Hα emitters in z∼ 2 protoclusters: evidence for faster evolution in dense environments

This is a study of Hα emitters in two dense galaxy protoclusters surrounding radio galaxies at z∼ 2. We show that the protocluster surrounding MRC 1138−262 contains 14 ± 2 times more Hα candidates than the average field (9σ significance), and the z= 2.35 radio galaxy 4C +10.48 is surrounded by 12 ± 2 times more emitters than the field (5σ), so it is also likely to reside in a dense protocluster environment. We compared these Hα emitters, situated in dense environments, to a control field sample selected from three separate fields forming a total area of 172 arcmin2. We constructed and compared Hα 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 protoclusters than the field at z∼ 2, so the total star formation rate within the central 1.5 Mpc of the protoclusters exceeds 3000 M⊙ yr−1. However, we found no significant difference in the shape of the Hα luminosity functions, implying that environment does not substantially affect the strength of the Hα line from strongly star-forming galaxies. The protocluster emitters are typically 0.8 mag 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 protocluster 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.