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

Abstract

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.

The structure and evolution of a forming galaxy cluster atz= 1.62

“…Wylezalek et al 2013;Planck Collaboration et al 2015). The handful of z > 1.5 protoclusters that have been studied in detail reveal a strongly starforming galaxy population, and accelerated mass growth compared to field galaxies (Steidel et al 2005;Hatch et al 2011b;Cooke et al 2014), which supports a picture of accelerated galaxy formation before the cluster has assembled. But in comparison to lower-redshift clusters, our understanding of z > 1.5 clusters and protoclusters is woefully poor.…”

Disentangling star formation and AGN activity in powerful infrared luminous radio galaxies at 1 <z< 4

“…10 -Also named the "Spiderweb galaxy", this galaxy is at z = 2.16 (van Ojik 1995). Its unusual morphology consists of a central massive galaxy surrounded by fainter companions (Pentericci et al 1998;Miley et al 2006;Kuiper et al 2010;Hatch et al 2011;Seymour et al 2012). The galaxy has an extension that is aligned with the radio jet axis.…”

Defying jet-gas alignment in two radio galaxies atz~ 2 with extended light profiles: Similarities to brightest cluster galaxies

“…Narrowband imaging is a standard way of identifying emissionline galaxies at high redshifts, and has been successfully used to identify over-densities of actively star-forming galaxies around high-redshift galaxies for almost two decades (e.g., Chambers et al 1996;Le Fevre et al 1996;Kurk et al 2004a;Venemans et al 2007;Hatch et al 2011;Koyama et al 2013;Hayashi et al 2012;Cooke et al 2014). Most of these observations were carried out for HzRGs at z 2, where Hα falls in the K band, which is relatively free from telluric night-sky lines.…”