On the Evolution of the Bi-Modal Distribution of Galaxies in the Plane
  of Specific Star Formation Rate versus Stellar Mass
  We have compared the observed distribution of galaxies in the plane of
specific star formation rate versus stellar mass with the predictions of the
Garching semi-analytic model at redshifts 0, 1 and 2. The goal is to test
whether the implementation of radio mode AGN feedback, which is responsible for
terminating the formation of stars in high mass galaxies, provides an adequate
match to current high-redshift observations. The fraction of quenched galaxies
as a function of stellar mass in the models is in good agreement with data at
z=0 and z=1. By z=2, there are too few quenched galaxies with low stellar
masses in the models. At z=2, the population of galaxies with no ongoing star
formation is clearly separated from the `main sequence' of star-forming
galaxies in the data. This is not found in the models, because z=2 galaxies
with stellar masses less than 10^11 solar masses are predicted to host black
holes with relatively low masses (less than 10^8 solar masses). The current
implementation of radio mode feedback from such black holes reduces the cooling
rates from the surrounding halo, but does not generate sufficient energy to
stop star formation entirely. We suggest that the models may be brought into
better agreement with the data if black hole growth is triggered by disc
instabilities in addition to major mergers, and if feedback mechanisms
associated with the formation of galactic bulges act to quench star formation
in galaxies.
