Black-hole masses, accretion rates and hot- and cold-mode accretion in
  radio galaxies at z~1
  Understanding the evolution of accretion activity is fundamental to our
understanding of how galaxies form and evolve over the history of the Universe.
We analyse a complete sample of 27 radio galaxies which includes both
high-excitation (HEGs) and low excitation galaxies (LEGs), spanning a narrow
redshift range of 0.9 < z < 1.1 and covering a factor of ~1000 in radio
luminosity. Using data from the Spitzer Space Telescope combined with
ground-based optical and near-infrared imaging, we show that the host galaxies
have masses in the range of 10.7 < log (M /M_sun) < 12.0 with HEGs and LEGs
exhibiting no difference in their mass distributions. We also find that HEGs
accrete at significantly higher rates than LEGs, with the HEG/LEG division
lying at an Eddington ratio of ~0.04, which is in excellent agreement with
theoretical predictions of where the accretion rate becomes radiatively
inefficient, thus supporting the idea of HEGs and LEGs being powered by
different modes of accretion. Our study also shows that at least up to L_151MHz
~3x10^27 W /Hz /sr, HEGs and LEGs are indistinguishable in terms of their radio
properties. From this result we infer that, at least for the lower radio
luminosity range, another factor besides accretion rate must play an important
role in the process of triggering jet activity.
