Subscribe now

Physics & Math

Home |Physics & Math |Space | News

Astrophile: Loner galaxy is seed of giant black hole


Object: NGC 4178, a pristine lone galaxy
Mass: 10,000 to 1 million solar masses

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.

Cosmic hermit

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.

If galaxies that have never been through a merger, like NGC 4178 – detectable by their lack of stellar bulges – have their own central black holes, their properties could help tell the story.

"We've been looking around and trying to find these galaxies with no bulges," Secrest says. "We've been slowly determining that many of them do in fact have black holes."

Missing middleweights found?

And some of them are in just the right mass range. Secrest and colleagues compiled observations of NGC 4178 in a range of wavelengths of light to narrow down the mass of its central black hole. It came in at between 10,000 and a million solar masses. If it turns out to be on the lower end of that range, it could be the smallest black hole ever seen in a pristine disc galaxy, and one that is still growing by sucking in material.

The fact that several such pristine galaxies turn out to have a small, still-expanding black hole at their core suggests that black holes can grow to intermediate size without mergers, but then need to pool their resources to get much bigger. This supports the idea that supermassive black holes form as a result of galactic unions, Secrest says. In other words, the black hole at NGC 4178's core could be an example of a seed from which larger black holes could grow.

But NGC 4178 will not be a singleton forever. The galaxy sits at the edge of the Virgo cluster of galaxies, and will enter the main part of the cluster in the next 100 million years or so.

"It will definitely at some point go through a merger," Secrest says. When it does it will probably develop a central bulge and a supermassive black hole, just like its more social neighbours. "In some ways, its pristineness will be lost," Secrest says – but at least it will have made some friends.

Journal reference: Astrophysical Journal, DOI: 10.1088/0004-637X/753/1/38


Does it hold a dark secret? <i>(Image: <a href="" target="ns">The Sloan Digital Sky Survey (SDSS) Collaboration</a>)</i>

Does it hold a dark secret? (Image: The Sloan Digital Sky Survey (SDSS) Collaboration)


Higgs Nobel bash: I was at the party of the universe

19:01 13 December 2013


Quark stars: How can a supernova explode twice?

16:00 09 December 2013


To kill a black hole, recruit a cosmic string army

15:45 06 December 2013



17:24 03 December 2013


Latest news

2013 review: The year in environment

09:00 20 December 2013


Dreams in art, from the Odyssey to Inception

18:00 17 December 2013


Turning back time: ageing reversed in mice

17:52 19 December 2013


Today on New Scientist

17:45 19 December 2013



© Copyright Reed Business Information Ltd.