Publication details for Prof David AlexanderYoung, 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. (2012). Variability-selected low-luminosity active galactic nuclei in the 4 Ms Chandra Deep Field-South. The astrophysical journal 748(2): 124.
- Publication type: Journal Article
- ISSN/ISBN: 0004-637X (print), 1538-4357 (electronic)
- DOI: 10.1088/0004-637X/748/2/124
- Keywords: Galaxies: active, X-rays: galaxies
- Further publication details on publisher web site
- Durham Research Online (DRO) - may include full text
Author(s) from Durham
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.