Issue |
A&A
Volume 694, February 2025
|
|
---|---|---|
Article Number | A218 | |
Number of page(s) | 15 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/202451448 | |
Published online | 18 February 2025 |
Revealing the hidden cosmic feast: A z = 4.3 galaxy group hosting two optically dark, efficiently star-forming galaxies
1
Cosmic Dawn Center (DAWN), Copenhagen, Denmark
2
DTU Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
3
Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretana 1111, Valparaíso, Chile
4
Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
5
Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 106216, Taiwan, ROC
6
Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
7
National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, Japan
8
AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
9
Leiden Observatory, Leiden University, NL-2300 RA Leiden, The Netherlands
10
Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
11
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⋆ Corresponding authors; malte.brinch@uv.cl; shuji@dtu.dk
Received:
10
July
2024
Accepted:
9
January
2025
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.
Key words: galaxies: evolution / galaxies: formation / galaxies: high-redshift / galaxies: ISM / galaxies: groups: individual: CCG-z4
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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