Intracluster light is a diffuse glow of stars stripped from galaxies during a galaxy cluster’s formation.
Abell 3667 is featured in this DECam image. Image credit: CTIO / NOIRLab / NSF / AURA / Anthony Englert, Brown University / T.A. Rector, University of Alaska Anchorage & NSF’s NOIRLab / M. Zamani & D. de Martin, NSF’s NOIRLab.
Galaxy clusters contain thousands of galaxies of all ages, shapes and sizes.
Typically, they have a mass of about one million billion times the mass of the Sun.
At one point in time galaxy clusters were believed to be the largest structures in the Universe — until they were usurped in the 1980s by the discovery of superclusters, which typically contain dozens of galaxy clusters and groups and span hundreds of millions of light-years.
However, clusters do have one thing to cling on to; superclusters are not held together by gravity, so galaxy clusters still retain the title of the biggest structures in the Universe bound by gravity.
“The histories of galaxy clusters not only help us understand how the Universe formed, but they also provide constraints on the properties of dark matter,” Brown University astronomer Anthony Englert and his colleagues said in a statement.
One clue the astronomers look for to understand the history of a galaxy cluster is intracluster light — the faint glow emitted by stars that have been stripped from their original galaxies by the immense gravity of a forming galaxy cluster.
These stars serve as whispering evidence of past galactic interactions, though most existing telescopes and cameras struggle to capture them.
The delicate intracluster light of galaxy cluster Abell 3667 shines prominently in the new image assembled from a total of 28 hours of observations with the 570-megapixel Dark Energy Camera (DECam) on NSF’s Víctor M. Blanco 4-m telescope at Cerro Tololo Inter-American Observatory, a program of NSF’s NOIRLab.
“Abell 3667 is more than 700 million light-years away from us,” the astronomers said.
“The great majority of faint sources of light in this image are very distant galaxies, and not foreground stars in our own Milky Way Galaxy.”
“Within Abell 3667, two smaller galaxy clusters are actively merging together, evidenced by the glowing bridge (yellow) of stars stretching across the center of this image.”
“This bridge connects the hearts of the two galaxy clusters, known as their brightest cluster galaxies, and forms out of material stripped from the galaxies as they merge to form one massive conglomerate.”
“Not only is this sequined sky full of faraway galaxies, but faint foreground features are also illuminated from its long exposure time.”
“Milky Way cirrus, or integrated flux nebulae, are faint, wispy clouds of interstellar dust that can be seen as faint bluish strands criss-crossing the image.”
“These cirrus are patches of dust illuminated by the combined light of stars within our own Galaxy.”
“They appear as diffuse, filamentary structures that can cover large areas of the sky.”
The findings appear in the Astrophysical Journal Letters.
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Anthony M. Englert et al. 2025. The Intracluster Light of Abell 3667: Unveiling an Optical Bridge in LSST Precursor Data. ApJL 989, L2; doi: 10.3847/2041-8213/ade8f1