Identification of the progenitors of rich clusters and member galaxies in rapid formation at z > 2

Abstract

We present the results of near-infrared spectroscopy of Hα emitters (HAEs) associated with two protoclusters around radio galaxies (PKS1138-262 at z=2.2 and USS1558-003 at z=2.5) with Multi-Object Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope. Among the HAE candidates constructed from our narrow-band imaging, we have confirmed membership of 27 and 36 HAEs for the respective protoclusters, with a success rate of 70 per cent of our observed targets. The large number of spectroscopically confirmed members per cluster has enabled us for the first time to reveal the detailed kinematical structures of the protoclusters at z>2. The clusters show prominent substructures such as clumps, filaments and velocity gradients, suggesting that they are still in the midst of rapid construction to grow to rich clusters at later times. We also estimate dynamical masses of the clusters and substructures assuming their local virialization. The inferred masses (∼10 14 M ⊙ ) of the protocluster cores are consistent with being typical progenitors of the present-day most massive class of galaxy clusters (∼10 15 M ⊙ ) if we take into account the typical mass growth history of clusters. We then calculated the integrated star formation rates of the protocluster cores normalized by the dynamical masses, and compare these with lower redshift descendants. We see a marked increase of star-forming activities in the cluster cores, by almost three orders of magnitude, as we go back in time to 11 billion years ago; this scales as (1+z) 6 .

Environmental dependence of galaxy formation explored by near-infrared spectroscopy of two protoclusters at z>2

“…The inferred dynamical masses of the protocluster cores are consistent with being the typical progenitors of present-day most massive clusters (Shimakawa et al 2014a). Also, those HAEs in the protoclusters show much higher [Oiii]/Hβ ratios than local star forming galaxies.…”

THE CANDIDATE CLUSTER AND PROTOCLUSTER CATALOG (CCPC). II. SPECTROSCOPICALLY IDENTIFIED STRUCTURES SPANNING 2 < z < 6.6

“…In the Millennium simulation, Chiang et al (2013) found that the first M ≥ 10 14 M halos are not present until z ≤ 2.3, and may not be virialized until a dynamical time later. However, there are some indications that subhalos in protoclusters can be virialized, as observed by Venemans et al (2007); Shimakawa et al (2014) and Topping et al (2016). These subhalos can be seen as bimodal distributions of protocluster galaxies along the LOS.…”

A HIGH-RESOLUTIONHUBBLE SPACE TELESCOPESTUDY OF APPARENT LYMAN CONTINUUM LEAKERS ATz∼ 3

“…Because the average surface density of LBGs down to R = 25.5 is roughly 1-2 galaxies per square arcminute (Steidel et al 1999(Steidel et al , 2004, the process of observing LBGs for LyC emission is greatly streamlined by observing galaxy protoclusters, which have an increased density of objects at a particular redshift. Several galaxy protoclusters have already been identified in the literature at 2 < z < 4 (see, e.g., Kodama et al 2007;Venemans et al 2007;Hatch et al 2011;Cucciati et al 2014;Lemaux et al 2014;Shimakawa et al 2014;Diener et al 2015). At the rate of one LyC detection per protocluster, observations of at least ∼10 protoclusters would be necessary to obtain a sample large enough to investigate systematic differences between LyC-leakers and non-leakers.…”