The Contribution of AGNs and Star-forming Galaxies to the Mid-Infrared as Revealed by Their Spectral Energy Distributions

Journal Article · Mon Sep 01 04:00:00 EDT 2008 · Astrophysical Journal

DOI:https://doi.org/10.1086/589848; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)· OSTI ID:21313629

Gruppioni, C; Zamorani, G; Comastri, A; Pozzetti, L ^[1]; Pozzi, F; Vignali, C ^[2]; Polletta, M ^[3]; La Franca, F; Sacchi, N ^[4]; Lonsdale, C; Surace, J; Shupe, D; Fang, F ^[5]; Rowan-Robinson, M ^[6]; Matute, I ^[7]; Berta, S ^[8]

---

1. INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna (Italy)
2. Dipartimento di Astronomia, Universita di Bologna, via Ranzani 1, I-40127 Bologna (Italy)
3. Institut d'Astrophysique de Paris, 98 bis Boulevard Arago, 75014 Paris (France)
4. Dipartimento di Fisica, Universita degli Studi 'Roma Tre', via della Vasca Navale 84, I-00146 Rome (Italy)
5. Infrared Processing and Analysis Center, Mail Stop 100-22, California Institute of Technology, Pasadena, CA 91125 (United States)
6. Astrophysics Group, Blackett Laboratory, Imperial College of Science Technology and Medicine, Prince Consort Road, London SW7 2BZ (United Kingdom)
7. INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Florence (Italy)
8. Dipartimento di Astronomia, Universita di Padova, vicolo Osservatorio 2, I-35122 Padua (Italy)

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 {mu}m 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 ({approx}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 S{sub 15{mu}}{sub m} {approx} 0.5-1 mJy and gradually increasing to 100% at S{sub 15{mu}}{sub m} > 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.

• OSTI ID: 21313629
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 684; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

99 GENERAL AND MISCELLANEOUS
COSMOLOGY
DUSTS
ENERGY SPECTRA
FLUX DENSITY
GALACTIC EVOLUTION
LUMINOSITY
RED SHIFT
SEYFERT GALAXIES
SPECTROSCOPY
STARS
UNIVERSE