Monthly Archives: February 2011

Cut down to size: supermassive black holes turn out not to be so “super” after all

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You might not be able to tell from wherever you are reading this, but black holes in the distant universe just shrunk down to as little as a tenth of their previous size. This is not some cosmic disappearing act; a new analysis of supermassive black holes at the centres of active galactic nuclei has revealed that their masses were previously overestimated by up to a factor of ten. The paper was published in Nature last week.

A composite image of active galaxy M82, using Hubble, Chandra and Spitzer data. Credit: NASA, ESA, CXC, and JPL-Caltech

Active galactic nuclei, or AGN, are among the most luminous objects in the universe and are powered by massive black holes millions of times the mass of the Sun. Gas clouds, known as “broad line regions” for reasons that will become clear later, surround the black holes. These gas clouds range from a few light days to hundreds of light days across; they are much wider than our solar system. Astronomers have been studying these clouds for over thirty years, but had not worked out the why some of them were flatter than others — until now.

Wolfram Kollatschny and Matthias Zetzl from the Institute for Astrophysics, at the University of Göttingen in Germany, looked into the relationship between the shape and width of spectral lines observed in the emission spectra of AGN. An emission spectrum is produced when an object, a gas cloud for example, blocks a light source, such as a star. The light coming the source gets absorbed into the gas cloud, and is eventually re-emitted. Astronomers can measure the intensity and wavelengths of the light that gets re-emitted. Spectral lines are spikes in the emission spectrum, and represent a lot of light at a certain wavelength. They can tell astronomers what elements are present in the gas cloud.

However, for the gas clouds surrounding black holes, it is not quite that simple. The regions are spinning very fast around the central black hole, and the light emitted from them is subject to the Doppler effect. The Doppler shift is seen, or rather heard, in more everyday situations too — when the pitch of an ambulance siren seems to rise as it speeds towards you in the street and fall as it gets further away. That’s the Doppler shift affecting the sound waves. It happens because the ambulance is moving as it emits the sound waves, so the frequency of the waves _appears_ to change.* When gas rotates around a black hole, the same thing is happening to the light rays because some of the gas is moving away from the observer quickly and some is moving towards the observer quickly. This makes the spectral line astronomers eventually observe broader — an effect known as Doppler broadening. This is the reason the gas clouds are called “broad line regions”.

Kollatschny and Zetzl looked at 37 active galactic nuclei. They worked out that fast rotating AGN created broader spectral lines, and slower ones made more narrow lines. From their observations, they saw that faster rotating AGN had flatter gas clouds surrounding them, and slower ones had more rounded gas clouds. As they now knew how fast AGN were spinning, they were also able to come up with new, more accurate estimates of the masses of their central black holes. Previous estimates used just the spectral lines to estimate masses. This is a problem, particularly for very distant AGN, as astronomers can usually only see one spectral line from these — so there’s nothing else to check any estimated against.

The new black holes masses came out between two and ten times smaller than the previous estimates. While this isn’t going to cause any major problems for the black holes themselves — they’re still the most massive objects in the universe — it may pose a problem for astronomers studying the formation of black holes.


*Wikipedia has a good animation illustrating the Doppler Effect.

Kollatschny, W., & Zetzl, M. (2011). Broad-line active galactic nuclei rotate faster than narrow-line ones Nature, 470 (7334), 366-368 DOI: 10.1038/nature09761


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Caught in the act: sneak preview of galaxy cluster that’s still forming

Galaxy clusters are some of the largest structures in the universe. Astronomers have found these clusters, which are large groups of galaxies bound together by gravity, as far back as only 4 billion years after the Big Bang (less than a third of the age of the universe). They know they contain stars that formed even earlier than that. But nobody had caught a cluster while it was still forming — until now.

Region around starburst galaxy COSMOS AzTEC-3. The green circle is 13 million light years across. Credit: Capak et al. / Nature

Astronomers have found a “protocluster” that was around only 1 billion years after the Big Bang (that’s a redshift of 5.3 for anyone that’s counting). It sits in a region that is 40 million light years across and is rich in young stars.

The protocluster was found in data from the Cosmological Evolution Survey, COSMOS. COSMOS uses the Hubble, Spitzer and Chandra space telescopes with the ground based Keck Observatory and Japan’s Subaru Telescope to get an good look at the universe. COSMOS looks at a tiny region of space — about 0.005% of the whole sky, or two square degrees — in all wavelengths of light, from radio to gamma waves.

Peter Capak, the lead author on the paper published in Nature last week, and colleagues knew that extremely bright objects such as starburst galaxies (galaxies with an unusually high amount of star formation) and quasars (the bit at the centre of a massive galaxy that surrounds the supermassive black hole) should exist in very young galaxy clusters, so they first looked for objects giving off a lot of radiation. They found objects emitting a lot of visible light by measuring optical and near-infrared radiation, starburst galaxies by taking radio wave measurements and quasars using X-rays. Once they had located these extreme objects, they looked in the areas surrounding them for unusually large numbers of galaxies given the size of the area — something they called “overdensities”. They then used Hubble and Subaru to measure how far away these extreme objects were, and the Keck II telescope in Hawaii to confirm the observations.

Capak and colleagues were particularly interested in an “overdensity” near a starburst galaxy known as COSMOS AzTEC-3. The area in question contained over 50 billion times the mass of the Sun in gas (and ten times more dark matter), and was brighter than 10 trillion Suns. Stars in the region are forming at a rate of over 1500 a year, more than a hundred times the average value.

Six of the 11 very bright objects in the protocluster. COSMOS AzTEC-3 is labelled "Starburst". Credit: Capak et al./Nature

Around COSMOS AzTEC-3 there were 11 bright galaxies — 10 more than would normally be expected. This led to the conclusion that what they were seeing was the beginnings of a galaxy cluster, known as a protocluster.

Chandra X-ray observations helped to pin down a quasar very close to the protocluster. It can be difficult to find quasars that are this far away because, although they are the most luminous objects in the universe, they’re not usually bright enough to be seen by the telescope. But this one was. The astronomers worked out that the quasar’s black hole must have a mass of between ten and a hundred million times the mass of the Sun.

Putting all of this information together, astronomers worked out the total mass of the protocluster. It weighs in at at least ten billion time the mass of the Sun, but could be up to a hundred billion Suns. This confirmed what the astronomers suspected: they were looking at one of the biggest and brightest objects at this distance.

Astronomers also measured the amount of gas in the protocluster. This is what will fuel the protocluster’s growth. They found more than enough to point to a very bright, and massive, future for the baby cluster. It will eventually evolve (or rather, confusingly, already has evolved — depending on which way you look at it) into a massive galaxy cluster. Massive clusters of galaxies have been found from around 4 billion years after the Big Bang, giving this one at least 3 billion years to grow up.

Capak PL, Riechers D, Scoville NZ, Carilli C, Cox P, Neri R, Robertson B, Salvato M, Schinnerer E, Yan L, Wilson GW, Yun M, Civano F, Elvis M, Karim A, Mobasher B, & Staguhn JG (2011). A massive protocluster of galaxies at a redshift of z ≈ 5.3. Nature, 470 (7333), 233-5 PMID: 21228776

It’s also on arXiv.


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A galaxy far, far away… (take two)

Last November I wrote about the most distant galaxy ever seen. Since then, one even further away has been found. This post is (slightly) adapted from an article I wrote for Felix about this new galaxy.

The newly discovered galaxy is circled in the top left hand corner. Credit: NASA/ESA

There haven’t always been stars and galaxies in the universe, and the time when they began to form — known as the reionisation epoch — is the subject of much interest in astrophysics. A paper published in Nature details a discovery that could tell us more about this mysterious time.

In the paper, Rychard Bouwens and colleagues say they have detected the most distant galaxy ever seen; the light from the galaxy was emitted only 500 million years after the Big Bang. This age puts it well within the epoch of reionisation.

The galaxy has the highest redshift ever observed; it was from this that the team were able to calculate the galaxy’s age.

Cosmological redshift is a measure of how fast an object is moving away from the Earth and is a consequence of the expanding Universe. Objects with higher redshift are moving away from the Earth faster than those with lower redshifts, which means they are further away from Earth and also further back in time. To put it simply, the higher the redshift the older the galaxy.

Bouwens and colleagues used something known as the Lyman-break technique to identify the galaxy. This technique relies on a sharp drop in the spectrum of a galaxy that is due to the absorption of energetic photons by neutral gas that surrounds galaxy forming regions. The discovery can then be confirmed by looking at optical images.

For a long time the observations required to study the reionisation epoch were out of reach, but recent images from Hubble are making the detection and study of far away galaxies possible for the first time. The new galaxy was discovered in images taken by Hubble’s Wide Field Camera 3.

Bouwens and colleagues also looked into the rate of star formation at the time just after the newly discovered galaxy. They discovered that in just 200 million years the rate of star formation increased tenfold. This confirms that the newly discovered galaxy is right in the heart of the reionisation epoch, and sheds new light on how the stars and galaxies we see today formed.

It seems that galaxies around at the same time as this new discovery may not have been able to fully reionise the universe. This leaves the means by which the universe went from being a neutral gas to an ionised one, with electrons and protons stripped away from each other, a mystery. However, the existence of galaxies at this time does point to the first stars forming 100 million years beforehand, or around 400 million years after the Big Bang.

“We’re seeing huge changes in the rate of star birth that tell us that if we go a little further back in time we’re going to see even more dramatic changes,” said Garth Illingworth, a co-author of the paper from the University of California at Santa Cruz. “We’re moving into a regime where there are big changes afoot. Another couple of hundred million years back towards the Big Bang, and that will be the time when the first galaxies really are starting to build up.”

Bouwens, R., Illingworth, G., Labbe, I., Oesch, P., Trenti, M., Carollo, C., van Dokkum, P., Franx, M., Stiavelli, M., González, V., Magee, D., & Bradley, L. (2011). A candidate redshift z ≈ 10 galaxy and rapid changes in that population at an age of 500 Myr Nature, 469 (7331), 504-507 DOI: 10.1038/nature09717

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