SURVEYING GALAXY PROTO-CLUSTERS IN EMISSION: A LARGE-SCALE STRUCTURE ATz= 2.44 AND THE OUTLOOK FOR HETDEX

Abstract

Galaxy proto-clusters at z 2 provide a direct probe of the rapid mass assembly and galaxy growth of present day massive clusters. Because of the need of precise galaxy redshifts for density mapping and the prevalence of star formation before quenching, nearly all the proto-clusters known to date were confirmed by spectroscopy of galaxies with strong emission lines. Therefore, large emission-line galaxy surveys provide an efficient way to identify proto-clusters directly. Here we report the discovery of a large-scale structure at z = 2.44 in the HETDEX Pilot Survey. On a scale of a few tens of Mpc comoving, this structure shows a complex overdensity of Lyα emitters (LAE), which coincides with broad-band selected galaxies in the COSMOS/UltraVISTA photometric and zCOSMOS spectroscopic catalogs, as well as overdensities of intergalactic gas revealed in the Lyα absorption maps of Lee et al. (2014). We construct mock LAE catalogs to predict the cosmic evolution of this structure. We find that such an overdensity should have already broken away from the Hubble flow, and part of the structure will collapse to form a galaxy cluster with 10 14.5±0.4 M by z = 0. The structure contains a higher median stellar mass of broad-band selected galaxies, a boost of extended Lyα nebulae, and a marginal excess of active galactic nuclei relative to the field, supporting a scenario of accelerated galaxy evolution in cluster progenitors. Based on the correlation between galaxy overdensity and the z = 0 descendant halo mass calibrated in the simulation, we predict that several hundred 1.9 < z < 3.5 proto-clusters with z = 0 mass of > 10 14.5 M will be discovered in the 8.5 Gpc 3 of space surveyed by the Hobby Eberly Telescope Dark Energy Experiment.in the largest halos first, i.e., in cluster progenitors during this epoch (Kereš et al. 2005;Dekel & Birnboim 2006;Dekel et al. 2009b). The subsequent virialization on both galaxy and cluster scales in about a dynamical timescale largely erases the signatures of the aforementioned processes, placing a fundamental limit on inferences based on the largely archaeological record of cluster formation based upon near-field studies. Direct studies of cluster progenitors thus provide irreplaceable probes to understand the formation of present day massive clusters.The search for high-redshift cluster progenitors is challenging due to their lack of mature cluster signatures such as extended X-ray emission (Fassbender et al. 2011), the Sunyaev-Zel'dovich effect (Bleem et al. 2015), and the prominent galaxy red sequence (Gladders & Yee 2005;Gilbank et al. 2011). The fundamental picture of gravitational structure formation implies that the most massive collapsed objects evolved from the densest regions in the early universe on a large scale (Kravtsov & Borgani 2012, and references therein). The finding of proto-clusters requires identifying galaxy overdensities in three-dimensions using precise redshift measurements (Chiang et al. 2013b).Active star formation in cluster progenitors implies that (at least ...

LARGE-SCALE STRUCTURE AROUND A z = 2.1 CLUSTER

“…Based on the spatial and redshift distribution traced by zCOSMOS SFGs and the population of DSFGs, it is possible that the large-scale structure associated with the ZFIRE cluster may extend beyond the ZFIRE survey area. In fact, Chiang et al (2015) also detect three LAEs a few arcmins offset from the ZFIRE cluster (their positions are also shown in Figure 2). Such a large spatial extent traced by these various galaxies is consistent with the cosmological simulations (e.g., Chiang et al 2013), where thez 2.1 cluster may still be actively accreting from large-scale structure while more mature cluster cores begin to assemble in the densest regions.…”

“…Red circles show galaxies with L IR  10 12 L e , and blue boxes indicate X-ray sources. Purple diamonds show the positions of three LAEs from Chiang et al (2015). Brown crosses mark the group candidates identified by Diener et al (2013).…”

SHADOW OF A COLOSSUS: A z = 2.44 GALAXY PROTOCLUSTER DETECTED IN 3D Lyα FOREST TOMOGRAPHIC MAPPING OF THE COSMOS FIELD

“…0 3 z ( ) s + » and <10% catastrophic failure rate. This target selection process was carried out in 2014 February, prior to the publications by Diener et al (2015) and Chiang et al (2015) reporting galaxy overdensities in the field. We observed these targets with the LRIS DoubleSpectrograph (Oke et al 1995;Steidel et al 2004) on the Keck I telescope at Maunakea, Hawai'i, during 2014 March 26-27/29-30 and April 18-20, using multi-object slitmasks.…”

“…Two other developments in the past year lend additional synergy: (i) the first data release of the MOSDEF survey (Kriek et al 2015, hereafter K15), whose near-infrared (NIR) nebular line redshifts reduce systemic redshift uncertainties hence allowing a cleaner comparison with our tomographic map; and (ii) the discovery of a z=2.45 galaxy protocluster within our target field (Diener et al 2015;hereafter D15;Chiang et al 2015; hereafter C15).…”

The VIMOS Ultra Deep Survey first data release: Spectra and spectroscopic redshifts of 698 objects up to zspec ~ 6 in CANDELS

“…In the COSMOS field the two most prominent peaks in the VUDS-DR1 N(z) are at z ∼ 2.45 and z ∼ 2.9. The strong over-density at z ∼ 2.45 was recently pointed out by Chiang et al (2015) and identified by Casey et al (2015) as a proto-cluster including seven star-bursting submillimeterluminous galaxies and five AGN. In the CANDELS-COSMOS area alone we find 22 galaxies with 2.45 ≤ z spec ≤ 2.5.…”