VARIABILITY-SELECTED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI IN THE 4 Ms CHANDRA DEEP FIELD-SOUTH

Journal Article · Sun Apr 01 04:00:00 EDT 2012 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/748/2/124· OSTI ID:22016069

Young, M; Brandt, W N; Xue, Y Q; Schneider, D P ^[1]; Paolillo, M ^[2]; Alexander, D M ^[3]; Bauer, F E ^[4]; Lehmer, B D ^[5]; Luo, B ^[6]; Shemmer, O ^[7]; Vignali, C ^[8]

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1. Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802 (United States)
2. Dipartimento di Scienze Fisiche, Universita Federico II di Napoli, Via Cinthia, 80126 Napoli (Italy)
3. Department of Physics, University of Durham, South Road, Durham DH1 3LE (United Kingdom)
4. Departamento de Astronomia y Astrofisica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22 (Chile)
5. Johns Hopkins University, Homewood Campus, Baltimore, MD 21218 (United States)
6. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
7. Department of Physics, University of North Texas, Denton, TX 76203 (United States)
8. Dipartimento di Astronomia, Universita di Bologna, Via Ranzani 1, 40127 Bologna (Italy)

The 4 Ms Chandra Deep Field-South (CDF-S) and other deep X-ray surveys have been highly effective at selecting active galactic nuclei (AGNs). However, cosmologically distant low-luminosity AGNs (LLAGNs) have remained a challenge to identify due to significant contribution from the host galaxy. We identify long-term X-ray variability ({approx}month-years, observed frame) in 20 of 92 CDF-S galaxies spanning redshifts z Almost-Equal-To 0.08-1.02 that do not meet other AGN selection criteria. We show that the observed variability cannot be explained by X-ray binary populations or ultraluminous X-ray sources, so the variability is most likely caused by accretion onto a supermassive black hole (SMBH). The variable galaxies are not heavily obscured in general, with a stacked effective power-law photon index of {Gamma}{sub stack} Almost-Equal-To 1.93 {+-} 0.13, and are therefore likely LLAGNs. The LLAGNs tend to lie a factor of Almost-Equal-To 6-80 below the extrapolated linear variability-luminosity relation measured for luminous AGNs. This may be explained by their lower accretion rates. Variability-independent black hole mass and accretion-rate estimates for variable galaxies show that they sample a significantly different black hole mass-accretion-rate space, with masses a factor of 2.4 lower and accretion rates a factor of 22.5 lower than variable luminous AGNs at the same redshift. We find that an empirical model based on a universal broken power-law power spectral density function, where the break frequency depends on SMBH mass and accretion rate, roughly reproduces the shape, but not the normalization, of the variability-luminosity trends measured for variable galaxies and more luminous AGNs.

• OSTI ID: 22016069
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 748; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
BLACK HOLES
GALAXY NUCLEI
LUMINOSITY
MASS
RED SHIFT
SPECTRAL DENSITY
X-RAY GALAXIES