DISCOVERY OF A LARGE NUMBER OF CANDIDATE PROTOCLUSTERS TRACED BY ∼15 Mpc-SCALE GALAXY OVERDENSITIES IN COSMOS

Abstract

To demonstrate the feasibility of studying the epoch of massive galaxy cluster formation in a more systematic manner using current and future galaxy surveys, we report the discovery of a large sample of proto-cluster candidates in the 1.62 deg 2 COSMOS/UltraVISTA field traced by optical/IR selected galaxies using photometric redshifts. By comparing properly smoothed 3D galaxy density maps of the observations and a set of matched simulations incorporating the dominant observational effects (galaxy selection and photometric redshift uncertainties), we first confirm that the observed ∼ 15 comoving Mpc scale galaxy clustering is consistent with ΛCDM models. Using further the relation between high-z overdensity and the present day cluster mass calibrated in these matched simulations, we found 36 candidate structures at 1.6 < z < 3.1, showing overdensities consistent with the progenitors of M z=0 ∼ 10 15 M ⊙ clusters. Taking into account the significant upward scattering of lower mass structures, the probabilities for the candidates to have at least M z=0 ∼ 10 14 M ⊙ are ∼ 70%. For each structure, about 15% − 40% of photometric galaxy candidates are expected to be true proto-cluster members that will merge into a cluster-scale halo by z = 0. With solely photometric redshifts, we successfully rediscover two spectroscopically confirmed structures in this field, suggesting that our algorithm is robust. This work generates a large sample of uniformly-selected proto-cluster candidates, providing rich targets for spectroscopic follow-up and subsequent studies of cluster formation. Meanwhile, it demonstrates the potential for probing early cluster formation with upcoming redshift surveys such as the Hobby-Eberly Telescope Dark Energy Experiment and the Subaru Prime Focus Spectrograph survey.

Passive galaxies as tracers of cluster environments atz~ 2

“…An overdensity at a consistent position and redshift is also visible in Scoville et al (2013) density maps. We also note the proximity (a few to 14 Mpc) of confirmed/candidate structures at similar redshift (Diener et al 2014;Castignani et al 2014;Chiang et al 2014;Yuan et al 2014). …”

“…In comparison with most other (proto-)cluster search techniques at these redshifts, e.g. the "IRAC selection" (e.g., Papovich et al 2010;Stanford et al 2012), targeted searches around radio galaxies (e.g., Venemans et al 2007;Chiaberge et al 2010;Wylezalek et al 2013), 3D mapping (with spectroscopic or photometric redshifts, e.g., Diener et al 2013;Scoville et al 2013;Chiang et al 2014;Mei et al 2014), or overdensities of optically red galaxies (e.g., Andreon et al 2009;Spitler et al 2012), the approach discussed in this work is likely, by definition, to favor the most evolved environments, thus allowing a better probe of the diversity of cluster progenitors at a crucial time for the formation of both clusters and their massive galaxies.…”

Mapping the Most Massive Overdensity Through Hydrogen (Mammoth). I. Methodology

“…There are a few other overdensities at z = 2-3 that can be used to conduct similar tests (e.g., protoclusters in COSMOS, Chiang et al 2014). The complete results of Lyα absorption around confirmed overdensities are summarized (Mukae et al 2016).…”

“…The density peaks on the (5 h −1 Mpc) 3 volume could have mass overdensities reaching δ m ≈10. Such high overdensity peaks mark regions of the most overdense environments at high redshifts (e.g., Steidel et al 2005;Chiang et al 2014 In the upper panel of Figure 25 we present the SDSS-III/ BOSS spectrum of CoSLA candidate J084259.37+365704.3 at z abs = 3.17. This CoSLA candidate is centered at 5076 Å and has an effective optical depth of t = -+ 2.42 eff 0.26 0.31 , which is a factor of 5.4× the mean optical depth at z = 3.17 (Dall'Aglio et al 2008;Faucher-Giguère et al 2008;Bolton et al 2009 .…”

Galaxy clusters and groups in the ALHAMBRA survey

“…Later on, Bellagamba et al (2011) used an optical and weak lensing search to detect galaxy clusters in the COSMOS field. Two recent works, Castignani et al (2014) and Chiang et al (2014) have provided a list of high redshift (z > 1) clusters and proto-clusters detections. The former is based on detecting over densities around radio galaxies to detect high-redshift (z ∼ 1 − 2) galaxy clusters and the latter, used the photometric redshift obtained from the combination of the optical and IR data to detect very high-redshift proto-clusters (z ∼ 2 − 7).…”