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.

Abstract

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.

Citation

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. https://doi.org/10.1088/0004-637x/748/2/124

• 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
• DOI: https://doi.org/10.1088/0004-637x/748/2/124
• Keywords: Galaxies, Active, X-rays.
• Public URL: https://durham-repository.worktribe.com/output/1505783