H-alpha emitters in z~2 proto-clusters: evidence for faster evolution in
  dense environments
  This is a study of H-alpha emitters in two dense galaxy proto-clusters
surrounding radio galaxies at z~2. We show that the proto-cluster surrounding
MRC 1138-262 contains 14+/-2 times more H-alpha candidates than the average
field (9 sigma significance), and the z=2.35 radio galaxy 4C+10.48 is
surrounded by 12+/-2 times more emitters than the field (5 sigma), so it is
also likely to reside in a dense proto-cluster environment. We compared these
H-alpha emitters, situated in dense environments, to a control field sample
selected from 3 separate fields forming a total area of 172 arcmin^2. We
constructed and compared H-alpha and rest-frame R continuum luminosity
functions of the emitters in both environments. The star formation density is
on average 13 times greater in the proto-clusters than the field at z~2, so the
total star formation rate within the central 1.5Mpc of the proto-clusters
exceeds 3000Msun/yr. However, we found no significant difference in the shape
of the H-alpha luminosity functions, implying that environment does not
substantially affect the strength of the H-alpha line from strongly star
forming galaxies. The proto-cluster emitters are typically 0.8mag brighter in
rest-frame R continuum than field emitters, implying they are twice as massive
as their field counterparts at the same redshift. We also show the
proto-cluster galaxies have lower specific star formation rates than field
galaxies, meaning the emitters in the dense environments formed more of their
stars earlier than the field galaxies. We conclude that galaxy growth in the
early Universe was accelerated in dense environments, and that cluster galaxies
differed from field galaxies even before the cluster had fully formed.
