Monthly Archives: January 2011

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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