a clear shift of peak position to longer wavelengths. In literature, this is generally interpreted as an indication of the deviations of the silicate composition, size and degree of crystallization of the AGN from that of the Galactic ISM. In this Letter, we show that the observed peak shift and profile broadening of the 9.7 μm silicate emission feature can be explained in terms of porous composite dust consisting of ordinary interstellar amorphous silicate, amorphous carbon and vacuum. Porous dust is naturally expected in the dense circumnuclear region around the AGN, as a consequence of grain coagulation.

Active galactic nuclei as seen by the Spitzer Space Telescope

“…The Spitzer IRS spectra have not only been used to test theories for the geometric distribution of the obscuring dust around the central AGN and role of host galaxy obscuration therein, but also to give insights into the possible effects of the AGN on the dust properties. Detailed modeling of the dust properties, in particular shifts in the wavelength of the 9.7 𝛍m silicate feature, both in emission and absorption, indicated that AGNs may be affecting the chemical composition, 63 the dust grain size distribution, 64 and/or the porosity of the dust grains, 65,66 though radiative transfer effects may also influence the position of the silicate emission feature. 55 The Spitzer IRS instrument crossed a new frontier by being the first instrument capable of obtaining midinfrared spectra of distant galaxies.…”

A tale of three galaxies: A “CLUMPY” view of the spectroscopically anomalous galaxies IRAS F10398+1455, IRAS F21013–0739 and SDSS J0808+3948

“…Researchers have been then calling for modified dust chemistry in AGN tori (e.g. Sturm et al 2005) or argued that radiative transfer effects are the cause of this shift (Nikutta et al 2009, but also see Li et al 2008, Smith et al 2010.…”

“…One possible reason for the mismatches may lie in the specific interstellar dust properties that we used: the Clumpy model assumes a mass mixture of 53% silicate dust and 47% graphite dust, with the optical constants of silicate and graphite respectively sourced from Ossenkopf et al (1992) and Draine (2003). We note that among those AGNs exhibiting the silicate emission features, a vast majority have their "10 µm" feature peaking at a longer wavelength compared to that of the diffuse ISM (e.g., see Hao et al 2005, Siebenmorgen et al 2005, Sturm et al 2005, Li et al 2008, Mason et al 2009, Smith et al 2010, Shi et al 2014. Nikutta et al (2009) showed that the Clumpy model with the Ossenkopf et al (1992) silicates can explain AGNs with longward-shifted silicate peaks through radiative transfer effects in a clumpy medium (which are observed with peaks at up to 11.6 µm!).…”

The mineralogy of newly formed dust in active galactic nuclei

“…Rhee and Larkin, 2006;Mason et al, 2006;Poncelet et al, 2006;López-Gonzaga et al, 2014;Alonso-Herrero et al, 2016;Lopez-Rodriguez et al, 2016), using standard astronomical silicate opacities such as those published by Draine and Li (2007). While it was already reported by Sturm et al (2005) that the silicates seen towards quasars differ from the Galactic interstellar silicates observed toward the Galactic Centre (e.g., Kemper et al, 2004), only a handful studies have since attempted to explain the difference in spectral appearance in a relatively small number of objects, using a different dust composition (Jaffe et al, 2004;Markwick-Kemper et al, 2007;Köhler and Li, 2010), variations in the silicate mineralogy (Xie et al, 2014(Xie et al, , 2015, different grain properties (Li et al, 2008;Smith et al, 2010) or optical depth effects (Nikutta et al, 2009). In this paper, we present the first results of a systematic study of AGN dust mineralogy by fitting the mid-infrared spectrum of a sample of Palomar Green (PG) quasars showing silicate emission following the method described by Markwick- Kemper et al (2007), and trying to correlate the results with the physical properties of the AGN.…”