2025A&A...694A.218B

We present the confirmation of a compact galaxy group candidate, CGG-z4, at z = 4.3 in the COSMOS field. This structure was identified by two spectroscopically confirmed z = 4.3 K_s-dropout galaxies with ALMA 870 μm and 3 mm continuum detections, surrounded by an overdensity of near infrared-detected galaxies with consistent photometric redshifts of 4.0 < z < 4.6. The two ALMA sources, CGG-z4.a and CGG-z4.b, have been detected with both CO(4-3) and CO(5-4) lines, whereby [CI](1-0) has been detected on CGG-z4.a, and H₂O(1_1, 0_–10, 1_) absorption detected on CGG-z4.b. We modeled an integrated spectral energy distribution (SED) by combining the far-infrared-to-radio photometry of this group and estimated a total star formation rate of ∼2000 M_⊙ yr^–1, making it one of the most star-forming groups known at z > 4. Their high CO(5-4)/CO(4-3) ratios indicate that each respective interstellar medium (ISM) is close to thermalization, suggesting either high gas temperatures, high densities, and/or high pressure; whereas the low [CI](1-0)/CO(4-3) line ratios indicate high star formation efficiencies. With the [CI]-derived gas masses, we found the two galaxies have extremely short gas depletion times of 99 Myr and < 63 Myr, respectively, suggesting the onset of quenching. With an estimated halo mass of log(M_halo [M_⊙]) ∼ 12.8, we find that this structure is likely to be in the process of forming a massive galaxy cluster.