The Cosmic History of Hot Gas Cooling and Radio AGN Activity in Massive
  Early-Type Galaxies
  We study the X-ray properties of 393 optically selected early-type galaxies
(ETGs) over the redshift range of z~0.0-1.2 in the Chandra Deep Fields. To
measure the average X-ray properties of the ETG population, we use X-ray
stacking analyses with a subset of 158 passive ETGs (148 of which were
individually undetected in X-ray). This ETG subset was constructed to span the
redshift ranges of z = 0.1-1.2 in the ~4 Ms CDF-S and ~2 Ms CDF-N and z =
0.1-0.6 in the ~250 ks E-CDF-S where the contribution from individually
undetected AGNs is expected to be negligible in our stacking. We find that 55
of the ETGs are detected individually in the X-rays, and 12 of these galaxies
have properties consistent with being passive hot-gas dominated systems (i.e.,
systems not dominated by an X-ray bright Active Galactic Nucleus; AGN). On the
basis of our analyses, we find little evolution in the mean 0.5-2 keV to B-band
luminosity ratio (L_X/L_B proportional to [1 + z]^1.2) since z~1.2, implying
that some heating mechanism prevents the gas from cooling in these systems. We
consider that feedback from radio-mode AGN activity could be responsible for
heating the gas. We select radio AGNs in the ETG population using their
far-infrared/radio flux ratio. Our radio observations allow us to constrain the
duty cycle history of radio AGN activity in our ETG sample. We estimate that if
scaling relations between radio and mechanical power hold out to z~1.2 for the
ETG population being studied here, the average mechanical power from AGN
activity is a factor of ~1.4-2.6 times larger than the average radiative
cooling power from hot gas over the redshift range z~0-1.2. The excess of
inferred AGN mechanical power from these ETGs is consistent with that found in
the local Universe for similar types of galaxies.
