Abstract
Using spectroscopic observations taken for the Visible Multi-Object Spectrograph (VIMOS) Ultra-Deep Survey (VUDS) we report here on the discovery of PCl J1001+0220, a massive proto-cluster of galaxies located at zspec ~ 4.57 in the COSMOS field. With nine spectroscopic members, the proto-cluster was initially detected as a ~12σ spectroscopic overdensity of typical star-forming galaxies in the blind spectroscopic survey of the early universe (2 < z ≲ 6) performed by VUDS. It was further mapped using a new technique developed which statistically combines spectroscopic and photometric redshifts, the latter derived from a recent compilation of incredibly deep multi-band imaging performed on the COSMOS field. Through various methods, the descendant mass of PCl J1001+0220 is estimated to be log (Mh/M⊙)z=0 ~ 14.5–15 with a large amount of mass apparently already in place at z ~ 4.57. An exhaustive comparison was made between the properties of various spectroscopic and photometric member samples and matched samples of galaxies inhabiting less dense environments at the same redshifts. Tentative evidence is found for a fractional excess of older galaxies more massive in their stellar content amongst the member samples relative to the coeval field, an observation which suggests the pervasive early onset of vigorous star formation for proto-cluster galaxies. No evidence is found for the differences in the star formation rates (SFRs) of member and coeval field galaxies either through estimating by means of the rest-frame ultraviolet or through separately stacking extremely deep Very Large Array 3 GHz imaging for both samples. Additionally, no evidence for pervasive strong active galactic nuclei (AGN) activity is observed in either environment. Analysis of Hubble Space Telescope images of both sets of galaxies as well as their immediate surroundings provides weak evidence for an elevated incidence of galaxy–galaxy interaction within the bounds of the proto-cluster. The stacked and individual spectral properties of the two samples are compared, with a definite suppression of Lyα seen in the average member galaxy relative to the coeval field (fesc, Lyα = 1.8−1.7+0.3% and 4.0−0.8+1.0%, respectively). This observation along with other lines of evidence leads us to infer the possible presence of a large, cool, diffuse medium within the proto-cluster environment evocative of a nascent intracluster medium forming in the early universe.
Highlights
The last decade and a half has seen a revolution in the study of overdensities in the early Universe
Using spectroscopic observations taken for the Visible Multi-Object Spectrograph (VIMOS) Ultra-Deep Survey (VUDS) we report here on the discovery of PCl J1001+0220, a massive proto-cluster of galaxies located at zspec ∼ 4.57 in the Cosmic Evolution Survey (COSMOS) field
While some phenomena exist in the early Universe, such as quasars or radio galaxies, which are so powerful and intrinsically bright that they have been able to serve as beacons to early searches near the epoch of H I reionization (z ∼ 5.5–10, Becker et al 2001, 2015; Planck Collaboration XIII 2016), the bulk of the galaxy population residing in the early Universe does not contain such phenomena (Miley & De Breuck 2008; Ouchi et al 2008; Lemaux et al 2014b; Ueda et al 2014; Talia et al 2017)
Summary
Such a cut allows us to keep a majority of high-stellar-mass galaxies in this redshift in the sample while forcing the average brightness of objects with 4 < zphot < 5 to those similar to the spectroscopic sample such that we can reasonably apply the statistics derived to this sample This cut yields a spectroscopic sampling rate (i.e., the number of objects targeted by all surveys versus the total number of objects) within the area covered by VUDS to a value roughly twice that of a [3.6]-limited sample (16.3 vs 9.8%) and at ∼10% for objects in the magnitude range which place them as potential members (21 < [3.6] < 25.4). These methods paint a coherent picture of a highly significant overdensity assembling in the early Universe
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