When galaxies run into each other, they often appear to merge gracefully, their cores pirouetting around one another, drawn by their mutual gravitational attraction as their spiral arms extend outward in repose and their disks dissolve into an eternal embrace.
But not in galaxy cluster PLCKG 287.0+32.9.
Like a high-speed multi-car pileup in a Hollywood blockbuster, with a drawn-out series of explosions and never-ending smash-ups, this epic collision between three packs of galaxies 5 billion light-years away is leaving astronomers slack-jawed at how it keeps going and going … and going.
It’s not easy to see what’s happening in the middle of this mess. But a team of astronomers has discovered new indications of how much carnage the collision is creating, including an enormous glowing cloud that surrounds the galaxy cluster, emitting in radio wavelengths.
Spanning almost 20 million light-years, this incandescent halo “is the largest ever observed in any galaxy cluster,” said the study’s lead author, Kamlesh Rajpurohit, a Smithsonian astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), as she presented the work at the 246th meeting of the American Astronomical Society in Anchorage, Alaska, on June 9. A preprint of one paper covering the research is available on the arXiv; another is in preparation.
The massive scale of the structure — or roughly 200 times the diameter of the Milky Way Galaxy — is unexpected, and hints at even more physics that lies just out of sight.
‘Memories of our early universe’
Galaxy clusters are the most visible part of the scaffolding that underlies all the matter in the universe. The universe’s largest structures are the filaments of the cosmic web, where gas and dust gather. From these titanic tendrils, galaxies are formed, with clusters of galaxies forming at the intersections of these filaments.
This is part of what makes galaxy clusters like PLCKG 287.0+32.9 (pronounced “Planck 287” for short) interesting to astronomers — “they carry memories of our early universe, therefore opening a direct window into how it all started and evolved,” said Rajpurohit.
PLCKG 287.0+32.9 is an ongoing collision of three different subclusters of galaxies that is “known to be one of the most massive and violent clusters” to date, said Rajpurohit. The new data her team analyzed, from NASA’s Chandra X-ray telescope and the MeerKAT radio telescope array in South Africa, show the carnage in greater detail.
Her team’s new high-resolution data from Chandra reveal where gas is colliding, forming a shock wave or churned up by turbulence. Some of these structures are related to the jets spat out by the feeding supermassive black holes at the centers of the cluster’s galaxies. All of these structures — where gas is being heated to temperatures high enough to emit X-rays — are indications of “very violent collisions,” said Rajpurohit.
Then, by combining the Chandra data with the MeerKAT observations, the team were able to get a fuller view of what’s happening. At radio wavelengths, the cluster appears to be what Rajpurohit called a “a beautiful mess,” with an additional shock front separated from the one visible to Chandra by a distance 12-million light years. Additionally, these shocks are not smooth, but contain a multitude of wiggle-like ripples.
But these shocks are just “the tip of the iceberg,” said Rajpurohit, compared to the massive radio glow that encompasses the entire cluster.
This raises a mystery: Radio emission in galaxies is often triggered by actively feeding supermassive black holes spitting out winds or jets of material, but those phenomena are not powerful enough to sustain a glowing halo as large as the one around PLCKG 287.0+32.9.
This is because this radio emission requires two things: highly energetic charged particles known as cosmic rays, and magnetic fields. Cosmic rays can be generated by shock waves. There, where the gas compresses, so do their magnetic fields, acting as particle accelerators that can propel electrons to speeds close to light. As these electrons spiral through magnetic fields, they emit radio emission — but eventually, this drains them of their energy.
The massive radio halo is so large that it doesn’t seem possible for cosmic rays to retain their energy while still emitting across such vast distances. “We still do not understand from where these cosmic rays come and how they survive there,” said Rajpurohit.
Instead, there must be some other energy source within the galaxy cluster — perhaps unseen shocks, turbulence, or other galaxies unseen by either Chandra or MeerKAT — that is reaccelerating them.
Rajpurohit suspects that there may be many more galaxies in the cluster than can only be seen at wavelengths longer than existing radio facilities can detect. “We just assume that they are like a ghost — they exist, but you are still not sensing them,” she told Astronomy.
Chandra’s uncertain future
The study would not have been possible without Chandra’s ability to take high-resolution X-ray images, which is unmatched by any other current or planned X-ray telescope.
Rajpurohit has more Chandra observations of PLCKG 287.0+32.9 pending — but the telescope may never get to take them. Although Chandra remains one of the most in-demand telescopes in NASA’s portfolio, the observatory has been on the chopping block for over a year due to budget difficulties whose roots predate the current administration. Pushback from Congress and a public awareness campaign from astronomers seemed to win the mission a reprieve last year.
But under the Trump administration’s presidential budget request released last month — which slashes funding for NASA science nearly in half — Chandra would be shuttered for good.
If that move is approved by Congress, “We will lose our X-ray eyes,” says Jaya Maithil, a CfA astronomer who also uses Chandra, but was not involved in the PLCKG 287.0+32.9 work.
At the AAS meeting, Maithil presented a surprisingly strong quasar jet seen by Chandra just 3 billion years after the Big Bang. “There is no other telescope that can actually resolve that jet — it’s only Chandra,” she told Astronomy. Even future X-ray telescopes currently on the drawing board won’t have comparable resolution, she noted. “All these things will be just one point source in the sky. … We’ll never be able to know how the universe formed as the way we see it today.”
