2025A&A...699A.324H

Protoclusters represent the densest regions of cosmic large-scale structure in the early Universe and are the environment where present-day massive elliptical galaxies were assembled. Millimetre continuum emission offers a powerful probe of obscured star formation at high redshifts across various environments. In this paper we present a deep ALMA 1.17 mm mosaic of the central 8 arcmin² (≈30 comoving Mpc²) region in the SSA22 protocluster at z = 3.09 to study the faint dusty star-forming galaxy (DSFG) population. The continuum map achieves an RMS noise level of ≈25 μJy beam^–1 at ≈1″ spatial resolution, which is approximately twice the depth of a previous observation of this field. We detect 53 sources with a signal-to-noise ratio above 4.2, doubling the number of detections. Utilizing optical to mid-infrared ancillary data, we searched for spectroscopic redshifts and identify 18 of the 53 as cluster members. For sources with more than two photometric data points in the near-infrared, the stellar mass (M_⋆) and star formation rate (SFR) from spectral energy distribution fitting are presented. The 1.17 mm number count shows a > 2× excess at flux density ≳1 mJy but is consistent with a blank field in fainter flux bins. The monochromatic far-infrared luminosity function of the SSA22 protocluster core region suggests a lack of faint DSFGs. All SSA22 protocluster member galaxies detected at 1.17 mm have SFRs within the M_⋆-SFR relation of general star-forming galaxies. Our results suggest that an early overdense environment such as the SSA22 protocluster predominantly accelerates the formation of massive early-type galaxies in present-day galaxy clusters, but that the star formation in individual member galaxies is likely driven by a gas supply along the cosmic web and occurs in a secular way.