Spectroscopic observations of PHz G237.01+42.50: A galaxy protocluster at z = 2.16 in the Cosmos field

Abstract

The Planck satellite has identified more than 2000 protocluster candidates with extreme star formation rates (SFRs). Here, we present the spectroscopic identification of a Planck-selected protocluster located in the Cosmos field, PHz G237.01+42.50. PHz G237.01+42.50 contains a galaxy overdensity of 31 spectroscopically identified galaxies at z ≃ 2.16 (significant at 5.4σ) in a 10′ × 11′ region. The overdensity contains two substructures or protoclusters at ⟨z⟩ ≃ 2.16 and 2.195 with estimated halo masses at z = 0 of ∼5–6 × 1014 M⊙, roughly consistent with Virgo-type clusters. The overdensity total SFR, ∼4000 M⊙ yr−1, is higher than predicted by simulations but much smaller than the SFR derived from the Planck data (i.e., 10 173 M⊙ yr−1). The analysis of the Herschel data in the field, in combination with the available ancillary data, shows that such a difference is due to an effect of source alignment along the line of sight that produces a 5σ overdensity of red Herschel sources in the field. We analyze the members’ ultraviolet (UV) spectra and UV-far-infrared spectral energy distributions to derive their SFR, stellar mass, and metallicity. Galaxy members include blue star-forming galaxies and Active galactic nuclei (AGN) with SFRs and stellar masses consistent with the main sequence. Active galactic nuclei, identified through optical spectroscopy or X-ray data, represent a significant fraction (20 ± 10%) of all members of the protocluster at z = 2.16, and they are powerful enough to produce radiative feedback. The core of this protocluster, besides being denser, includes members that are, on average, more massive and star-forming and contains a larger fraction of AGN and Herschel-detected galaxies than the full sample, suggesting an environmental effect on galaxy growth. A comparison between PHz G237.01+42.50 and other protoclusters in the literature at similar redshifts reveals some common traits and differences that reflect both observational biases and a diversity in intrinsic properties that is not yet fully understood.

Questioning Planck-selected star-forming high-redshift galaxy protoclusters and their fate

“…More recently, a third PHz source, PHz G237.01+42.50 has been deeply investigated by Polletta et al (2021). This source is located in the COSMOS field and contains at least 31 spectroscopically confirmed galaxies at redshift around z ∼ 2.16.…”

“…Galaxies of these two structures (called ss1 and ss2) are mostly blue SF galaxies with SFR and stellar masses consistent with the main sequence, and with a significant fraction (20 ± 10%) of Active galactic nuclei (AGN). Galaxy member SFR and stellar masses used in this study are reported in Table 8 of Polletta et al (2021).…”

“…1. As discussed in Polletta et al (2021), the total SFR from the SF galaxies, identified spectroscopically or through narrow-band imaging, in a small redshift range (z = [2.15 − 2.2]) is much smaller than the SFR derived from Planck sub-mm measurements, implying the need of integrating the signal along a larger redshift range to reproduce the Planck -derived SFR of the PHz sources. Indeed, they reproduce the Planck flux densities measured in the Planck beam of a PHz source by summing the Herschel flux densities of all the Herschel sources in the same region.…”

“…More specifically, the overdensity in the PHz source G237.01+42.50 has been found by combining spectroscopic observations at optical and near-IR wavelengths over a 10 × 10 region. These observations were biased in favor of SF galaxies with sufficiently bright emission lines at rest-frame ultraviolet or optical wavelengths to enable a spectroscopic redshift measurement (Polletta et al 2021). Once an overdensity at a specific redshift was found, narrow-band imaging observations in a smaller region of the field were carried out to identify SF galaxies with a strong Hα line in emission at the same redshift (Koyama et al 2021).…”

“…-Two overdensities have been found in PHz G237.01+42.50, both in the same 10 × 10 arcmin 2 region, one containing 20 galaxies at z = 2.155 ± 0.005 (ss1 ) and anther with 8 galaxies at z = 2.195 ± 0.005 (ss2 ) (Polletta et al 2021). We therefore consider cylinders with parameters θ = 10 arcmin and δ z = 0.01.…”