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The 4 Ms Chandra Deep Field-South (CDF-S) and other deep X-ray surveys havebeen highly effective at selecting active galactic nuclei (AGN). However,cosmologically distant low-luminosity AGN (LLAGN) have remained a challenge toidentify due to significant contribution from the host galaxy. We identifylong-term X-ray variability (~month-years, observed frame) in 20 of 92 CDF-Sgalaxies spanning redshifts z~0.08-1.02 that do not meet other AGN selectioncriteria. We show that the observed variability cannot be explained by X-raybinary populations or ultraluminous X-ray sources, so the variability is mostlikely caused by accretion onto a supermassive black hole. The variablegalaxies are not heavily obscured in general, with a stacked effectivepower-law photon index of Gamma_stack~1.93+/-0.13, and are therefore likelyLLAGN. The LLAGN tend to lie a factor of ~6-80 below the extrapolated linearvariability-luminosity relation measured for luminous AGN. This may beexplained by their lower accretion rates. Variability-independent black-holemass and accretion-rate estimates for variable galaxies show that they sample asignificantly different black-hole mass-accretion rate space, with masses afactor of 2.4 lower and accretion rates a factor of 22.5 lower than variableluminous AGN at the same redshift. We find that an empirical model based on auniversal broken power-law PSD function, where the break frequency depends onSMBH mass and accretion rate, roughly reproduces the shape, but not thenormalization, of the variability-luminosity trends measured for variablegalaxies and more luminous AGN.

Variability Selected Low-Luminosity Active Galactic Nuclei in the 4 Ms Chandra Deep Field-South