Radio observations shed more light on the properties of Pandora's Cluster
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
associate editor
Using the Very Large Array (VLA), astronomers have performed radio observations of a galaxy cluster Abell 2744, nicknamed Pandora's Cluster. As a result, they obtained the deepest high-resolution radio image of this cluster, which provides more insights into its properties. The were published June 25 on the arXiv pre-print server.
Galaxy clusters contain up to thousands of galaxies and are the largest structures in the universe that are self-bound by their own gravity. Observations show that galaxy clusters generally form as a result of mergers and grow by accreting sub-clusters.
Abell 2744 is a massive X-ray galaxy cluster located some four billion light years away from the Earth. The cluster has a virial mass of about 740 trillion solar masses and showcases a strong central halo, along with an extended tail.
Pandora's Cluster has been a target of several observational programs at multiple radio and millimeter wavelengths with different angular resolutions and sensitivities, reaching a resolution of 3.5 by 3.5 arcseconds with a sensitivity of approximately 100 mJy/beam.
Now, a team of astronomers led by Esteban A. Orozco of the National Autonomous University of Mexico, reports that they managed to obtain a better quality radio image of this cluster.
"With an rms [root-mean-square] noise of ≈ 1 µJy/beam, at the phase center, and sub-arcsec angular resolution (θ1/2 = 0′′.82), this is the deepest and most detailed radio image of Abell 2744 ever obtained," the researchers wrote in the paper.
The observations detected 93 radio sources in Pandora's Cluster, out of which 46 have optical or near-infrared counterparts. The vast majority of them (88) appear to be point-like sources, while five of them turned out to be extended or multi-component sources.
According to the paper, the identified sources are galaxies with a median effective radius of 6,550 light years and stellar masses spanning from 660,000 to 160 billion solar masses. The median star formation rate for these galaxies was found to be approximately 1.9 solar masses per year, while the median flux density was measured to be 15.6 µJy/beam.
Moreover, the astronomers detected nine active galactic nuclei (AGN) candidates in the cluster. They calculated that the AGN fraction for the Pandora's Cluster is at a level of 10–20%, which is consistent with the value of 14% derived from simulations and models.
The study found that the Pandora's Cluster upper limit to the radio luminosity at 6.0 GHz is 4.1 duodecillion erg/s. The astronomers estimate that the cluster has a radio spectral index of approximately 0.7.
The authors of the paper have also conducted a search for radio counterparts of the so-called "little red dots" (LRD) galaxies as part of the study. In general, LRDs are compact galaxies with red optical colors and even broad hydrogen-alpha emission lines, suggesting the presence of type I AGNs. However, the researchers found no radio counterparts of these galaxies in the VLA data.
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More information: Esteban A. Orozco et al, The VLA Frontier Fields Survey: A 6GHz High-resolution Radio Survey of Abell 2744, arXiv (2025).
Journal information: arXiv
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