A distant — and lonely — galaxy appears to have pulled in and assimilated all of its former companion galaxies. This result made with NASA’s Chandra X-ray Observatory and the International Gemini Observatory may push the limits for how quickly astronomers expect galaxies to grow in the early universe.

The unexpectedly solo galaxy is located about 9.2 billion light-years from Earth and contains a quasar, a supermassive black hole pulling in gas at the center of the galaxy and driving powerful jets of matter seen in radio waves. The environment of this galaxy, known as 3C 297, appears to have the key features of a galaxy cluster, enormous structures that usually contain hundreds or even thousands of galaxies. Yet 3C 297 stands alone.

“It seems that we have a galaxy cluster that is missing almost all of its galaxies,” said Valentina Missaglia of the University of Torino in Italy, who led the study. “We expected to see at least a dozen galaxies about the size of the Milky Way, yet we see only one.”

Missaglia and her colleagues see two key traits of a galaxy cluster in the Chandra X-ray data. First, the X-ray data reveals the lone galaxy is surrounded by large quantities of gas with temperatures of tens of millions of degrees — something normally seen in galaxy clusters.

Second, the supermassive black hole’s jet has created an intense source of X-rays about 140,000 light-years away, implying that it has plowed into gas surrounding the galaxy. A third trait of galaxy clusters possessed by 3C 297, previously reported in Karl G. Jansky Very Large Array data, is that one of the radio jets is bent, showing that it has interacted with its surroundings.

Despite having these important features of a galaxy cluster, Missaglia’s team’s data from the Gemini Observatory in Hawaii revealed that none of the 19 galaxies that appear close to 3C 297 in a Gemini optical image, and that have accurate distance measurements, are actually at the same distance as the lonely galaxy.

“The question is, what happened to all of these galaxies?” said co-author Juan Madrid of the University of Texas, Rio Grande Valley. “We think the gravitational pull of the one large galaxy combined with interactions between the galaxies was too strong, and they merged with the large galaxy. For these galaxies apparently resistance was futile.”

The researchers think 3C 297 is no longer a galaxy cluster, but a “fossil group.” This is the end stage of a galaxy pulling in and merging with several other galaxies. While many other fossil groups have been detected before, this one is particularly distant, at 9.2 billion light-years away. (The previous record holders for fossil groups were at distances of 4.9 and 7.9 billion light-years.)

“It may be challenging to explain how the universe can create this system only 4.6 billion years after the big bang,” said co-author Mischa Schirmer of the Max Planck Institute for Astronomy. “This doesn’t break our ideas of cosmology, but it begins to push the limits on how quickly both galaxies and galaxy clusters must have formed.”

The authors cannot rule out the presence of dwarf galaxies around 3C 297, but their presence would still not explain the lack of larger galaxies like the Milky Way. Nearby examples are M87 in the Virgo Cluster, which has had large galactic companions for billions of years. However, 3C 297 will spend billions of years essentially alone.

The new study was published in the January 2023 issue of The Astrophysical Journal, available here.

Earlier Chandra observations lasting only three hours showed hints of the hot gas seen in the new study, as reported by co-author Chiara Stuardi in a paper published in the April 2018 issue of The Astrophysical Journal Supplement Series, available here. Much deeper Chandra observations, however, were required to confirm it. The Chandra observations of 3C 297 were taken over a total time of 2.5 days in April and August of 2021 and 2022.

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts. The International Gemini Observatory is operated by NSF's NOIRLab.