DECOMPOSING STAR FORMATION AND ACTIVE GALACTIC NUCLEUS WITH SPITZER MID-INFRARED SPECTRA: LUMINOSITY FUNCTIONS AND CO-EVOLUTION

Journal Article · Sun Oct 10 00:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/722/1/653· OSTI ID:21464687

Hai, Fu; Scoville, N Z ^[1]; Lin, Yan ^[2]; Capak, P ^[3]; Aussel, H; Le Floc'h, E ^[4]; Ilbert, O ^[5]; Salvato, M ^[6]; Kartaltepe, J S ^[7]; Frayer, D T ^[8]; Sanders, D B ^[9]; Sheth, K ^[10]; Taniguchi, Y ^[11]

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1. Astronomy Department, California Institute of Technology, MS 249-17, Pasadena, CA 91125 (United States)
2. Spitzer Science Center, California Institute of Technology, MS 220-06, Pasadena, CA 91125 (United States)
3. Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125 (United States)
4. Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu-CNRS-Universite Paris Diderot, CE-Saclay, pt courrier 131, F-91191 Gif-sur-Yvette (France)
5. Laboratoire d'Astrophysique de Marseille, BP 8, Traverse du Siphon, 13376 Marseille Cedex 12 (France)
6. IPP-Max-Planck-Institute for Plasma Physics, Boltzmannstrasse 2, D-85748, Garching (Germany)
7. National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85721 (United States)
8. National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944 (United States)
9. Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 (United States)
10. Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
11. Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577 (Japan)

We present Spitzer 7-38 {mu}m spectra for a 24 {mu}m flux-limited sample of galaxies at z {approx} 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 {mu}m and 15 {mu}m 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 {mu}m) LF from 70 {mu}m selected galaxies shows a shallower bright-end slope than the bolometrically corrected SF 15 {mu}m 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{sub BH} - M{sub bulge} relation and the M{sub BH} - M{sub host} relation at z {approx} 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, {approx}90% of them can be identified with their near-infrared spectral indices.

• OSTI ID: 21464687
• Journal Information: Astrophysical Journal, Vol. 722, Issue 1; Other Information: DOI: 10.1088/0004-637X/722/1/653; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
BLACK HOLES
ENERGY SPECTRA
GALACTIC EVOLUTION
INFRARED SPECTRA
LUMINOSITY
MASS
QUASARS
STAR EVOLUTION
STARS
UNIVERSE
X-RAY GALAXIES
COSMIC RADIO SOURCES
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
EVOLUTION
GALAXIES
OPTICAL PROPERTIES
PHYSICAL PROPERTIES
SPECTRA