COSMOS2020: Ubiquitous AGN Activity of Massive Quiescent Galaxies at
  $0<z<5$ Revealed by X-ray and Radio Stacking
  We characterize the average X-ray and radio properties of quiescent galaxies
(QGs) with $\log{(M_\star/M_\odot)}>10$ at $0<z<5$. QGs are photometrically
selected from the latest COSMOS2020 catalog. We conduct the stacking analysis
of X-ray images of the Chandra COSMOS Legacy Survey for individually undetected
QGs. Thanks to the large sample and deep images, the stacked X-ray signal is
significantly detected up to $z\sim5$. The average X-ray luminosity can not be
explained by the X-ray luminosity of X-ray binaries, suggesting that the
low-luminosity active galactic nuclei (AGNs) ubiquitously exist in QGs.
Moreover, the X-ray AGN luminosity of QGs at $z>1.5$ is higher than that of
star-forming galaxies (SFGs), derived in the same manner as QGs. The stacking
analysis of the VLA-COSMOS images is conducted for the identical sample, and
the radio signal for QGs is also detected up to $z\sim5$. We find that the
radio AGN luminosity of QGs at $z>1.5$ is also higher than SFGs, which is in
good agreement with the X-ray analysis. The enhanced AGN activity in QGs
suggested by the individual analysis in the X-ray and radio wavelength supports
its important role for quenching at high redshift. Their enhanced AGN activity
is less obvious at $z<1.5$, which can be interpreted as an increasing role of
others at lower redshifts, such as environmental quenching.
