CO in the ALMA Radio-Source Catalogue (ARC): the molecular gas content
  of radio galaxies as a function of redshift
  To evaluate the role of radio activity in galaxy evolution, we designed a
large archival CO survey of radio galaxies (RGs) to determine their molecular
gas masses at different epochs. We used a sample of 120 RGs representative of
the NVSS 1.4GHz survey, when flux limited at 0.4Jy. Of those, 66 galaxies
belonged to the ALMA Radio-source Catalogue (ARC) of calibrators and had
spectral window tunings around CO (1-0), (2-1), (3-2), or (4-3). We reduced
their ALMA data, determined their H2 mass contents, and combined the results
with similar results for the remaining 54 galaxies from the literature. We
found that, while at all epochs the majority of RGs have undetectable
reservoirs, there is a rapid increase in the H2 mass content of the CO-detected
RGs with z. At 1<z<2.5, 1/4 of the RGs have at least as much molecular gas as
simulations would indicate for a typical halo mass of that epoch. These
galaxies plausibly have 'normal' or even starbursty hosts. Taking into account
the completeness correction of the sample, we created the corresponding H2 mass
functions at 0.005<z<0.3 and 1<z<2.5. The local mass function reveals that the
number density of low-z RGs with detectable molecular gas reservoirs is only a
little lower (a factor of ~4) than that of type 1 and 2 AGN in simulations. At
1<z<2.5, there is a significant decrease in the number density of high-z RGs
due to the rarity of bright radio galaxies. An estimate for the missing faint
RGs would, nonetheless, bring populations close again. Finally, we find that
the volume density of molecular gas locked up in the brightest 1/5000-1/7000
RGs is similar in the examined z bins. This result likely indicates that the
inflow rate on one hand and the star-formation depletion rate plus the
jet-driven expulsion rate on the other hand counteract each other in the most
luminous RGs of each epoch.
