Massive starburst galaxies in a z = 2.16 proto-cluster unveiled by panoramic Hα mapping

Abstract

We present a panoramic narrow-band study of Hα emitters in the field of the z = 2.16 proto-cluster around PKS 1138-262 using MOIRCS on the Subaru Telescope. We find 83 Hα emitters down to a star formation rate of SFR (Hα) ∼ 10 M_⊙ yr^-1 across a ∼ 7 × 7 arcmin² region centred on the radio galaxy, and identify ∼10-Mpc scale filaments of emitters running across this region. By examining the properties of Hα emitters within the large-scale structure, we find that galaxies in the higher density environments at z = 2.16 tend to have redder colours and higher stellar masses compared to galaxies in more underdense regions. We also find a population of Hα emitters with red colours [(J - K_s) ≳ 1], which are much more frequent in the denser environments and which have apparently very high stellar masses with M_* ≳ 10¹¹ M_⊙, implying that these cluster galaxies have already formed a large part of their stellar mass before z ∼ 2. Spitzer Space Telescope 24-μm data suggest that many of these red Hα emitters are bright, dusty starbursts (rather than quiescent sources). We also find that the proto-cluster galaxies follow the same correlation between SFR and M_* (the `main sequence') of z ∼ 2 field star-forming galaxies, but with an excess of massive galaxies. These very massive star-forming galaxies are not seen in our similar, previous study of z ∼ 1 clusters, suggesting that their star formation activity has been shut off at 1 ≲ z ≲ 2. We infer that the massive red (but active) galaxies in this rich proto-cluster are likely to be the products of environmental effects, and they represent the accelerated galaxy formation and evolution in a biased high-density region in the early Universe.