Thursday, July 02, 2009

"Discovery" of a new class of black holes

Illustration of the possible mid-sized black hole HLX-1

Illustration credit: Heidi Sagerud

When reading about scientific discoveries, it is always important to remember Professor Astronomy's Discovery Law: The last person to discover something gets the credit.

Yesterday, a news story was released on a nice bit of research that is "the first solid evidence of a new class of medium-sized black holes." Only many other astronomers who have claimed to discover medium-sized black holes would argue that they had already discovered the first solid evidence of such things. Now, let me make it clear. The authors do not claim in their paper to have discovered mid-sized black holes; that claim is made in the European Space Agency's press release. And such hyped claims are often made by NASA and other US agencies, so ESA is not doing anything unusual. But this is not the first claim of a mid-sized black hole, and it won't be the final word, either.

An artist's conception of how the black hole, called HLX-1, might look if our eyes could see both X-rays and optical light is at the top of this post. It looks like a photo, but it is just an illustration.

So, forgetting the cultural aspects of the story, let's look at the science. Stories like this always raise the questions, "How can we see a black hole if light cannot escape it?" and "How do we know how big a black hole is?" These two questions are actually very closely related.

It is true that light cannot escape a black hole, if the light gets too close. How close is too close? For a black hole with the mass of the sun, the light would have to come within about 2 miles of the center of the black hole to be captured. The further away from the black hole you go, the more "normal" things are. If we replaced the sun by a black hole with a mass the same as the sun, the Earth and all the planets would orbit exactly the same as they do now, and we would still see all the stars in the heavens, except for those appearing the tiniest fraction of a degree away from the black hole in the sky.

So, stuff in orbit around a black hole will move in almost perfect ellipses around the black hole, just like it would around a normal star. They obey what we call "Kepler's Laws of Planetary Motion" first described by astronomer Johannes Kepler in the first decade of the 17th century. Kepler's Laws provided some of the first evidence for "small" black holes, those just a few times the mass of the sun. An example is the star system Cygnus X-1 (meaning the first X-ray source discovered in the constellation Cygnus). In that star system, a star with about 30 times the mass of the sun is losing some of its outer layers to a nearby unseen companion.

Now this invisible companion to Cygnus X-1 could have been a normal star, a white dwarf, a neutron star, or a black hole; all three would be impossible to see next to the very bright star. However, we can measure the movement of the bright star due to the gravitational pull of the companion star, much in the same way we find planets. The mass of the unseen companion is roughly nine times the mass of the sun. A normal star of that mass would be visible in optical light, but we don't see it. White dwarfs explode if they get bigger than 1.4 times the mass of the sun, so we know it's not a white dwarf. We aren't positive how big neutron stars can get before they collapse under their own gravity; but we know that limit is somewhere between 2 and 5 times the mass of the sun. So the invisible companion in Cygnus X-1 must be a black hole, and that black hole is about nine times the mass of the sun (but only about 30 miles in diameter!). Dozens of systems like Cygnus X-1 are known, and all the black holes have masses of a few times the mass of the sun to a few tens of solar masses.

Astronomers have also used Kepler's Laws in interpreting the movements of stars in the center of the Milky Way galaxy. They are able to determine that there must be something about 2 million times the mass of the sun in a volume no larger than the size of our Solar System at the very center of our galaxy. But we don't see any infrared light from that spot, even though we can see individual stars moving around that spot! That is some of the best evidence we have of a giant black hole at the center of the Milky Way.

Astronomers can also use the motions of stars to get the masses of black holes in other galaxies. This is harder to do, because we can't see the individual stars in the centers of most other galaxies. But we can measure average velocities of these stars, and we find that most galaxies have "supermassive" black holes at their centers, with masses of a million to over a billion times the mass of the sun!

So, we have lots of little black holes (ten times the mass of the sun, probably hundreds in each galaxy), and we have lots of ginormous black holes (millions of times the mass of the sun, but seemingly limited to one or two per galaxy). The first thing that pops into my mind would be that we probably have to build supermassive black holes out of sun-sized black holes, so I think it's natural to assume that there must be black holes with sizes in between, say maybe a few hundred times the mass of the sun to ten thousand times the mass of the sun, and there would be several of those in each galaxy.

But it's been hard to find candidate black holes of these intermediate masses. Some candidates have been claimed. One group, led by astronomer Karl Gebhardt here at the University of Texas, has found evidence of intermediate-mass black holes at the centers of some globular star clusters. Their evidence is from measuring the motions of individual stars, much like in Cygnus X-1 and in the center of the Milky Way.

Another way to find intermediate-mass (mid-size) black holes is to look in with X-ray telescopes. As gas and dust orbit around a black hole, they heat up due to friction. And, since they are moving so fast in their orbits, the gas and dust glow in energetic radiation, including ultraviolet light, X-ray light, and even gamma rays. Numerous studies (like this one) of X-ray sources have suggested that intermediate-mass black holes exist, and yesterday's press release on HLX-1 falls into that category.

The main difference of the HLX-1 black hole with other mid-size black hole candidates is that it is brighter in the X-rays than the earlier candidates. A brighter X-ray source means that it cannot be a stellar-mass black hole, because if those smaller black holes produced as much X-ray light as HLX-1, the X-rays would push all the gas away from the black hole, and it would stop emitting light. The astronomers studying HLX-1 also rule out that the X-rays come from a distant galaxy, because they don't detect any radio or optical light; supermassive black holes often (but not always) emit light in one or both of these kinds of light.

But I think the jury is still out on whether HLX-1 is any more convincing of a mid-sized black hole than any previous candidate. I won't list off the many questions I have. Frankly, solid evidence is very difficult to come by, and we may just not yet have the necessary tools to convince ourselves that mid-sized black holes exist. Future X-ray and optical telescopes may help a lot by allowing us to see fainter and get better data.

Finally, it may not be true that supermassive black holes are made out of coagulations of small black holes; some people argue that they are formed in completely different ways. I think that this idea is intriguing and could explain the apparent paucity of mid-sized black holes. But the lack of known intermediate-mass black holes may just mean that, in most cases, these black holes don't have any gas or dust to eat. If gas and dust are not falling into a black hole, they are virtually invisible and nearly impossible to detect.

In short, mid-sized black holes may exist and be hard to prove, or maybe they are very rare or even absent. HLX-1 is another important piece in the puzzle, but it is not convincing proof of mid-sized black holes nor the first evidence of mid-sized black holes.

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1 comment:

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