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

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

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The 4Ms 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.

Original languageEnglish (US)
Article number124
JournalAstrophysical Journal
Issue number2
StatePublished - Apr 1 2012

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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