Newly discovered group hosts two optically dark star-forming galaxies

An international team of astronomers reports the discovery of a new compact galaxy group using the Atacama Large Millimeter/submillimeter Array (ALMA). The new group, designated CGG-z4, hosts two optically dark star-forming galaxies. The finding was detailed in a research paper published Jan. 9 on the pre-print server arXiv.

Galaxy groups are the smallest aggregates of galaxies, typically containing up to 50 members. For astronomers, overdense structures like protoclusters or galaxy groups are prime targets to help them investigate the growth of massive galaxies.

Recent observations performed by a group of astronomers led by Malte Brinch of the Technical University of Denmark, have uncovered the presence of a new galaxy group. They identified the new group with ALMA in the Cosmic Evolution Survey (COSMOS) field.

“Through the use of ALMA and ancillary data in the COSMOS field, we have discovered a compact galaxy group, CGG-z4,” the researchers wrote in the paper.

Brinch’s team identified CGG-z4 as a sky overdensity of galaxies within a small region, at a redshift of approximately 4.3. In total, the observations uncovered the presence of 13 galaxies over a region of 13 by 31 arcseconds.

According to the study, CGG-z4 has a stellar mass of about 100 billion solar masses, while its dark matter halo mass is estimated to be over one trillion solar masses. The group has a total star-formation rate at a level of 2,837 solar masses per year.

The observations found that CGG-z4 contains two optically and near-infrared dark galaxies, which received designation CGG-z4.a and CGGz4.b. They both turned out to be efficiently star-forming galaxies with huge gas reservoirs, large stellar masses, and short gas depletion times (of about 100 million years).

The short gas depletion times of CGG-z4.a and CGGz4.b mean that the two galaxies have high star-forming efficiencies, which suggests an onset of quenching of star-formation. The astronomers suppose that these galaxies would become about two times more massive and would enter a quiescent phase at a redshift of around 4.0.

The researchers conclude that the results indicate that CGG-z4 is likely in the process of forming a massive galaxy cluster, noting that CGG-z4 is expected to grow into a Virgo- or Coma-like cluster over the next 10 billion years.

“Comparisons with the simulations suggest that CGG-z4 is a forming protocluster, which is likely to form a cluster with M > 10¹⁴ solar masses by z ∼ 0, and the two massive dusty galaxies will form BCGs in the cluster,” the authors wrote.

They added that CGG-z4 requires follow-up observations in order to improve the completeness of its membership.

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