229720
229719
Skywatching

Wandering black holes

There is a really large black hole at the centre of our galaxy, the Milky Way.

It has a mass millions of times that of the Sun. It pulls in anything that comes too close, and the radiation produced as it sucks in material would be a hazard for any life-bearing worlds in the neighbourhood.

Fortunately for us, it lies some 25,000 light years away from us, and is not showing any signs of moving from its central location.

It seems that most large galaxies host one of these supermassive black holes (SMBHs) at their centres. They were probably formed at the same time as their host galaxies, although we don't know that for sure, and have probably grown a bit over time from engulfing material from their surroundings.

Black holes are probably the most bizarre objects in nature. They are the result of matter becoming extremely compressed.

This can happen by making a big enough lump of matter so that its own gravity can compress its core to the point where it becomes a black hole, or more usually, when a big star explodes, and shock waves compress the core.

We don't think the black holes at the cores of galaxies formed in this way. However, a black hole forms when, due to a combination of the amount of matter it contains and the degree of compression, its gravity becomes intense enough to close off a bubble of space-time.

Nothing can get out except gravity, which pulls stuff in on a one-way trip. No light can get out, so the object is completely black. They are completely invisible.

The only way we can detect them is how they interact with their surroundings. Their gravity tugs at nearby stars, and they are surrounded by a huge accretion disc, made up of the disintegrated remains of anything: stars, planets or other bodies, which got too close.

As the material spirals in, it becomes extremely hot, millions of degrees in some cases. This produces X-rays, radio waves and other electromagnetic waves, which we can detect.

We can detect SMBHs in distant galaxies because the huge mass of the black holes in their centres distorts the way their host galaxies rotate.

An SMBH is usually the biggest concentration of mass in a galaxy. It sits still because there is nothing around strong or massive enough to move it. Basically the only known object that would fit the bill is another SMBH.

This agrees with our observation that all the SMBH's in galaxies are sitting happily at their centres. Now we have an exception.

That exception lies in the distant spiral galaxy J0437+2456, which is located some 228 million light-years away.

The galaxy is odd because it seems to be being generally churned up, rather than showing a stable structure like other galaxies, including ours.

However, the oddest thing is that its central black hole is moving at a speed different from the galaxy in which it is located. It is wandering around.

Whether the black hole is moving away from its galaxy or the other way round, we are still posed with the question as to how this could arise. Whatever happened is probably uncommon, since we have observed a very large number of galaxies and this wanderer is the only we have found.

Of course, now we know what we are looking for, more might turn up.

We know that even back in the youth of the universe, when galaxies we see were newly formed, they seem to have had large black holes in their centres. So SMBHs are probably bizarre but normal things to find in the cores of galaxies.

Despite this recent discovery of a wandering SMBH, almost all of them stay home. So the SMBH in our galaxy poses no threat to us, yet.

  • Mars is high in the southwest after dark.
  • You might catch Saturn, Jupiter and maybe, with luck Mercury, low in the southeast just before dawn.
  • The Moon will be full on the 28th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



More Skywatching articles

225769
About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



227089
The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

Previous Stories



228921
218881


232116