Star-forming fractions and galaxy evolution with redshift in rich
  X-ray-selected galaxy clusters
  We have compared stacked spectra of galaxies, grouped by environment and
stellar mass, among 58 members of the redshift z = 1.24 galaxy cluster RDCS
J1252.9-2927 (J1252.9) and 134 galaxies in the z = 0.84 cluster RX J0152.7-1357
(J0152.7). These two clusters are excellent laboratories to study how galaxies
evolve from star-forming to passive at z ~ 1. We measured spectral indices and
star-forming fractions for our density- and mass-based stacked spectra. The
star-forming fraction among low-mass galaxies (< 7 x 10^10 M_sun) is higher in
J1252.9 than in J0152.7, at about 4 sigma significance. Thus star formation is
being quenched between z = 1.24 and z = 0.84 for a substantial fraction of
low-mass galaxies. Star-forming fractions were also found to be higher in
J1252.9 in all environments, including the core. Passive galaxies in J1252.9
have systematically lower D_n4000 values than in J0152.7 in all density and
mass groups, consistent with passive evolution at modestly super-solar
metallicities.
