Spectroscopic observations of PHz G237.01+42.50: A galaxy protocluster at <i>z</i> = 2.16 in the Cosmos field
M. Polletta, G. Soucail, H. Dole, M. D. Lehnert, É. Pointecouteau, G. Vietri, M. Scodeggio, L. Montier, Yusei Koyama, G. Lagache, Brenda Frye, F. Cusano, M. Fumana
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 5.4 σ ) 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 × 10 14 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 5 σ 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.