The bright, churning heart of the ancient galaxy 3C 297, visible just to the right of center in a new NASA image, has fascinated astronomers. And when Valentina Missaglia, a postdoctoral researcher at the Institute of Astrophysics at FORTH in Greece, led a team to peer into the invisible world of X-ray and radio emission that surrounds the galaxy, she found something unexpected. Inspired by the data, the team titled their study December 2022 Astrophysical Journal With vivid descriptors for the galaxy: “Mighty but lonely.”

The supermassive black hole at the center of galaxy 3C 297 is quite spectacular. It superheats the material around it to glow as brightly as a quasar and churns out a 140,000 light-year-long jet larger than the Milky Way and galaxies. But it turns out that the region around 3C 297 is full of caches of emission data that suggest researchers that this solitary object may have companions that no longer exist.

Missaglia hypothesizes that this one bright galaxy is a fossil, a single epic remnant of a former galaxy cluster whose stellar mass coalesced into a single galaxy. “The environment of this galaxy … appears to have the core characteristics of a galaxy cluster, a massive structure that typically contains hundreds or even thousands of galaxies. Yet 3C 297 stands alone,” Chandra officials wrote in a statement released Wednesday.

Observations by NASA’s Chandra X-ray Observatory and the two Gemini telescopes in Chile and Hawaii have helped reveal otherwise unseen clues about 3C 297, located 9.2 billion light-years away in the constellation Virgo.

“It looks like we have a galaxy cluster that’s missing almost all of its galaxies,” Missaglia shared in Chandra’s statement.

The problem is the gas. Galaxy clusters contain hot gas. It reaches a temperature high enough to emit X-rays, when a cluster is formed. This halo of gas is long-lived, visible long after its formation due to its long cooling time and interaction with active galactic nuclei such as the supermassive black hole at the center of a solitary galaxy.

And Misaglia’s team found plenty of evidence of this hot gas in lunar observations.

Lunar officials wrote in the statement that the X-ray data revealed “a large amount of gas with temperatures in the tens of millions of degrees—typically seen in galaxy clusters.”

More clues come from the galaxy-sized jets of black holes. It reveals the existence of gas around the galaxy as it “plows” into the material to create “an intense source of X-rays about 140,000 light-years away” from the black hole. Another indication is the curvature of another black hole jet, whose shape suggests that it has interacted with some matter.

The signs of a galaxy cluster were there, and indeed, the lunar X-ray data supported the expectation of a cluster filled with at least 100 galaxies. But when the Gemini telescopes looked in optical light, the team found no galaxies at the same distance as a single bright galaxy.

Astronomers use the term “fossil” to describe when a massive elliptical galaxy – the last stage of galactic evolution – is left alone, like 3C 297, without a companion but with hot gas spread out. “(Galactic) companions did, in fact, come together to form one entity in the early universe,” Misaglia wrote in his Chandra blog post published Tuesday.

It is possible that small invisible galaxies could be unseen members of these ghost clusters. But lunar officials wrote that their presence near 3C 297 “still does not explain the lack of larger galaxies” that should be there.

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