Â鶹ÒùÔº

Using the James Webb Space Telescope (JWST), an international team of astronomers has performed deep and high spectral resolution imaging of a distant protocluster of galaxies, designated A2744-z7p9OD. Results of the new observations, July 8 on the arXiv preprint server, shed more light on the properties of this protocluster, revealing that it hosts a remarkably evolved core.

Galaxy clusters are collections of hundreds to thousands of galaxies bound together by gravity. Such clusters are the most immense gravitationally bound structures in the universe, and therefore they could serve as excellent laboratories for studying galaxy evolution and cosmology.

Of special interest for astronomers are studies of protoclusters of galaxies—the progenitors of clusters. These objects, found at high redshifts (over 2.0), could provide essential information about the early phases of the universe.

A2744-z7p9OD (A2744-PC for short) is a galaxy protocluster at a redshift of 7.88. Previous observations of this protocluster have suggested that its core hosts at least one relatively evolved galaxy.

Recently, a group of astronomers led by Callum Witten of the University of Geneva in Switzerland decided to further investigate this matter using JWST's Near-Infrared Camera (NIRCam).

"Utilizing recent deep and relatively high spectral resolution NIRCam imaging of the Abell 2744 lensing field, we study the A2744-z7p9OD protocluster," the researchers wrote in the paper.

First of all, the observations resulted in the detection of seven new members of A2744-PC, which expanded the list of known galaxies in this protocluster to 23. Afterward, the astronomers obtained the intrinsic spectral properties of all the protocluster-resident galaxies (PRGs), such as the Balmer break and ultraviolet slopes.

The study found that the investigated PRGs showcase stronger Balmer breaks and shallower ultraviolet slopes relative to field galaxies at this redshift, which corresponds to about 650 million years after the Big Bang. This suggests that most of the protocluster residents are already at a more advanced stage of their evolution than is typical.

Furthermore, the observations of A2744-PC show that its core is composed of massive, dusty galaxies with declining star-formation histories, while the outskirts of the protocluster consists of younger galaxies experiencing ongoing bursts of star formation.

According to the authors of the paper, these results indicate that A2744-PC has a highly evolved core and is in general at a more advanced stage of its evolution than would typically be expected from simulations.

"By z < 5 [redshift lower than 5], the cores of protoclusters are expected to be the first regions to show evidence of quenching and dense intracluster gas, with more spatially extended star formation. Such properties are already in place in A2744-PC at z ∼ 8," the scientists concluded.