TY - JOUR
T1 - Revealing the hidden cosmic feast
T2 - A z = 4.3 galaxy group hosting two optically dark, efficiently star-forming galaxies
AU - Brinch, Malte
AU - Jin, Shuowen
AU - Gobat, Raphael
AU - Sillassen, Nikolaj B.
AU - Algera, Hiddo
AU - Gillman, Steven
AU - Greve, Thomas R.
AU - Gomez-Guijarro, Carlos
AU - Gullberg, Bitten
AU - Hodge, Jacqueline
AU - Lee, Minju
AU - Liu, Daizhong
AU - Magdis, Georgios
AU - Valentino, Francesco
N1 - Publisher Copyright:
© The Authors 2025.
PY - 2025
Y1 - 2025
N2 - 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 Ks-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(11,0–10,1) 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(Mhalo [M ]) ∼ 12.8, we find that this structure is likely to be in the process of forming a massive galaxy cluster.
AB - 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 Ks-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(11,0–10,1) 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(Mhalo [M ]) ∼ 12.8, we find that this structure is likely to be in the process of forming a massive galaxy cluster.
KW - Galaxies: evolution
KW - Galaxies: formation
KW - Galaxies: groups: individual: CCG-z4
KW - Galaxies: high-redshift
KW - Galaxies: ISM
U2 - 10.1051/0004-6361/202451448
DO - 10.1051/0004-6361/202451448
M3 - Journal article
AN - SCOPUS:85218413291
SN - 0004-6361
VL - 694
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
M1 - A218
ER -