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The contribution of AGNs and star-forming galaxies to the mid-infrared as revealed by their spectral energy distributions

Gruppioni, C. and Pozzi, F. and Polletta, M. and Zamorani, G. and La Franca, F. and Sacchi, N. and Comastri, A. and Pozzetti, L. and Vignali, C. and Lonsdale, C. and Rowan-Robinson, M. and Surace, J. and Shupe, D. and Fang, F. and Matute, I. and Berta, S. (2008) The contribution of AGNs and star-forming galaxies to the mid-infrared as revealed by their spectral energy distributions. Astrophysical Journal, 684 (1). pp. 136-152. ISSN 0004-637X. doi:10.1086/589848.

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We present the broadband SEDs of the largest available highly complete (72%) spectroscopic sample of MIR-selected galaxies and AGNs at intermediate redshift. The sample contains 203 extragalactic sources from the 15 μm ELAIS-SWIRE survey, all with measured spectroscopic redshift. Most of these sources have full multiwavelength coverage from the FUV (GALEX) to the FIR (Spitzer) and lie in the redshift range 0.1 < z < 1.3. This large sample allows us for the first time to characterize the spectral properties of sources responsible for the strong evolution observed in the MIR. Based on SED-fitting, we have classified the MIR sources, identifying AGN signatures in about 50% of them. This fraction is significantly higher than that derived from optical spectroscopy (~29%) and is due in particular to the identification of AGN activity in objects spectroscopically classified as galaxies (the spectroscopic classification may be somewhat unreliable because of host galaxy dilution in the optical). It is likely that in most of our objects, the AGN is either obscured or low luminosity, and thus dominates the energetic output only in the MIR, showing up just in the range where the host galaxy SED has a minimum. The fraction of AGNs strongly depends on flux density, with that derived through the SED-fitting about 20% at S15μm ~ 0.5–1 mJy and gradually increasing to 100% at S15μm > 10 mJy, while that obtained from optical spectroscopy is never >30%, even at higher flux densities. Our results will be very useful for updating all models aimed at interpreting the deep IR survey data and in particular for constraining the nature and role of dust-obscured systems in the intermediate/high-redshift universe.

Item Type:Article
Related URLs:
URLURL TypeDescription
Gruppioni, C.0000-0002-5836-4056
Polletta, M.0000-0001-7411-5386
Comastri, A.0000-0003-3451-9970
Vignali, C.0000-0002-8853-9611
Lonsdale, C.0000-0003-0898-406X
Surace, J.0000-0001-7291-0087
Shupe, D.0000-0003-4401-0430
Matute, I.0000-0003-1177-3896
Berta, S.0000-0002-0320-1532
Additional Information:© 2008 The American Astronomical Society. Received 2007 June 29; accepted 2008 April 25. The authors acknowledge financial contribution from the contracts PRIN-INAF, ASI-INAF I/023/05/0, and PRINMIUR 2006025203.We thank F. Fiore and M. Mignoli for useful discussions. We are grateful to the anonymous referee for useful comments that improved the quality of the paper.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Istituto Nazionale di Astrofisica (INAF)
Agenzia Spaziale Italiana (ASI)I/023/05/0
Ministero dell'Istruzione, dell'Universita e della Ricerca (MIUR)2006025203
Subject Keywords:cosmology: observations; galaxies: evolution; galaxies: Seyfert; galaxies: starburst; infrared: galaxies
Issue or Number:1
Record Number:CaltechAUTHORS:20090408-080213990
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13886
Deposited By: Tony Diaz
Deposited On:24 Apr 2009 15:08
Last Modified:08 Nov 2021 22:41

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