Evidence for Widespread AGN Activity among Massive Quiescent Galaxies at z ~ 2

TL;DR

This study reveals widespread AGN activity in massive, quiescent galaxies at z~2, suggesting AGN play a key role in halting star formation during galaxy evolution.

Contribution

It provides the first comprehensive evidence that a large fraction of massive quiescent galaxies at z~2 host AGN, including low-luminosity ones, highlighting AGN's role in galaxy quenching.

Findings

41% of galaxies detected in X-rays, indicating AGN presence

Up to 100% of quiescent galaxies contain low- or high-luminosity AGN

AGN activity is independent of galaxy stellar mass

Abstract

We quantify the presence of Active Galactic nuclei (AGN) in a mass-complete (M_* >5e10 M_sun) 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 AGN (L_0.5-8keV > 3e42 ergs/s). The latter fraction is similar for star-forming and quiescent galaxies, and does not depend on galaxy stellar mass, suggesting that perhaps luminous AGN 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 AGN. Among the quiescent galaxies, the excess suggests that as many as 70-100% of these contain low- or high-luminosity AGN, while the corresponding fraction is lower among star-forming galaxies (43-65%). The ubiquitous presence of AGN in massive, quiescent z ~ 2 galaxies that we find provides observational support for the importance of AGN in impeding star formation during galaxy evolution.