Astrophile: Loner galaxy is seed of giant black hole

NGC 4178 enjoyed the single life. Even though the flat, disc-shaped galaxy was getting on a bit, it had a svelte spiral figure to be proud of. Its central black hole was perfect: not too small, not too large. It had never been involved in a major merger with another galaxy, and wanted to keep it that way. None of the unsightly bulges and warps associated with too much socialising for NGC 4178.

But other, more gregarious, galaxies were getting together all around it. They merged into grand spiral galaxies in a firework display of star formation which left them with impressive bulging bellies. They pooled their central black holes until they were billions of times larger than the sun. NGC 4178 watched it all from the sidelines, glad to maintain its trim appearance, although it couldn't help wondering if it wasn't missing out on something.

Unsociable galaxies are unusual. Astronomers think that galaxies grow from scraggly clusters of stars to elegant spirals like the Milky Way by merging and pooling their resources. Loners like NGC 4178, which has spent most of the lifetime of the universe avoiding the company of other galaxies, are useful tools for disentangling how this happens. They are rare snapshots of a simpler time.

"They are more representative of the initial stuff, from when structure started to form in the universe," says Nathan Secrest, a graduate student at George Mason University in Fairfax, Virginia. Galaxies like NGC 4178 are about "as pristine as you can get".

One of the puzzles they can help solve is the origin of supermassive black holes. Most large galaxies seem to have a giant black hole, millions or billions of times larger than the sun, at their centres. How these black holes got so big is still a mystery: did they grow gradually from mergers of smaller black holes, coalescing when their host galaxies merged? Or did they form when gas clouds collapsed in the early universe?

If these giants did grow by devouring their more diminutive counterparts, then the universe should also be riddled with middleweight black holes, tens of times the size of the sun. But only a few of these have ever been spotted.


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