Growing black holes and galaxies: black hole accretion versus star
  formation rate
  We present a new suite of hydrodynamical simulations and use it to study, in
detail, black hole and galaxy properties. The high time, spatial and mass
resolution, and realistic orbits and mass ratios, down to 1:6 and 1:10, enable
us to meaningfully compare star formation rate (SFR) and BH accretion rate
(BHAR) timescales, temporal behaviour and relative magnitude. We find that (i)
BHAR and galaxy-wide SFR are typically temporally uncorrelated, and have
different variability timescales, except during the merger proper, lasting
~0.2-0.3 Gyr. BHAR and nuclear (<100 pc) SFR are better correlated, and their
variability are similar. Averaging over time, the merger phase leads typically
to an increase by a factor of a few in the BHAR/SFR ratio. (ii) BHAR and
nuclear SFR are intrinsically proportional, but the correlation lessens if the
long-term SFR is measured. (iii) Galaxies in the remnant phase are the ones
most likely to be selected as systems dominated by an active galactic nucleus
(AGN), because of the long time spent in this phase. (iv) The timescale over
which a given diagnostic probes the SFR has a profound impact on the recovered
correlations with BHAR, and on the interpretation of observational data.
