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Variability-selected low-luminosity active galactic nuclei in the 4 Ms Chandra Deep Field-South

Young, M.; Brandt, W.N.; Xue, Y.Q.; Paolillo, M.; Alexander, D.M.; Bauer, F.E.; Lehmer, B.D.; Luo, B.; Shemmer, O.; Schneider, D.P.; Vignali, C.

Variability-selected low-luminosity active galactic nuclei in the 4 Ms Chandra Deep Field-South Thumbnail


M. Young

W.N. Brandt

Y.Q. Xue

M. Paolillo

F.E. Bauer

B.D. Lehmer

B. Luo

O. Shemmer

D.P. Schneider

C. Vignali


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 (~month-years, observed frame) in 20 of 92 CDF-S galaxies spanning redshifts z ≈ 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 Γstack ≈ 1.93 ± 0.13, and are therefore likely LLAGNs. The LLAGNs tend to lie a factor of ≈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.


Young, M., Brandt, W., Xue, Y., Paolillo, M., Alexander, D., Bauer, F., …Vignali, C. (2012). Variability-selected low-luminosity active galactic nuclei in the 4 Ms Chandra Deep Field-South. Astrophysical Journal, 748(2), Article 124.

Journal Article Type Article
Publication Date Jan 1, 2012
Deposit Date Jun 13, 2012
Publicly Available Date Dec 23, 2013
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 748
Issue 2
Article Number 124
Keywords Galaxies, Active, X-rays.
Public URL


Published Journal Article (2.1 Mb)

Copyright Statement
© 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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