An ALMA spectroscopic survey of the Planck high-redshift object PLCK G073.4−57.5 confirms two protoclusters

Planck’s High-Frequency Instrument observed the whole sky between 350 μm and 3 mm, discovering thousands of unresolved peaks in the cosmic infrared background. The nature of these peaks is still poorly understood – while some are strong gravitational lenses, the majority are overdensities of star-forming galaxies but with almost no redshift constraints. PLCK G073.4−57.5 (G073) is one of these Planck-selected peaks. ALMA observations of G073 suggest the presence of two structures (one around redshift 1.5 and one around redshift 2) aligned along the line of sight, but this analysis lacked robust spectroscopic confirmation. Characterizing the full redshift distribution of the galaxies within G073 is needed in order to better understand this representative example of Planck-selected objects, and connect them to the emergence of galaxy clusters. We used ALMA Band 4 spectral scans to search for CO(3–2), CO(4–3), and CI(1–0) line emission, targeting eight red Herschel-SPIRE sources in the field, as well as four bright SCUBA-2 sources. We find 15 emission lines in 13 galaxies, and using existing photometry information, we determined the spectroscopic redshift of all 13 galaxies. Eleven of these galaxies are SPIRE-selected and lie in two structures at ⟨z⟩  =  1.53 and ⟨z⟩  =  2.31 (both with a standard deviation in redshift of 0.02), while the two SCUBA-2-selected galaxies are at z  =  2.61. Using multi-wavelength photometry, we constrained stellar masses and star formation rates, and using the CO and CI emission lines we constrained gas masses. Our protocluster galaxies exhibit typical depletion timescales (Mgas/SFR) for field galaxies at the same redshifts but higher gas-to-stellar mass ratios, potentially driven by emission line selection effects. The two structures confirmed in our survey are reproduced in cosmological simulations of star-forming halos at high redshifts; the simulated halos have a 60–70% probability of collapsing into galaxy clusters, implying that the two structures in G073 are genuinely protoclusters.