Tag Archives: black holes

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

This post was chosen as an Editor's Selection for ResearchBlogging.org

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.

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*Wikipedia has a good animation illustrating the Doppler Effect.

Reference
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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Black holes are not fed by colliding galaxies after all

This post was chosen as an Editor's Selection for ResearchBlogging.org

It’s not a question you’re likely to have ever considered, but the source of “food” for some of the most active black holes has been a longstanding line of inquiry for the astrophysics community. Many thought they had the answer when several studies seemed to show a link between collisions of similarly sized galaxies and the formation of a very active black hole at the centre of the merged galaxy. But a new survey of 1400 galaxies has answered the question once and for all and it turns out that, in most cases, this link doesn’t actually exist.

Jet powered by a black hole at the centre of the galaxy M87, 50 million light years from Earth. Image: NASA

It’s long been known that at the centre of most galaxies lies a black hole. Some are relatively quiet, like the one in our own galaxy, but others manage to take in some of the matter that surrounds them and then spit it out in the form of huge amounts of energy. There’s a name for what’s created by the most lively ones: Active Galactic Nuclei, or AGN for short. Scientists don’t quite understand why some black holes in galaxies are more active than others, but research done last year by NASA’s Chandra X-ray Observatory did shed some light on how black holes grow in galaxies, by looking at how many of them are active at any one time and the size of the galaxies they inhabit. A new study, published in the Astrophysical Journal today, goes one step further and tests whether galaxy mergers trigger the creation of energetic AGN.

AGN give themselves away by emitting radiation. To do this, they take in gas and dust from their surroundings and heat it up before pouring it out again in the form of X-rays, radio, UV or other radiation. What we don’t know is how they take in this matter in the first place. Contrary to popular belief, black holes don’t actually suck in everything that surrounds them. In fact, unless some matter is heading in a straight line towards a black hole, it is much more likely to end up in orbit around it than falling into the black hole itself. This may sound counterintuitive, but it happens for the same reason that Earth and all the other planets in the solar system orbit the Sun — as long as an object has some sideways motion, or angular momentum, it will orbit rather than fall into the more massive object. Scientists have some idea of how matter could overcome this angular momentum on smaller and larger scales, but not in this situation.

Active and inactive galaxies showing varying levels of distortion. A large amount of distortion would give them them away as having undergone a major merger with another galaxy. Image credit: NASA/ESA and M. Cisternas (MPIA)

This new paper uses galaxies imaged by the COSMOS survey. What makes this study different from all the others is that the researchers included a control sample. As well as images of 140 galaxies with active black holes at the centre, they used 1264 images of galaxies they knew to be inactive. The pictures were given to ten galaxy experts at eight different institutions who were asked to classify them as “distorted” or “not distorted”. A galaxy with an active galactic nucleus will have certain tell tale signs giving away its AGN, so these giveaways were removed to make the trial “blind” — a technique always used in medical trials and other branches of science, but not often required in physics where the work is usually done by computers. Blinding the study makes sure that the people judging whether the galaxies are distorted are not, consciously or subconsciously, biased by any belief that active galaxies are more likely to be distorted than inactive ones.

The researchers found that, in the majority of the cases, there was no evidence that galaxy merger trigger the creation of active galactic nuclei. This means that someone will need to some up with another, more peaceful suggestion of how AGN get fed. A few ideas exist, such as “galactic harassment” — the fly by of another galaxy that gets close enough to disturb but not to do any damage or merge with the original galaxy — or the collisions of clouds of gas within the galaxy. But more research is needed to establish which, if any, of these ideas are the right one.

However, only galaxies around in the last eight billion years were included in the study, so the questions still remains of whether AGN created in the more distant past were triggered by mergers. In fact, this is the next problem on the groups to-do list.

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Reference
Cisternas, M., Jahnke, K., Inskip, K., Kartaltepe, J., Koekemoer, A., Lisker, T., Robaina, A., Scodeggio, M., Sheth, K., Trump, J., Andrae, R., Miyaji, T., Lusso, E., Brusa, M., Capak, P., Cappelluti, N., Civano, F., Ilbert, O., Impey, C., Leauthaud, A., Lilly, S., Salvato, M., Scoville, N., & Taniguchi, Y. (2011). THE BULK OF THE BLACK HOLE GROWTH SINCE z ~ 1 OCCURS IN A SECULAR UNIVERSE: NO MAJOR MERGER-AGN CONNECTION*
The Astrophysical Journal, 726 (2) DOI: 10.1088/0004-637X/726/2/57

 

Click here to read the paper on arXiv.

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Thousand light year long bubble surrounds black hole in nearby galaxy

The Eddington luminosity is the exact brightness a black hole has when the outwards and inwards forces on it balance. It may seem strange to talk about the brightness of a black hole, as usually we think of them as not letting anything – including light – escape their gravitational pull, but in reality this is not the case. Black holes do draw in all the material that surrounds them, but as they do this they become surrounded by disks of gas in a process known as accretion. This gas can become so hot that it emits vast amounts of X-rays.

So, black holes above the Eddington luminosity emit light radiation that is more powerful that their gravitational pull. Such black holes are rare and not much is known about them. Very bright X-ray sources are the most likely candidates to harbour these types of black hole, so researchers from the University of Strasbourg and University College London went looking for X-ray sources around unusually large remnants of supernovae. They found what they were looking for in a galaxy not too far away…

NGC 7793, a spiral galaxy in the constellation Sculptor. Image: NASA

There is a spiral galaxy known as NGC 7793 12.7 million light years away from us in the Sculptor constellation. In one of its spiral arms is a very bright nebula with a power source that is almost certainly a black hole with a luminosity above the Eddington limit. Unusually for a black hole such as this, it appears to dispel most of its accretion-generated energy mechanically rather than through radiation. This means that coming out of the black hole’s disk in opposing directions are two huge jets of very energetic particles – something that has not been seen very often before in these types of nebulae. These jets are the most powerful of their kind ever found, and are ten thousand times more energetic than the X-rays emitted from the core of the black hole. Around the jets has formed a bubble of plasma that is a thousand light years long.

X-ray/optical image of the plasma bubble surrounding the jets. 190pc (parsecs) is equal to approximately 600 light years. Image: Pakull, M., Soria, R., & Motch, C.

Above is an image of the bubble of plasma that surrounds the jets. In the image you can see the core and the northern and southern hotspots, where the jets interact with the ambient medium. Jets such as these are needed to explain many things in astrophysics, but their origin is not yet well understood.

Reference:
Pakull, M., Soria, R., & Motch, C. (2010). A 300-parsec-long jet-inflated bubble around a powerful microquasar in the galaxy NGC 7793 Nature, 466 (7303), 209-212 DOI: 10.1038/nature09168

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