TheSpitzerdiscovery of a galaxy with infrared emission solely due to AGN activity

Abstract

Aims. We present an analysis of a galaxy (SAGE1CJ053634.78-722658.5) at a redshift of 0.14 of which the infrared (IR) emission is entirely dominated by emission associated with the active galactic nucleus. Methods. We present the 5−37 μm Spitzer/IRS spectrum and broad wavelength spectral energy distribution (SED) of SAGE1CJ053634.78-722658.5, an IR point-source detected by Spitzer/SAGE. The source was observed in the SAGE-Spec program and was included to determine the nature of sources with deviant IR colours. The spectrum shows a redshifted (z = 0.14 ± 0.005) silicate emission feature with an exceptionally high feature-to-continuum ratio and weak polycyclic aromatic hydrocarbon (PAH) emission bands. We compare the source with models of emission from dusty tori around AGNs. We present a diagnostic diagram that will help to identify similar sources based on Spitzer/MIPS and Herschel/PACS photometry. Results. The SED of SAGE1CJ053634.78-722658.5 is peculiar because it lacks far-IR emission due to cold dust and a clear stellar counterpart. We find that the SED and the IR spectrum can be understood as emission originating from the inner ∼10 pc around an accreting black hole. There is no need to invoke emission from the host galaxy, either from the stars or from the interstellar medium, although a possible early-type host galaxy cannot be excluded based on the SED analysis. The hot dust around the accretion disk gives rise to a continuum, which peaks at 4 μm, whereas the strong silicate features may arise from optically thin emission of dusty clouds within ∼10 pc around the black hole. The weak PAH emission does not appear to be linked to star formation, as star formation templates strongly over-predict the measured far-IR flux levels. Conclusions. The SED of SAGE1CJ053634.78-722658.5 is rare in the local universe but may be more common in the more distant universe. The conspicuous absence of host-galaxy IR emission places limits on the far-IR emission arising from the dusty torus alone.

The VMC Survey

“…In addition, two QSOs used by Anguita et al (2000) and one by Pedreros et al (2002) to study the proper motion of the LMC are attributed to a private communication by Maza in 1989 and/or do not appear in previous studies. More recently, 1 QSO was confirmed by Hony et al (2011) and 145 QSOs by Kozłowski et al (2012), the latter identified from OGLE-III light-curves. The entire MACHO database, with light-curves spanning a time range of ∼7.5 yr, was searched by Kim et al (2011Kim et al ( , 2012 who trained a support vector machine model with diagnostic features based on mid-IR colours, spectral energy distribution red-shifts and X-ray luminosity of previously known QSOs to identify 663 high confidence candidates, but none of them are at present spectroscopically confirmed.…”

The Interstellar Dust Properties of Nearby Galaxies

“…They can be seen in emission in galaxies, when the dust is hot enough (T 150 K). It is the case near the central engine of AGNs (Wu et al 2009;Hony et al 2011), even low-luminosity AGNs, like the nucleus of M 31 (Hemachandra et al 2015). The MIR spectra of earlytype galaxies also show clear silicate emission features, but likely of circumstellar origin (e.g.…”

The SAGE-Spec Spitzer Legacy program: the life-cycle of dust and gas in the Large Magellanic Cloud. Point source classification – III

“…It is SSTISAGEMC J053634.77−722658.6 (OB-JID 151), however, which turned out to be the most intriguing. Hony et al (2011) argued that the lack of far-IR emission and the optical and near-IR faintness indicate that the mid-IR emission is dominated by the AGN torus; this galaxy has the largest silicate-to-continuum ratio known. Subsequently, van Loon & Sansom (2015) obtained optical spectra with the Southern African Large Telescope (SALT), refining the redshift to z = 0.1428 and discovering strong, broad Hα emission from near the AGN and relatively narrow stellar absorption in the spectrum of an early-type galaxy.…”