STELLAR MASSES OF LYMAN BREAK GALAXIES, Lyα EMITTERS, AND RADIO GALAXIES IN OVERDENSE REGIONS ATz= 4-6

Abstract

We present new information on galaxies in the vicinity of luminous radio galaxies (RGs) and quasars at z 4, 5, and 6. These fields were previously found to contain overdensities of Lyman Break Galaxies (LBGs) or spectroscopic Lyα emitters, which were interpreted as evidence for clusters-in-formation ("protoclusters"). We use Hubble Space Telescope and Spitzer data to infer stellar masses from stellar synthesis models calibrated against the Millennium Run simulations, and contrast our results with large samples of LBGs in more average environments as probed by the Great Observatories Origins Deep Survey (GOODS). The following results were obtained. First, LBGs in both overdense regions and in the field at z = 4-5 lie on a very similar sequence in a z -[3.6] versus 3.6 μm color-magnitude diagram. This is interpreted as a sequence in stellar mass (M * ∼ 10 9 -10 11 M ) in which galaxies become increasingly red due to dust and age as their star formation rate (SFR) increases, while their specific SFR stays constant. Second, the two RGs are among the most massive objects (M * ∼ 10 11 M ) known to exist at z 4-5, and are extremely rare based on the low number density of such objects as estimated from the ∼25× larger area GOODS survey. We suggest that the presence of the massive (radio) galaxies and associated supermassive black holes has been boosted through rapid accretion of gas or merging inside overdense regions. Third, the total stellar mass found in the z = 4 protocluster TN1338 accounts for <30% of the stellar mass on the cluster red sequence expected to have formed at z 4, based on a comparison with the massive X-ray cluster Cl1252 at z = 1.2. Although future near-infrared observations should determine whether any massive galaxies are currently being missed by our UV/Lyα selections, one possible explanation for this mass difference is that TN1338 evolves into a smaller cluster than Cl1252. This raises the interesting question of whether the most massive protocluster regions at z > 4 remain yet to be discovered.

Discovery of a protocluster at z ∼ 6

“…In earlier epochs, at z ∼ 4−5, Overzier et al (2009b) found no significant difference in the stellar mass between protocluster galaxies and field galaxies. Additionally, Overzier et al (2009b) showed that the total stellar mass of protocluster galaxies at z ∼ 4 is much smaller than that of massive cluster galaxies at z ∼ 1 which are expected to have formed at z 4. This may suggest that there are more massive structures at z > 4, which will become rich cluster in the local universe.…”

Ultra-red Galaxies Signpost Candidate Protoclusters at High Redshift

“…This is reminiscent of present-day massive cD ETGs, which dominate the centers of present-day galaxy clusters (Kelvin et al 2014). However, we stress that without optical/near-IR imaging of these ultra-red galaxy environments, we are potentially missing many galaxies, each of which could contribute » worth of stars to the final system (Overzier et al 2009b;Casey et al 2015); thus the possible stellar masses of these systems are largely unconstrained, and all these results should be regarded as firm lower limits.…”

The Emission-Line Spectra of Major Mergers: Evidence for Shocked Outflows

“…Regardless of how these galaxies get their gas, comparable SFRs are only found among ULIRGs and the Lyman-Break Analogs (Overzier et al 2008(Overzier et al , 2009a(Overzier et al , 2009bHeckman et al 2011) in the local universe, so these environments provide the best local laboratories for studying the feedback processes that shape the evolution of high-redshift galaxies.…”