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Decomposing Star Formation and Active Galactic Nucleus with Spitzer Mid-infrared Spectra: Luminosity Functions and Co-evolution

Fu, Hai and Yan, Lin and Scoville, N. Z. and Capak, P. and Aussel, H. and Le Floc'h, E. and Ilbert, O. and Salvato, M. and Kartaltepe, J. S. and Frayer, D. T. and Sanders, D. B. and Sheth, K. and Taniguchi, Y. (2010) Decomposing Star Formation and Active Galactic Nucleus with Spitzer Mid-infrared Spectra: Luminosity Functions and Co-evolution. Astrophysical Journal, 722 (1). pp. 653-667. ISSN 0004-637X. doi:10.1088/0004-637X/722/1/653.

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We present Spitzer 7-38 μm spectra for a 24 μm flux-limited sample of galaxies at z ~ 0.7 in the COSMOS field. The detailed high-quality spectra allow us to cleanly separate star formation (SF) and active galactic nucleus (AGN) in individual galaxies. We first decompose mid-infrared luminosity functions (LFs). We find that the SF 8 μm and 15 μm LFs are well described by Schechter functions. AGNs dominate the space density at high luminosities, which leads to the shallow bright-end slope of the overall mid-infrared LFs. The total infrared (8-1000 μm) LF from 70 μm selected galaxies shows a shallower bright-end slope than the bolometrically corrected SF 15 μm LF, owing to the intrinsic dispersion in the mid-to-far-infrared spectral energy distributions. We then study the contemporary growth of galaxies and their supermassive black holes (BHs). Seven of the thirty-one luminous infrared galaxies with Spitzer spectra host luminous AGNs, implying an AGN duty cycle of 23% ± 9%. The time-averaged ratio of BH accretion rate and SF rate matches the local M_(BH) – M_(bulge) relation and the M_(BH) – M_(host) relation at z ~ 1. These results favor co-evolution scenarios in which BH growth and intense SF happen in the same event but the former spans a shorter lifetime than the latter. Finally, we compare our mid-infrared spectroscopic selection with other AGN identification methods and discuss candidate Compton-thick AGNs in the sample. While only half of the mid-infrared spectroscopically selected AGNs are detected in X-ray, ~90% of them can be identified with their near-infrared spectral indices.

Item Type:Article
Related URLs:
URLURL TypeDescription
Fu, Hai0000-0001-9608-6395
Yan, Lin0000-0003-1710-9339
Scoville, N. Z.0000-0002-0438-3323
Capak, P.0000-0003-3578-6843
Aussel, H.0000-0002-1371-5705
Ilbert, O.0000-0002-7303-4397
Salvato, M.0000-0001-7116-9303
Kartaltepe, J. S.0000-0001-9187-3605
Frayer, D. T.0000-0003-1924-1122
Sanders, D. B.0000-0002-1233-9998
Sheth, K.0000-0002-5496-4118
Taniguchi, Y.0000-0003-2247-3741
Additional Information:© 2010 The American Astronomical Society. Received 2010 June 15; accepted 2010 August 17; published 2010 September 22. We thank Knud Jahnke, Mark Sargent, Giovanni Zamorani, and the anonymous referee for helpful comments. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech under grant JPL-1344606.
Group:COSMOS, Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:galaxies: active; galaxies: luminosity function, mass function; infrared: galaxies; quasars: general; X-rays: galaxies
Issue or Number:1
Classification Code:PACS: 97.10.Bt; 98.54.Cm; 98.62.Qz; 98.62.Js; 98.62.Ai; 95.30.Ky
Record Number:CaltechAUTHORS:20101104-083052443
Persistent URL:
Official Citation:Hai Fu et al 2010 ApJ 722 653 doi: 10.1088/0004-637X/722/1/653
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:20659
Deposited On:04 Nov 2010 15:57
Last Modified:09 Nov 2021 00:02

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