Bizarre star bridge links galaxy to runaway black hole

HubbleSite said on April 6, 2023, that scientists have identified a 200,000-light-years-long bridge of hot, young, blue stars. It spans twice the diameter of our Milky Way galaxy. This star bridge – located halfway across the universe – appears to stretch between a runaway supermassive black hole and the galaxy the black hole is escaping. Astronomers think the black hole is causing gas to compress along its wake. And this process has been causing the new stars – the star bridge – to form. Nothing like this star bridge has ever been seen before. HubbleSite said:

Here’s an invisible monster on the loose, barreling through intergalactic space so fast that if it were in our solar system, it could travel from Earth to the moon in 14 minutes.

This supermassive black hole, weighing as much as 20 million suns, has left behind a never-before-seen 200,000-light-year-long ‘contrail’ of newborn stars … Likely the result of a rare, bizarre game of galactic billiards among three massive black holes.

Yale astronomer Pieter van Dokkum and his colleagues used the Hubble Space Telescope to spy this bridge of stars. They believe their explanation is plausible, despite the fact that no one has seen anything like this. The Astrophysical Journal Letters published the peer-reviewed study explaining the black hole idea on April 6, 2023.

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A black hole with a trail of stars

HubbleSite said:

The universe is so capricious that even the slightest things that might go unnoticed could have profound implications. That’s what happened to Pieter van Dokkum, when he was looking through Hubble Space Telescope images and noticed a suspected blemish that looked like a scratch on photographic film.

For Hubble’s electronic cameras, cosmic rays skimming along the detector look like ‘scratches.’ But once spectroscopy was done on the oddball streak, van Dokkum realized it was really a 200,000-light-year-long chain of young blue stars …

Rather than gobbling up stars ahead of it, like a cosmic Pac-Man, the speedy black hole is plowing into gas in front of it to trigger new star formation along a narrow corridor.

Van Dokkum said:

We think we’re seeing a wake behind the black hole where the gas cools and is able to form stars. So, we’re looking at star formation trailing the black hole. What we’re seeing is the aftermath. Like the wake behind a ship, we’re seeing the wake behind the black hole.

The trail must have lots of new stars, given that it is almost half as bright as the host galaxy it is linked to, these scientists said.

The star bridge is a serendipitous discovery

Van Dokkum wasn’t looking for black holes, or new stars, when he found the stellar bridge. Instead, he was looking for globular star clusters in a nearby dwarf galaxy. He said:

This is pure serendipity that we stumbled across it. I was just scanning through the Hubble image and then I noticed that we have a little streak. I immediately thought, ‘Oh, a cosmic ray hitting the camera detector and causing a linear imaging artifact.’ When we eliminated cosmic rays, we realized it was still there. It didn’t look like anything we’ve seen before.

Because it was so weird, van Dokkum and his team followed up with ground-based spectroscopic observations at the W. M. Keck Observatories in Hawaii. He describes the bridge of stars as:

Quite astonishing, very, very bright and very unusual.

The additional analysis made the team conclude that they were looking at the aftermath of a black hole flying through a halo of gas surrounding the host galaxy of the black hole.

Multiple collisions of supermassive black holes

The astronomers believe the black hole and its wake of stars originated in a merger of two galaxies, each with a supermassive black hole at its core.

They think the two galaxies might have merged 50 million years ago. If so, the two supermassive black holes at their centers would have come together, forming a binary or double black hole.

Then another galaxy might have come along, also with a supermassive black hole at its core. Now there were three black holes in close proximity, leading to an unstable configuration whereby one of the black holes robbed momentum from the other two. At that point – POW – that black hole was propelled out of the host galaxy.

The original binary may have remained intact, the scientists said. Or the new interloper black hole may have replaced one of the two that were in the original binary, and kicked out the previous companion. The scientists explained:

When the single black hole took off in one direction, the binary black holes shot off in the opposite direction. There is a feature seen on the opposite side of the host galaxy that might be the runaway binary black hole.

Circumstantial evidence for this is that there is no sign of an active black hole remaining at the galaxy’s core.

The next step is to do follow-up observations with NASA’s James Webb Space Telescope and the Chandra X-ray Observatory to confirm the black hole explanation.

Will we find more star bridges?

As many astronomers do, these scientists also pointed to the upcoming Nancy Grace Roman Space Telescope, scheduled to launch in May 2027. They said it’ll be able to see as clearly as the Hubble Space Telescope. But it’ll have a wide-angle view of the universe. The scientists said:

As a survey telescope, the Roman observations might find more of these rare and improbable ‘star streaks’ elsewhere in the universe. This may require machine learning using algorithms that are very good at finding specific weird shapes in a sea of other astronomical data.

Bottom line: Scientists have identified a 200,000-light-years-long trail of young blue stars – a star bridge located halfway across the universe – linking a runaway black hole and the galaxy it’s escaping.

Source: A Candidate Runaway Supermassive Black Hole Identified by Shocks and Star Formation in its Wake

Via HubbleSite