BAN #415: An ORC in the galactic neighborhood?

4 April 2022 Issue #415

April 04, 2022

Blog Jam

[Hubble spied Earendel, what may be the farthest star ever seen, an incredible 12.9 BILLION light-years from Earth. Credit: SCIENCE: NASA, ESA, Brian Welch (JHU), Dan Coe (STScI); IMAGE PROCESSING: NASA, ESA, Alyssa Pagan (STScI)]

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

On the blog last week I wrote a follow-up story about ORCs — Odd Radio Circles — which are mysterious, faint, and huge circles on the sky seen in radio waves. I first wrote about them just after they were discovered, which was only a couple of years ago because they’re so difficult to detect.

I wrote the follow-up because a more sensitive radio telescope observed the first one (ORC1) and got deeper, higher resolution images of it. When I got the press release I was excited because these are very cool objects and the initial observations didn’t give too many clues about their origin. The new observations showed that ORC1 has a galaxy right in its center, so it’s likely the ORC is gas blown out by that galaxy, making it a brain-stomping 2 million light-years across.

But a funny thing happened on the way to that article: The press release didn’t give a link to the journal paper, so I went to the online arXiv, where astronomers publish can publish articles after they’re peer reviewed so the community can see them immediately; it can take months before the journals actually publish them. When I searched on the authors’ names I had a moment of confusion: There were two papers listed, and only one of them was mentioned in the press release.

The second paper, though, was to me much more interesting: They reported on the discovery of J0624–6948, an ORC that was located near the Large Magellanic Cloud (or LMC) on the sky. The LMC is a companion galaxy of our Milky Way, orbiting us at a distance of roughly 170,000 light-years. This ORC is located so close to the LMC — about 3° from the edge, while the LMC itself is over 10° across — that pondering a physical connection isn’t too ridiculous; it would be on the outskirts of the LMC if true. It would be about 8,500 light-years from the edge in real spatial terms. If it’s at that distance its physical size is about 150 light-years across, which is friggin’ huge.

So what is it? They speculate that it might be a supernova remnant, the expanding cloud of gas blasted out by an exploding star. But its location is odd. If it’s really a remnant, then the star must have existed in intergalactic space, outside both the LMC and Milky Way!

That’s. So. Cool!

[Images of the ORC at various radio frequencies show its structure and weird almost perfectly circular nature. Some images do shows a faint source in the center which is likely a distant galaxy and the astronomers figure it’s unlikely to be the ORC source. Credit: Filipović et al. 2022]

Mind you, the evidence is pretty shaky, because it’s difficult to pin down what might have created this ORC. After all, the distance isn’t known, and that’s the key factor to understanding it. That gives you its physical size (a nearby tree and a distant mountain might look the same size to you, but once you know their actual distances you’ll find the mountain is bigger), which is important. It also gives you the total amount of energy it gives off — more distant objects would have to be more luminous to be as bright as a closer object to us. So without that the evidence is indirect.

If it’s at the LMC distance it could be a supernova remnant. 150 light-years is big, but in the relatively empty space between galaxies the immense momentum of a few solar masses worth of material expanding outward at hundreds if not thousands of kilometers per second would expand freely, and after a while would get this big. It would be close to a sphere in shape, but as it plows through the thin gas between galaxies it would snowplow that material up, creating a shell, and seen from any angle that would look like a circle.

[The location on the sky of the ORC (pink star and circles) with the Milky Way above. The Large and Small (another satellite galaxy) Magellanic Clouds are both indicated as well. Credit: Filipović et al. 2022]

But then why is it between galaxies? Stars can be ejected out of galaxies, sometimes at high speed. In fact some high-velocity cannonball stars have been seen screaming away from the LMC. A massive star in a binary pair with another star can explode, and if they were orbiting close enough together the sudden loss of mass in the system means the gravity gets so weak the other star is flung away like from a slingshot (this is all explained in that link just above). If the second star is massive, it too may explode a million or two years later, and boom: Intergalactic supernova.

Or, less dramatically, it could be from a white dwarf exploding as a supernova. When a star like the Sun dies it leaves behind a white dwarf, what was once its hot dense core. If the star was a binary, the white dwarf can draw material off the companion star, and once it gains enough mass it explodes (I’m eliding over some details, duh). These systems take a long time to produce a supernova, but systems like this are common enough and could easily be in the LMC suburbs. So that fits too, but it’s all pretty circumstantial.

So, maaaybe.

There’s nothing really at the center, though. If the explosion left behind a neutron star or black hole it might be hard to detect, but it would not be slowed by intergalactic gas, so it would actually be moving faster than the gas it expelled, and could be outside the ORC completely by now.

The astronomers do wonder about other potential sources for the ORC. If it were some blast from a distant galaxy like with ORC1 then there’d be some galaxy there near the center, probably a decently bright one. But nope. There is what may be a galaxy near the center, but it’s likely too low-energy to make the ORC. They posit a few other possibilities, but if it really is at the distance of the LMC none match the characteristics seen.

If it’s not at that distance then most if not all bets are off. In that case it could be any number of things. What’s needed are more observations, but that’s tough. ORCs are  faint. There’s no obvious visible light from these things, and they’re so big that it’s hard to get deep of the whole structure all at once. The Advanced Camera for Surveys on Hubble is just barely large enough to see this whole thing at once, but getting time on Hubble for something that may not even show up in images is a tough sell. With everyone on the planet squabbling for James Webb Space Telescope time that’s probably out as well.

So for now these objects will remain a mystery. I do love a mystery, but you know what’s even more fun? Answers to them.

Patience. It’s an attribute astronomers get used to developing.

Et alia

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