Radio galaxies in ZFOURGE/NMBS: no difference in the properties of
  massive galaxies with and without radio-AGN out to z = 2.25
  In order to reproduce the high-mass end of the galaxy mass-distribution, some
process must be responsible for the suppression of star-formation in the most
massive of galaxies. Commonly Active Galactic Nuclei (AGN) are invoked to
fulfil this role, but the exact means by which they do so is still the topic of
much debate, with studies finding evidence for both the suppression and
enhancement of star-formation in AGN hosts. Using the ZFOURGE and NMBS galaxy
surveys, we investigate the host galaxy properties of a mass-limited
(M$_{\odot}$ $\ge$ 10$^{10.5}$ M$_{\odot}$), high-luminosity (L$_{1.4}$ $>$
10$^{24}$ W Hz$^{-1}$) sample of radio-loud Active Galactic Nuclei to a
redshift of z = 2.25. In contrast to low redshift studies, which associate
radio-AGN activity with quiescent hosts, we find that the majority of z $>$ 1.5
radio-AGN are hosted by star-forming galaxies. Indeed, the stellar populations
of radio-AGN are found to evolve with redshift in a manner that is consistent
with the non-AGN mass-similar galaxy population. Interestingly, we find the
radio-AGN fraction is constant across a redshift range of 0.25 $\le$ z $<$
2.25, perhaps indicating that the radio-AGN duty cycle has little dependence on
redshift or galaxy type. We do however see a strong relation between the
radio-AGN fraction and stellar mass, with radio-AGN becoming rare below $\sim$
10$^{10.5}$ M$_{\odot}$ or a halo-mass of 10$^{12}$ M$_{\odot}$. This halo-mass
threshold is in good agreement with simulations that initiate radio-AGN
feedback at this mass limit. Despite this we find that radio-AGN host
star-formation rates are consistent with the non-AGN mass-similar galaxy
sample, suggesting that while radio-AGN are in the right place to suppress
star-formation in massive galaxies they are not necessarily responsible for
doing so.
