EVIDENCE FOR WIDESPREAD ACTIVE GALACTIC NUCLEUS ACTIVITY AMONG MASSIVE QUIESCENT GALAXIES AT z ∼ 2

We quantify the presence of active galactic nuclei (AGNs) in a mass-complete (M-* > 5 x 10(10) M-circle dot) sample of 123 star-forming and quiescent galaxies at 1.5 <= z <= 2.5, using X-ray data from the 4 Ms Chandra Deep Field-South (CDF-S) survey. 41% +/- 7% of the galaxies are detected directly in X-rays, 22% +/- 5% with rest-frame 0.5-8 keV luminosities consistent with hosting luminous AGNs (L0.5-8 keV > 3 x 10(42) erg s(-1)). The latter fraction is similar for star-forming and quiescent galaxies, and does not depend on galaxy stellar mass, suggesting that perhaps luminous AGNs are triggered by external effects such as mergers. We detect significant mean X-ray signals in stacked images for both the individually non-detected star-forming and quiescent galaxies, with spectra consistent with star formation only and/or a low-luminosity AGN in both cases. Comparing star formation rates inferred from the 2-10 keV luminosities to those from rest-frame IR+UV emission, we find evidence for an X-ray excess indicative of low-luminosity AGNs. Among the quiescent galaxies, the excess suggests that as many as 70%-100% of these contain low-or high-luminosity AGNs, while the corresponding fraction is lower among star-forming galaxies (43%-65%). Our discovery of the ubiquity of AGNs in massive, quiescent z similar to 2 galaxies provides observational support for the importance of AGNs in impeding star formation during galaxy evolution.