Tag Archives: galaxies

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.

Reference
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.”

References
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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First Planck results: from the coldest to the largest objects in the universe

Last week astronomers working on the European Space Agency’s Planck experiment convened in Paris to talk about their first results, and they weren’t short of things to say. No less than 25 papers were announced on Tuesday 11th January — and this is before work has even started on the mission’s main aim of putting together a detailed picture of the Cosmic Microwave Background, or CMB.

The CMB is a uniform glow of microwave radiation, with only tiny fluctuations, that gives us a snapshot of the universe around 380,000 years after the Big Bang. We’ve seen it before, courtesy of WMAP in 2003 and COBE in 1992. But Planck has the power to look at this faint glow in never-before-seen detail, revealing more about the universe than every before.

Video showing locations of the different compact objects found by Planck.

Before they can get to work on this new view of the CMB, however, astronomers must study the foreground noise of the picture in detail. This “noise” is made up of structures formed after the CMB: galaxies, galaxy clusters, and matter within the Milky Way, such as gas and dust.

Planck astronomers studied this “noise” in order to better understand how they can remove it from the picture, and just see the CMB. It’s called noise because it gets in the way when we try to look at the CMB, but actually it’s very interesting in its own right. These structures can tell us a lot about the formation of stars, galaxy clusters and even the universe itself — and that’s what some of the 25 new papers announced last week are about.

One of the superclusters detected by Planck and confirmed by XMM-Newton. Credit: ESA/Planck Collaboration

First up, we have galaxy clusters. They’re the largest structures in the universe, and the Planck mission has just completed the first all-sky survey of them using something called the Sunyaev-Zel’dovich effect (SZE). Galaxy clusters don’t just contain galaxies; they also hold hot gas and a large amount of dark matter. The SZE arrises when high energy particles in the hot gas interact with the CMB and distort it. Astronomers can see this distortion in the CMB and use it to detect galaxy clusters.

In total, 189 galaxy clusters have been detected by Planck using the SZE. This includes 169 that had already been detected using other methods, and 20 brand new ones. The really interesting thing about these galaxy clusters is the huge range of masses they encompass — between one and fifteen hundred trillion times the mass of the Sun. Galaxy clusters are extremely sensitive to the underlying framework that describes our universe, and so can shed light on the evolution and structure of the universe.

By working together with another experiment at the ESA, the XMM Newton X-ray observatory, 11 of the newly discovered galaxy clusters have already been confirmed. XMM-Newton has been able to get a closer look at some and reveal that two of the new clusters are in fact superclusters. That is, they are clusters of galaxy clusters, rather than simply clusters of galaxies.

Map showing cold, dense clumps of dust in the Milky Way. Credit: ESA/Planck Collaboration

Not content with studying the largest structures in the universe, Planck has also taken a look at the coldest.

Thanks to Planck, we can now detect material at lower temperatures than ever before. And we can do it more accurately than ever before, too. Astronomers working on the Planck mission have just finished looking at the results of the first all-sky survey of compact cold dust clumps in the Milky Way, and cool dust in other galaxies. These clumps are some of the coldest objects in the universe, and are key to understanding some of the hottest — cold, dusty clumps, like the ones seen by Planck, are believed to be sites of star formation.

These clumps have temperatures of only 7 to 16 degrees above absolute zero. Most of the clumps Planck found were only a few light years away from Earth, but some were up to eight thousand light years away*.

Though Planck was only able to look in detail at this dust in our own galaxy, the results are vital to understanding the behaviour of similar dust in other galaxies. When we look at galaxies that are further away, we also see them as they were further back in time. As we learn more about star formation in our own galaxy from these cold clumps, we will begin to have a better understanding of star formation in galaxies that are further away — and further back in time.

However, Planck has limitations, and these mean that it cannot look into the heart of these cold objects. This is where the ESA’s Herschel space observatory comes in. It has a much higher resolution than Planck and has no problem seeing the detailed structure of the clumps. Between them, Herschel and Planck can form a complete picture of the clumps at both small and large scales. With their help, we can effectively reel in far away galaxies for a closer look and learn about star formation throughout the history of the universe.

Cold dust clumps and galaxy clusters are just two of the interesting discoveries Planck has made in the two full sky surveys it’s completed since it launched in May 2009. It will continue to survey the sky until at least the end of 2011, but full results, including that new detailed picture of the CMB minus the noise, will not be published before early 2013. That might seem like a long way away, but these first results should keep astronomers busy for a little while yet. Then they can get on with the job of studying the CMB, which will no doubt keep them busy for an even longer time to come.

*A light year is roughly three thousand billion miles. For comparison, the Sun is just less than a hundred billion miles, or eight light minutes, from Earth


More on Planck
Watch the press conference
More about Planck’s findings at the BBC
The 25 papers published by the Planck team

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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.

*

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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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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New limit on neutrino mass… from cosmology, not particle physics

Physicists at University College London have found a new upper limit on the mass of a neutrino – one of the tightest constraints yet from either particle physics or cosmology.

Neutrinos are elementary particles that travel close to the speed of light, but are very difficult to detect because they are not electrically charged. In fact, in the time it takes you to read this sentence, thousands of billions of neutrinos will have passed through your body – and you won’t have felt a thing.

According to the Standard Model of particle physics, neutrinos should be massless – just like the photons that make up light – but in reality they do have a very small mass. What the Standard Model failed to take into account is the fact that neutrinos undergo something known as oscillations, or mixing.

Neutrinos come in three “flavours”: electron neutrinos, mu neutrinos and tau neutrinos (as well as a corresponding antiparticle for each). When a neutrino is created, for example in the Sun during nuclear fusion, it has a specific flavour. Over time the neutrino can change flavour, but only if its mass is non-zero. All of the neutrinos created in the Sun are electron neutrinos. However, by the time they reach the Earth, we detect equal amounts of each flavour of neutrino; this is how we know that neutrinos must have mass.

Sudbury Neutrino Observatory: underground detector of solar neutrinos. Image: NASA

Shaun Thomas and colleagues at UCL have now found an upper limit on that mass. It’s the tightest constraint ever placed on the neutrino mass, and comes from cosmology rather than particle physics. Thomas et al took redshift data from over 700,000 galaxies using the Sloan Digital Sky Survey and combined this with results from Wilkinson Microwave Anisotropy Probe, which measured the radiation left over from the Big Bang. They were then able to compare density distributions from the early and more recent Universe.

A Universe where neutrinos are massless would look a bit different to our own. Having massive neutrinos means that the total mass in the Universe is higher than it otherwise would be, and the evolution of the Universe is affected. Larger neutrinos suppress the growth of galaxies more than smaller ones; the more mass neutrinos have, the fewer galaxies will be able to form.

From this relationship, Thomas et al were able to provide the first limit on neutrino mass to come from a galaxy redshift survey, and the tightest constraint yet. The new limit found will complement future neutrino experiments such as KATRIN, and provides hope that the next generation of galaxy surveys may be able to put even tighter constraints on the mass.

Reference:
Thomas, S., Abdalla, F., & Lahav, O. (2010). Upper Bound of 0.28 eV on Neutrino Masses from the Largest Photometric Redshift Survey Physical Review Letters, 105 (3) DOI: 10.1103/PhysRevLett.105.031301

PDF available here.

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