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Published July 2010 | Published
Journal Article Open

HerMES: Far infrared properties of known AGN in the HerMES fields


Nuclear and starburst activity are known to often occur concomitantly. Herschel-SPIRE provides sampling of the far-infrared (FIR) spectral energy distributions (SEDs) of type 1 and type 2 AGN, allowing for the separation between the hot dust (torus) and cold dust (starburst) emission. We study large samples of spectroscopically confirmed type 1 and type 2 AGN lying within the Herschel Multi-tiered Extragalactic Survey (HerMES) fields observed during the science demonstration phase, aiming to understand their FIR colour distributions and constrain their starburst contributions. We find that one third of the spectroscopically confirmed AGN in the HerMES fields have 5σ detections at 250 μm, in agreement with previous (sub)mm AGN studies. Their combined Spitzer-MIPS and Herschel-SPIRE colours (specifically S_(250)/S_(70) vs S_(70)/S_(24)) quite clearly separate them from the non-AGN, star forming galaxy population, as their 24 μm flux is dominated by the hot torus emission. However, their SPIRE colours alone do not differ from those of non-AGN galaxies. SED fitting shows that all those AGN need a starburst component to fully account for their FIR emission. For objects at z > 2 we find a correlation between the infrared luminosity attributed to the starburst component, LSB, and the AGN accretion luminosity, L_(acc), with L_(SB) ∝ L_(acc)^(0.35). Type 2 AGN detected at 250 μm show on average higher LSB than type 1 objects but their number is still too low to establish whether this trend indicates stronger star formation activity.

Additional Information

© 2010 ESO. Received 31 March 2010; Accepted 19 April 2010. Published online 16 July 2010. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. SPIRE has been developed by a consortium of institutes led by Cardiff University (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCLMSSL, UKATC, Univ. Sussex (UK); and Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); Stockholm Observatory (Sweden); STFC (UK); and NASA (USA). The data presented in this paper will be released through the Herschel Database in Marseille HeDaM2. This work makes use of data taken with the Spitzer Space Telescope, the Sloan Digital Sky Survey (http://www.sdss.org) and the Two Micron All Sky Survey (http://www.ipac.caltech.edu/2mass/overview/access.html). This work made use of Virtual Observatory tools and services for catalogue searches, crosscorrelation and plotting namely TOPCAT (http://www.star.bris.ac.uk/~mbt/topcat/), and VizieR (http://vizier.u-strasbg.fr/cgi-bin/VizieR). E.H. would like to thank Kambiz Fathi and Jacopo Fritz for the very useful discussions. We thank the anonymous referee for the very insightful comments.

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