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.

*

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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Scientific American Guest Blog post

I have a post up on the Scientific American Guest Blog today. It’s all about strange exoplanet discoveries, and what they can tell us about our own solar system. I really enjoyed researching and writing it, so I hope you’ll enjoy reading it too…

“Habitable and not-so-habitable exoplanets: how the latter can tell us more about our origins than the former”

Beta Pictoris and its planet

The image above is of one of the planets I talk about, and its star. It’s called Beta Pictoris b and is only around 60 light years from Earth. You’ll have to go read my post to find out what’s unusual about it, though.

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Saturn’s rings get spontaneously shaken up

From far away Saturn’s rings look pretty solid – I’m sure I’m not the only person who, as a child, imagined it’d be possible to skate around the planet on them. In reality, though, they’re made up of millions and millions of bits of ice and dust, ranging in size from micrometres to metres. Until recently, scientists thought that the occasionally odd behaviour of the most massive ring, known as the B ring, was solely due to the pull of one of Saturn’s moons, Mimas. However, new research published in the December issue of the Astronomical Journal explains that Mimas is not the only reason for the variations that we see in this ring…

Saturn as seen by the Cassini Orbiter. Image: NASA

Joseph Spitale and Carolyn Porco from the Space Science Institute at Boulder, Colorado looked at four years worth of images of Saturn’s rings from the Cassini mission. They saw evidence of wave patterns in the B ring that seemed to have arisen spontaneously – without being forced by Mimas. The waves are thought to come about because of the high density of the B ring, and are given a boost by its sharp edge which reflects and amplifies the waves. Spitale and Porco also found small moons, known as “moonlets”, near the outer edge of the B ring.

Cassini image of Saturn's B ring, taken in 2009. Image: NASA

The small chunks of ice and dust that make up Saturn’s rings may be left over from the formation of the planet itself, or could be all that is left of a moon that strayed too close to its parent and got broken up by Saturn’s gravity.* Either way, these new findings show that the rings are anything but the static bands of ice we sometimes imagine them to be, and that their motion doesn’t even always come from outside influences.

But these findings don’t just tell us about the behaviour of Saturn’s rings. They also offer insight into other systems in the universe that may have similar oscillations, such as spiral galaxies and protoplanetary disks. This is an example of one of the amazing things about physics. By observing something close to us, we can learn about the behaviour of systems on the other side of the universe.

*There’s something known as the Roche limit that dictates how close a moon can get to its planet before it’s broken up by tidal forces caused by the planet itself.

References:
Joseph N. Spitale, & Carolyn C. Porco (2010). Free Unstable Modes and Massive Bodies in Saturn’s Outer B Ring Astron.J.140:1747-1757,2010 arXiv: 0912.3489v2

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Huge leap forward in quantum computing

Originally published on 19/11/10 in felix, the student newspaper of Imperial College London.

Do you think you could make sense of this sentence if every fourth word was missing? How about trying to hold a conversation when you can only hear three quarters of what the other person is saying? Cutting out a fraction of the information being transferred in a given situation may make life slightly difficult, but it certainly doesn’t stop the meaning being conveyed in most cases. This is because of the redundancy built into language. However, redundancy is not only useful for conversations on a dodgy phone line – it can also come in handy in the world of quantum computing, as two researchers explained in a paper published in Physical Review Letters last week.

"Cosmic internet"

I've no idea what this picture is supposed to represent, but it you have to admit it does look pretty.

The research was carried out by Sean Barrett, of Imperial College, and Thomas Stace, at the University of Queensland in Brisbane, Australia. They found that if a quarter of the qubits (the quantum equivalent of bits, which store information in a classical computer) are lost, the computer can still function as normal. Barrett and Stace looked at the remaining information and used a code that could check for errors to decipher what was missing. “It’s surprising, because you wouldn’t expect that if you lost a quarter of the beads from an abacus that it would still be useful,” said Dr Barrett.

One of the main differences between a classical bit and its quantum equivalent is that the latter can exhibit entanglement. This means that, no matter how far away two entangled qubits are, if one changes so will the other – instantaneously. Quantum computers take advantage of this effect, as well as another property of quantum systems known as superposition, to perform complicated calculations much faster than classical computers. At the moment, though, the largest quantum computers have only two or three qubits.

It had previously been thought that large quantum computers would be very sensitive to missing information, but this research shows that they should be much more robust than we’d imagined. At this stage, the work is theoretical and scientists must do a lot more in order to make quantum computers bigger than a few qubits in the lab.

When large quantum computers are a reality, they may have the potential to revolutionise fields as far apart as drug modelling, electronics and code breaking. However, we won’t know exactly what applications quantum computers will be best suited to until we’re able to make one.

“At the moment quantum computers are good at particular tasks, but we have no idea what these systems could be used for in the future,” said Dr Barrett. “They may not necessarily be better for everything, but we just don’t know. They may be better for very specific things that we find impossible now.”

Reference:
Sean D. Barrett, & Thomas M. Stace (2010). Fault tolerant quantum computation with very high threshold for loss
errors Phys. Rev. Lett. 105, 200502 (2010) arXiv: 1005.2456v2

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A galaxy far, far away…

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

When we look up into the sky at night, we see stars (even in London I can usually spot a few!). But there haven’t always been stars and galaxies in the universe. In a period known as the dark ages – not to be confused with the other dark ages – there was no light at all. After the the ionised gas that filled the universe in its very early life cleared, there was a very long period of, well, nothing. The universe was transparent, but contained no stars or galaxies for just less than 400 million years. The process that allowed stars and galaxies to eventually form is known as reionisation, and new research published in Nature last month details a discovery that may open a new window on to this time.

Galaxy UDFy-38135539 shown in the Hubble Ultra Deep Field. Image: NASA

In the paper, Lehnert and colleagues reported detecting the most distant object physicists have ever seen: a galaxy, the light from which was emitted less than 600 million years after the Big Bang. It’s the first galaxy known to have lived fully within the epoch of reionisation.

The galaxy, which goes by the catchy name UDFy-38135539, has a redshift* of z = 8.6 – the highest ever observed – and it was from this that the team were able to calculate the galaxy’s age. UDFy-38135539 was first spotted by Hubble’s Wide Field Camera 3, but Lehnert and colleagues made ground based observations using an instrument called SINFONI on the Very Large Telescope in Chile to look at the galaxy in more detail.

The instrument helped by splitting up the light from the galaxy in a process known as spectroscopy, allowing the team to look for a feature called the Lyman-α line. Each photon making up the Lyman-α line would have been emitted when an electron in a hydrogen ion dropped down from a higher energy level to a lower one. The photons that Lehnert and colleagues observed were ultraviolet when they were emitted from the galaxy, but when they reached Earth had wavelengths in the infrared region, giving the high redshift mentioned above. This stretching of the wavelength occurs because of the length of time the photons took to travel here – they were created just 600 million years after the big bang. This may sound like a long time, but if you consider that the age of the Universe is 13.7 billion years, you’ll appreciate that 600 million years is actually very early on in the grand scheme of things, and certainly very, very long ago. Physicists know that reionisation started within 600 million years of the Big Bang, so Lehnert and colleagues came to the conclusion that galaxy UDFy-38135539 must have lived within the epoch of reionisation.

Diagram showing the universe from the Big Bang, to now. Image: Wikipedia

At the beginning, the Universe was very hot and dense, with conditions similar to that in a particle accelerator. After three minutes, the Universe had expanded and cooled enough to have formed all of the elementary particles, as well as protons and neutrons (also in these first three minutes was the matter-antimatter annihilation that destroyed most of the antimatter in the universe). For 400,000 years after this the universe was full of ionised gas, and was opaque. Then, in a period known as recombination, the electrons and protons in the gas got together and formed atoms; this made everything a lot clearer. For around 400 million years after that, the universe remained transparent and rather empty, with no stars or galaxies. It was only when astronomical objects, possibly quasars or small galaxies, began to form due to gravitational collapse that things began to get a bit more interesting.

These objects poured radiation out into the universe, causing reionisation. During reionisation, electrons were stripped back off the hydrogen atoms that were formed when they first joined up with the protons, and the universe began to turn into an ionised gas once again. Fortunately, due to the expansion that had taken place in the time between recombination and reionisation, the universe was able to remain transparent despite these bubbles of plasma that had begun to form all over the place. This process led to the formation of the stars and galaxies we see today, but physicists are not yet sure exactly how this all happened.

Because the galaxy UDFy-38135539 lived within the epoch of reionisation, it may be able to help us explain how reionisation started, and how these objects that formed in the cosmological dark ages were able to transform the universe from a mostly neutral one, to one filled with ionised gas.

There are already a few other faraway candidates lined up for study too, and Lehnert and colleagues have shown that such study is possible with current instruments, but astronomers will have to wait for the new wave of telescopes to really study reionisation in detail. The James Webb Space Telescope (JWST), which is the successor to Hubble, and the Extremely Large Telescope (ELT), which is the successor of the VLT, are two that will allow for this more detailed investigation. Both are due to be up and running later on this decade.

* 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, and are further away too. See an earlier post of mine for an image showing redshift in action.

Reference

M. D. Lehnert, N. P. H. Nesvadba, J. -G. Cuby, A. M. Swinbank, S. Morris, B. Clement, C. J. Evans, M. N. Bremer, & S. Basa (2010). Spectroscopic confirmation of a galaxy at redshift z=8.6 Nature, 467 arXiv: 1010.4312v1

Further reading

BBC article, including short phone interview with Malcolm Bremer who was one of the physicists on the team.

Blog post at Cosmic log, which includes a Q&A with lead researcher Matt Lehnert.

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MACHOs, WIMPs and the mystery of the missing mass

A couple of weeks ago I was at my mum’s house when she asked me to explain the concept of dark matter to her. I did, to the best of my ability, but her request got me thinking: stories about scientific research don’t always have a full explanation of the concepts in question, for good reasons, but maybe it would be good to have something to link to that does explain concepts in a bit more detail?

So I decided to write this post, as a kind of experiment. It is intended to be a brief* introduction to dark matter, for my mum and anyone else that would like a bit more background information.

Coma galaxy cluster. Image: NASA. Large version here, but be warned it is really big.

In 1937, a physicist called Fritz Zwicky worked out the mass of the Coma galaxy cluster and noticed that it did not match up with the amount of light coming from it. In fact, he’d worked out that there was a lot more mass in the galaxy cluster than its radiated light would suggest. This missing mass is what we now call “dark matter”, and it makes up 23% of the mass-energy density of the observable universe and around 90% of the total matter.

It’s called dark matter because it does not interact via electromagnetic radiation (light). As you can probably imagine, this makes detecting it a rather hard task.

All the experiments trying to directly detect dark matter are underground, to reduce the amount of background interference from cosmic rays. One of these detectors, the Cryogenic Dark Matter Search (CDMS), caused a bit of a fuss last year when some people thought it looked ready to announce a discovery. Unfortunately, it turned out that they didn’t see anything significant after all.

The CDMS experiment is searching for weakly interacting massive particles (WIMPs). These are a class of particles that, as their name suggests, interact via the weak nuclear force and have mass. There are many possible WIMPs, but the favourite is the lightest stable supersymmetric particle – the neutralino. The neutralino is the superpartner of the neutrino, a particle we will meet later on as it is also a possible dark matter constituent.

WIMPs are known as “cold” dark matter. This means that, when the large scale structures in the universe formed, WIMPs weren’t travelling very fast and would not have interfered with the formation of galaxies. Some other dark matter candidates are “hot”, meaning they would have been travelling at relativistic speeds in the early universe and could have ironed out any fluctuations in the matter density of the universe. The fact that there are galaxies in the universe today means these fluctuations cannot have been completely wiped out, which causes a problem if we want to consider these “hot” candidates.

While they are the favourite candidates at the moment, WIMPs are not the only class of particles that could make up dark matter. In the rest of this post, I’ll detail some of the other contenders.

Massive, compact halo objects (MACHOs) are large stellar objects that don’t give out light, and were probably named to contrast with WIMPs (you can’t say physicists don’t have a sense of humour). The “halo” in their title refers to the position that MACHOs occupy in a galaxy; the halo of a galaxy is a roughly spherical component that extends beyond the visible part, and is where most of the missing mass in galaxies resides.

Unlike most dark matter candidates, MACHOs are made from the same stuff as you and me – protons and neutrons that make up ordinary atoms. This is useful because we know a portion of dark matter must be made from this baryonic matter.

MACHOs can be any non-luminous stellar object, including black holes, neutron stars and brown dwarfs. White dwarfs and very faint red dwarfs have also been considered, but these are not totally “dark” as they do emit small amounts of light. Because of their mass, MACHOs can be detected using gravitational microlensing – a method that looks at how light coming from a distant star has been bent by the object in question. Using this technique, limits have been set on the mass of individual MACHOs in the galactic halo of our own galaxy, the Milky Way. There are some disagreements between results from different groups (see here for results from the MACHO project, and here for results from the Eros-2 project). However, overall, it has been found that stellar objects make up no more than a few percent of the mass in our galactic halo.

This means that MACHOs can’t explain the whole dark matter problem.

The most favoured hypothesis when it comes to non-baryonic dark matter, which we know constitutes the lion’s share of dark matter, is that it is made up of particles that were created in the early universe and were stable enough to survive to this day.

Neutrinos are one such particle. They were produced in the early universe in similar numbers to photons, positrons and electrons and, as my particle physics textbook amusingly puts it, “they have the great advantage that they are known to exist”. It used to be thought that they were massless (meaning that they would be useless when it came to explaining the missing mass in our universe), but it turns out that they do have a very small mass.

The problem with neutrinos as a dark matter candidate is that they are “hot”: they would have ironed out fluctuations in the early universe and prevented the large-scale structure that we see today. For this reason, they can only account for up to 30% of dark matter. Our search must go on.

The final candidates are axions. These hypothetical particles were first dreamt up to solve a problem in Quantum Chromodynamics, the theory that describes the strong nuclear force. If they exist, axions would almost certainly have been produced in large numbers in the early universe. As a bonus, they fall into the category of cold dark matter, so would have posed no problems for galaxy formation.

Some experiments are searching for axions, but as yet there is no experimental support for the axion model. It has recently been suggested that since the model may cause a bigger problem than the one it was invented to solve, the model should be scrapped. The future of the axion looks uncertain.

At the moment we cannot say for certain which, if any, of these candidates actually make up the ridiculously large percentage of the universe we know next to nothing about. But physicists will keep on searching, and in the next few years maybe we’ll get a couple of steps closer to figuring out the mystery of the missing mass in our universe.

* Ok, so it didn’t turn out quite as brief as I’d intended, but it still really is only a whistle-stop tour around the four main candidates. There are alternate theories I didn’t get chance to mention, including modified gravity laws and solutions in the form of quantum gravity theories. If you’re interested in reading more I’d recommend checking out this NASA page, and also this page by the Berkeley Cosmology Group, for starters.

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Solar system might be older than we thought…

Researchers from Arizona State University have found the oldest solar system object ever discovered. In fact, it’s so old that it formed up to two million years before the solar system did, according to current estimates. It might be time for a rethink of when and how our little place in the Universe came into existence…

Planets and dwarf planets in the solar system. Sizes to scale, distances (obviously) not. Image: NASA

Coming up with a successful model for the formation of the solar system is not an easy task. Such a model must explain everything we know about the solar system today, from the fact that all the planets revolve the same way around the Sun and in the same plane, to the composition of the planets themselves.

The most generally accepted model is the Solar Nebula Disk Model (SNDM), which is a modern variant of the Nebular Hypothesis originally put forward by Laplace and Swedenborg in the 16th Century. In the SNDM, stars form in huge rotating clouds of molecular hydrogen. Our own Sun started out its life as a proto-star in one of these clouds, and formed when a small part of the cloud underwent gravitational collapse. Most of the collapsing mass went into the formation of the proto-Sun, with the rest making the protoplanetary disk that surrounded it. Next came planetesimals, which are believed to be the starting point of planets. It is thought that they grow when bits of material in the disk stick together after collisions, and once they reach a certain size, around a kilometer across, gravity takes over and they attract more and more mass. Not all planetesimals become fully-fledged planets; only the largest are able to survive long enough to make it.

When our solar system was evolving, the planetesimals that didn’t get swept up to form planets likely became asteroids instead. It is these asteroids that large meteorites found on earth are believed to originate from. Because they were created at the birth of the solar system, meteorites can give us some clues about its formation and age.

Artists impression of a protoplanetary disk. Image: NASA

Audrey Bouvier and colleague Meenakshi Wadhwa looked at something known as the calcium-aluminium rich inclusions (CAIs) in a meteorite found in the Sahara desert. The CAIs range in size from a few centimeters down to sub-millimeter lengths, and are believed to have formed in the protoplanetary disk as the solar system was beginning to take shape.

Several different radioactive decays can be investigated to determine how old a piece of rock is. The half-life of the each decay is the key to finding out the rock’s age. Researchers can look at how much of an isotope is present in the sample and compare it with how much there is of whatever it decays into, and then use the decay’s half-life to find the age of the sample. By looking at several different decays and combining the age estimates found for each it’s possible to get an even more precise estimate.

Bouvier and Wadhwa did this for the CAIs in their meteorite and found that it was 4,568.2 million years old. That’s between 0.3 and 1.9 million years older than previous research suggests the solar system is.

During their research, Bouvier and Wadhwa also learnt about how the solar system started. By comparing the time of formation of the CAIs with the time of formation of small, round grains of rock known as chondrules, they were able to determine the concentration of an isotope of iron, Fe-60, at the beginning of the solar system. Pushing back the formation of the solar system means that the concentration of Fe-60 at its beginning was twice as much as estimated using previous knowledge. Fe-60 is only made in the end stages of a star’s life, and is then scattered into space when the star dies. The high concentration found makes it very likely that the source of the Fe-60 was the death of a nearby star: our solar system evolved out of the remnants of a supernova.

Reference:
Audrey Bouvier, & Meenakshi Wadhwa (2010). The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion Nature Geoscience : 10.1038/ngeo941

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