Tag Archives: cosmology

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.

Advertisements

2 Comments

Filed under Physics

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

3 Comments

Filed under Physics

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.

10 Comments

Filed under Physics

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.

13 Comments

Filed under Physics, Primers

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.

1 Comment

Filed under Physics