Clusters

The images above show the galaxy cluster Abell 2034 in the optical, X-ray and radio. These show that the cluster contains 328 individual galaxies (including two massive brightest cluster galaxies) a disturbed ICM and several distinct sites of particle acceleration.

As the Universe evolves, gravity brings together hundreds, sometimes thousands of galaxies together to form galaxy clusters. The galaxies within these clusters usually account for about 1% of the total mass. They are encompassed by a hot low density gas (a million to 10 million Kelvin) known as the intra-cluster medium (ICM) which contains about 9% of the cluster mass. The other, approximately 90% of the mass is in the surrounding dark matter halo.

Observations at different frequencies

Observations at different frequencies help us to form a comprehensive picture of the structure and evolution of galaxy clusters. Optical telescopes, for example, can detect individual galaxies allowing us to determine the dynamics of the galaxies and infer the distribution of dark matter. X-rays observatories are used to measure thermal emission from the ICM. Radio telescopes offer a completely different view. They detect non-thermal emission, which reveals the cluster’s magnetic field and the sites of extreme particle acceleration in the ICM.

ASTRON interests

Our group at ASTRON is interested in studying the particle acceleration processes and magnetic fields within the tenuous ICM. We wish to understand the formation of radio halos which are characterised by cluster-wide radio emission and are thought to be caused by turbulence throughout the cluster. We also aim to understand the conditions that lead to the formation of radio relics. These objects are characterised by their peripheral location and are thought to be generated by large shock waves. Finally, we are studying other unusual structures showing intense particle acceleration in the ICM and the interaction between the ICM and discrete radio sources such as tailed radio galaxies.

Research staff: Tim Shimwell

Source: ASTRON-Netherlands Institute for Radio Astronomy/Astronomy

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LOFAR detects gigantic radio sources in the universe

Artistic representation of the large-scale structure of the Universe above the core of the LOFAR telescope. The inset shows a zoom into a galaxy cluster where a megahalo is observed (orange emission, from LOFAR observations).

An international research team, led by the Observatory of Universität Hamburg has, using LOFAR, discovered four radio sources of up to ten million light years in size: megahalos.

Seen from a great distance, the universe is not evenly distributed; it actually resembles a net-like structure, somewhat similar to the way neurons are connected to one another in the brain. At the nodes of this so-called cosmic web hundreds, sometimes even thousands of galaxies are crowded together into galaxy clusters. Sometimes, two galaxy clusters collide with each other and merge into a single cluster. In the process, they release enormous amounts of energy, so large that they are the most powerful events happening in our Universe after the Big Bang. During these collisions, tiny, charged particles are accelerated to near-lightspeed, emitting radio waves that can be detected with radio telescopes.

Using the Low Frequency Array (LOFAR), scientists have now discovered four galaxy clusters where a faint radio emission envelopes the entire clusters even reaching their outskirts. Dr. Virginia Cuciti led the international research team: “Megahalos extend up to ten million light years) in size, which means that they cover a volume that is about 30 times larger than the volume of the radio sources known so far in galaxy clusters. This implies that with megahalos we can now observe the peripheral regions of galaxy clusters which were previously almost inaccessible.”

Computer simulation of the large-scale structure of the Universe. The inset shows a zoom into a galaxy cluster where a megahalo is observed (orange emission, from LOFAR observations).

Cuciti’s team used LOFAR Two-metre Sky Survey (LoTSS) observations of these four galaxy clusters. While analysing the data of one of the clusters, she and her teammates saw some significant hints of radio emission on exceptionally large scales, Cuciti says. “So, we decided to re-inspect all the images of a sample of 310 clusters that we were studying with the aim of looking for similar emission. When we discovered that three other clusters of this sample showed emission on similar scales and with similar characteristics, it became clear that we discovered a new type of cosmic phenomenon that opens the possibility to explore the external region of galaxy clusters through radio observations.”

This discovery could not have been made without LOFAR, Cuciti says. “It is not by chance that megahalos have been discovered with LOFAR. They are very large, and their emission is very faint. Moreover, the synchrotron spectrum of megahalos is steep, which basically means that they are brighter at low radio frequency, therefore a sensitive radio telescope operating at low radio frequency, such as LOFAR, is the ideal instrument to detect them.”

But even then, it was not easy, co-author and astronomer at ASTRON Timothy Shimwell says: “Even in the very sensitive and wide area LOFAR surveys dataset these objects were very hard to find because they are so faint and a very careful analysis of large quantities of data was required to identify them.”

LOFAR 2.0
A region of the LOFAR core seen from above. The two antenna types of LOFAR are visible.

With LOFAR currently undergoing an upgrade to LOFAR2.0, making it an even more sensitive instrument, even more valuable information can be found about megahalos. Cuciti: “With more sensitive observations we could be able to detect megahalos in a much larger number of clusters. This is actually one of the most interesting aspects of this work, because it means that, if megahalos are present in a large fraction of clusters, if not all of them, we are opening a new field of research, a new way to systematically explore the periphery of galaxy clusters with radio observations. The LOFAR 2.0 upgrade will increase the sensitivity of LOFAR, especially in the LBA (at 50 MHz), and will therefore allow us to answer to the question: how many clusters host megahalos?"

The Nature-article Galaxy clusters enveloped by vast volumes of relativistic electrons can be found here.

Source: ASTRON-Netherlands Institute for Radio Astronomy

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LOFAR detects gigantic radio sources in the universe

Artistic representation of the large-scale structure of the Universe above the core of the LOFAR telescope. The inset shows a zoom into a galaxy cluster where a megahalo is observed (orange emission, from LOFAR observations).

An international research team, led by the Observatory of Universität Hamburg has, using LOFAR, discovered four radio sources of up to ten million light years in size: megahalos.

Seen from a great distance, the universe is not evenly distributed; it actually resembles a net-like structure, somewhat similar to the way neurons are connected to one another in the brain. At the nodes of this so-called cosmic web hundreds, sometimes even thousands of galaxies are crowded together into galaxy clusters. Sometimes, two galaxy clusters collide with each other and merge into a single cluster. In the process, they release enormous amounts of energy, so large that they are the most powerful events happening in our Universe after the Big Bang. During these collisions, tiny, charged particles are accelerated to near-lightspeed, emitting radio waves that can be detected with radio telescopes.

Using the Low Frequency Array (LOFAR), scientists have now discovered four galaxy clusters where a faint radio emission envelopes the entire clusters even reaching their outskirts. Dr. Virginia Cuciti led the international research team: “Megahalos extend up to ten million light years) in size, which means that they cover a volume that is about 30 times larger than the volume of the radio sources known so far in galaxy clusters. This implies that with megahalos we can now observe the peripheral regions of galaxy clusters which were previously almost inaccessible.”

Computer simulation of the large-scale structure of the Universe. The inset shows a zoom into a galaxy cluster where a megahalo is observed (orange emission, from LOFAR observations).

Cuciti’s team used LOFAR Two-metre Sky Survey (LoTSS) observations of these four galaxy clusters. While analysing the data of one of the clusters, she and her teammates saw some significant hints of radio emission on exceptionally large scales, Cuciti says. “So, we decided to re-inspect all the images of a sample of 310 clusters that we were studying with the aim of looking for similar emission. When we discovered that three other clusters of this sample showed emission on similar scales and with similar characteristics, it became clear that we discovered a new type of cosmic phenomenon that opens the possibility to explore the external region of galaxy clusters through radio observations.”

This discovery could not have been made without LOFAR, Cuciti says. “It is not by chance that megahalos have been discovered with LOFAR. They are very large, and their emission is very faint. Moreover, the synchrotron spectrum of megahalos is steep, which basically means that they are brighter at low radio frequency, therefore a sensitive radio telescope operating at low radio frequency, such as LOFAR, is the ideal instrument to detect them.”

But even then, it was not easy, co-author and astronomer at ASTRON Timothy Shimwell says: “Even in the very sensitive and wide area LOFAR surveys dataset these objects were very hard to find because they are so faint and a very careful analysis of large quantities of data was required to identify them.”

LOFAR2.0

A region of the LOFAR core seen from above. The two antenna types of LOFAR are visible.

With LOFAR currently undergoing an upgrade to LOFAR2.0, making it an even more sensitive instrument, even more valuable information can be found about megahalos. Cuciti: “With more sensitive observations we could be able to detect megahalos in a much larger number of clusters. This is actually one of the most interesting aspects of this work, because it means that, if megahalos are present in a large fraction of clusters, if not all of them, we are opening a new field of research, a new way to systematically explore the periphery of galaxy clusters with radio observations. The LOFAR 2.0 upgrade will increase the sensitivity of LOFAR, especially in the LBA (at 50 MHz), and will therefore allow us to answer to the question: how many clusters host megahalos?"

The Nature-article Galaxy clusters enveloped by vast volumes of relativistic electrons can be found here.

Source: ASTRON-Netherlands Institute for Radio Astronomy

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Cosmic flashes discovered in a surprising location in space

Extremely fast radio signals from a surprising source. A cluster of ancient stars (left) close to the spiral galaxy Messier 81 (M81) is the source of extraordinarily bright and short radio signals. The image shows in blue-white a graph of how one flash’s brightness changed over the course of only tens of microseconds. (Image: Daniëlle Futselaar/ASTRON, artsource.nl)

Source of mysterious radio signals: an artist's impression of a magnetar in a cluster of ancient stars (in red) close to the spiral galaxy Messier 81 (M81). (Image: Daniëlle Futselaar/ASTRON, artsource.nl)

Astronomers have observed mysterious flashes in the sky from an unexpected source, a globular cluster in the galaxy M81. It is the closest source of fast radio bursts that has been located so far. The results are described in two papers to be published this week in Nature and Nature Astronomy.

Fast radio bursts (FRBs) are unpredictable, extremely short flashes of light from space. Astronomers have struggled to understand them ever since they were first discovered in 2007. So far, they have only been seen by radio telescopes. Each flash lasts only a thousandth of a second. Yet each flash emits as much energy as the Sun gives out in a day. Every day, there are several hundred flashes across the sky. Most are located far away from Earth, in galaxies billions of light years away. Only a few have been observed so far.

In two papers published in parallel this week in the journals Nature and Nature Astronomy, an international team of astronomers presents observations that take scientists a step closer to solving the mystery, while also raising new puzzles. The team is led by Franz Kirsten (Chalmers, Sweden, and the Netherlands Institute for Radio Astronomy ASTRON, the Netherlands) and Kenzie Nimmo (ASTRON and the University of Amsterdam).

Close but surprising location

The team traced the repeating bursts to the outskirts of the nearby spiral galaxy Messier 81 (M81), about 12 million light years from Earth. This makes it the closest source of FRBs ever found. The discovery had another surprise in store: its location corresponded exactly to the site of a globular cluster, a dense cluster of very old stars.

"It is amazing to find fast radio bursts from a globular cluster. This is a place in space where you only find old stars. Further out in the universe, fast radio bursts have been found in places where stars are much younger. This had to be something else," says Franz Kirsten.

The scientists believe that the source of the radio flashes is an object that has been predicted but never seen before: a magnetar that formed after a white dwarf star collapsed under its own weight.

Many stars in clusters form binary stars. Some are so close together that one star attracts material from the other. Once one of the white dwarfs has absorbed enough extra mass from its companion, the star ends its life as a neutron star. "This is a rare occurence, but in a cluster of ancient stars, it is the simplest way of making fast radio bursts," says team member Mohit Bhardwaj from McGill University in Canada.

Fastest ever To the team's surprise, some of the flashes were shorter than expected. "The flashes flickered in brightness within as little as a few tens of nanoseconds. That means they must have come from a tiny volume in space, smaller than a football field and perhaps only tens of metres across," says Kenzie Nimmo.

Future observations of the globular cluster in M81 will have to reveal whether the source is really an unusual magnetar, or something else, like an unusual pulsar or a black hole closely orbiting a massive star.

"These fast radio bursts seem to give us new and unexpected insights into how stars live and die. Like supernovas, they could tell us things about the life cycle of stars in the universe," says Nimmo.

To study the source with the highest possible resolution and sensitivity, the scientists combined measurements from 12 radio telescopes in the European VLBI network (EVN) spread halfway around the globe, including ASTRON's Westerbork Synthesis Radio Telescope and telescopes in Sweden, Latvia, Russia, Germany, Poland, Italy and China. This allowed them to pinpoint the exact location of the source of FRBs in the sky.

Papers:

The research is published in two articles in the journals Nature en Nature Astronomy.

* A repeating fast radio burst source in a globular cluster, by Franz Kirsten et al: www.nature.com/articles/s41586-021-04354-w

* Burst timescales and luminosities link young pulsars and fast radio bursts, by Kenzie Nimmo et al: https://arxiv.org/abs/2105.11446

Original press release: www.astronomie.nl

 

Source:  ASTRON-Netherlands Institute for Radio Astronomy

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Astronomers find largest radio galaxy ever

This fiery pair of plasma plumes, named Alcyoneus, forms the largest known structure made by a galaxy.
c) Martijn Oei et al.

A supermassive black hole lurks in the centre of many galaxies, which slows down the birth of new stars and therefore strongly influences the lifecycle of the galaxy as a whole. Sometimes, this leads to tumultuous scenes: the black hole can create two jet streams, that catapult the building material for baby stars out of the galaxy at almost the speed of light. In this violent process, the stardust heats up so much that it dissolves into plasma and glows in radio light. The international team of researchers from Leiden (The Netherlands), Hertfordshire, Oxford (both UK), and Paris (France) have now collected that light – with the pan-European LOFAR telescope, whose epicentre lies in a marshy Dutch ‘radio-dark’ nature reserve, where your smartphone deliberately loses signal. 

Record length

The picture of the two plasma plumes is special, because never before scientists saw a structure this big made by a single galaxy. The discovery shows that the sphere of influence of some galaxies reaches far from their direct environment. How far, exactly? That is hard to determine. Astronomical pictures are taken from a single viewpoint (Earth), and therefore do not contain depth.[1] As a result, scientists can only measure a part of the radio galaxy length: a low estimate of the total length. But even that lower bound, of more than 16 million light-years, is gargantuan, and comparable to one hundred Milky Ways in a row. Or: consider every living insect on Earth, and blow up each of them to the size of Mount Everest, before asking them to stand in a row.[2] This row also suffices (flying off not allowed).

Visible with the naked radio eye

Because Earth does not occupy a special place in the Universe, it was never very likely that such a largest galactic structure would reside in our own backyard. And indeed: the radio giant is three billion light-years away from us. Despite that mind-boggling distance, the giant looms as large in the sky as the Moon – an indication that the structure had to have a record length. The fact that the radio eyes of the LOFAR telescope only saw the giant just now, is because the plumes are relatively faint. By reprocessing a set of existing images in such a way that subtle patterns stood out, the scientists were suddenly able to spot the giant.

The giant Alcyneus

The researchers named the giant structure Alcyoneus, after the son of Ouranos, the Greek primordial god of the sky. This mythological Alcyoneus was a giant that fought against Heracles and the Olympian gods for supremacy over the cosmos. In the world-famous Pergamon Altar in Berlin, a sculpture of this Alcyoneus is carved out.

Ghostly dance

Alcyoneus’ plumes possibly reveal information about the mostly elusive filaments of the Cosmic Web. The Cosmic Web is another name for the contemporary, grown-up Universe, that looks like a network of threads and nodes that astronomers call filaments and clusters, respectively. The galaxies in filaments and clusters are clearly visible themselves, but detecting the medium between galaxies has only been successful in clusters – barring a handful of exceptions. Could Alcyoneus change this? Because Alcyoneus, just like the Milky Way, inhabits a filament, its plumes feel a headwind while moving through the medium. This subtly changes the direction and shape of the plumes: they perform a slow dance with an invisible partner. For many years, scientists have proposed that the shapes of and pressures in the plumes of radio galaxies could relate to filament properties, but never before did they find an example where that connection is as plausible as with Alcyoneus. Namely, Alcyoneus’ plumes are so big and rarefied that the surrounding medium can relatively easily mold them.

Black hole are cosmic mainstays

The Cosmic Web retains its form because the attractive force of gravity is compensated by the heat pressure of the medium in filaments and clusters. In the past two decennia it has become clear that the glowing stardust that jet streams eject from galaxies, keeps the Web warm. In this way, the central black holes in galaxies contribute to sustain the large-scale structure of our Universe. That is extra noteworthy because black holes are very small compared to filaments and clusters. It is as if something the size of a marble regulates the Earth’s temperature. Marvellous thermometres, in short.

Mysterious origin

What has given Alcyoneus its record length, remains a mystery for now. The scientists first thought of an exceptionally massive black hole, an extensive stellar population (and so a lot of stardust), or extraordinarily powerful jet streams. Surprisingly enough, Alcyoneus appears to be less than average on all these aspects compared to its smaller sisters and brothers. In the times ahead, the team will therefore now investigate whether the environments of radio galaxies could explain the growth of giants instead.

Paper:

The discovery of a radio galaxy of at least 5 Mpc. By: Martijn S.S.L. Oei, Reinout J. van Weeren, Martin J. Hardcastle, Andrea Botteon, Tim W. Shimwell, Pratik Dabhade, Aivin R.D.J.G.I.B. Gast, Huub J.A. Röttgering, Marcus Brüggen, Cyril Tasse, Wendy L. Williams, Aleksandar Shulevski. Accepted for publication in: Astronomy & Astrophysics (preprint).

Original press release: www.astronomie.nl

Source:  ASTRON-Netherlands Institute for Radio Astronomy/News

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Gigapixel radio image of the Universe using Europe as a radio telescope

An illustration of the difference in resolving power between using only the Dutch LOFAR stations and all of the international stations throughout Europe, arranged in the formation of an HBA core stations. The animation fades from an angular resolution of 6" to a resolution of 0.3". Credit: Frits Sweijen

An international team of astronomers has created one of the largest and most detailed radio maps at megahertz frequencies thanks to Dutch supercomputers. This research, led by Frits Sweijen at Leiden University, has been published in Nature Astronomy on Thursday. Using the International LOFAR Telescope they have, in the wake of tremendous progress late last year, mapped an area of the sky the size of 25 full moons in great detail, with a resolving power comparable to optical telescopes on Earth. The resulting radio image contains nearly seven billion pixels and contains just shy of 2500 radio galaxies.

LOFAR

The sky is filled with radiation invisible to the naked eye, including radio waves at frequencies ten million times lower than red light. With tens of thousands of antennas across Europe, the LOFAR telescope listens to those cosmic radio waves at a frequency of 144 MHz, just above the FM radio band. Through these antennas the European continent transforms into an almost 2000 km big radio telescope. This tremendous size means that LOFAR can see exquisite and unprecedented detail at such low radio frequencies, with a resolving power high enough to make out the Great Pyramid if it were on the Moon. The combined area of all the antennas make it sensitive enough to detect a mobile phone ringing all the way out on Mars.

This resolving power is challenged by Earth's atmosphere however. Ultraviolet radiation from the Sun creates a layer of charged particles in the upper atmosphere. This so-called ``ionosphere'' distorts radio waves from space before they reach the telescope. For LOFAR it therefore is like looking at the sky from the bottom of the ocean. With advanced techniques these distortions can be corrected, focusing the telescope across its entire field of view, thus allowing it to be mapped.

Super computers

Determining these corrections and subsequently converting radio waves into an image, requires modern algorithms and a lot of compute power. Thanks to super computers this was not an issue. Locally, the Academic Leiden Interdisciplinary Cluster Environment (ALICE) lent its power to the scientists. Nationally, SURF in the Netherlands, provided early access to the new platform data processing platform named Spider. This platform is specifically designed for data-intensive projects like this. Lengthy calculations could be run in a massively parallel fashion thanks to these supercomputers. The final image was too large to be made in one go. To image the full field of view, it was processed in 25 smaller chunks each covering an area the size of a full moon. Each of these chunks was turned into an image over seven days, using software recently developed at ASTRON. Piece by piece, on a single computer, this process would have taken more than 175 days to complete. Thanks to the large scale compute infrastructure at Leiden and SURF, however, it only took seven days effectively.

The sharp eyes of the International LOFAR Telescope allow scientists to study the evolution of black holes and their host galaxies in more detail than before. Galaxies in the early Universe, for example, that would otherwise be too small to resolve due to their distance or young age, can have their spatial structure studied. The published results enable this for thousands of sources at once. With its near seven billion pixels this single image contains almost as many pixels as radio surveys from the past did covering the entire sky. These new results explore a tip of the ice berg with a detailed map of the entire Northern sky as the future goal.

This work made use of the Dutch national e-infrastructure with the support of the SURF Cooperative using grant no. EINF-251; the ERC Starting Grant ClusterWeb 804208; the Medical Research Council grant MR/T042842/1; the UK STFC ST/R000972/1 and ST/V000594/1 grants and the Academic Leiden Interdisciplinary Cluster Environment (ALICE) provided by Leiden University.

Article in Nature

Source:ASTRON-Netherlands Institute for Radio Astronomy/News

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Ultra-sensitive radio images reveal thousands of star-forming galaxies in early Universe

The deepest LOFAR image ever made, in the region of sky known as ‘Elais-N1’, which is one of the three fields studied as part of this deep radio survey. This image arises from a single LOFAR pointing observed repeatedly for a total of 164 hours. Over 80,000 radio sources are detected; this includes some spectacular large-scale emission arising from massive black holes, but most sources are distant galaxies like the Milky Way, forming their stars. Credit: Philip Best & Jose Sabater, University of Edinburgh.

An international team of astronomers has published the most sensitive images of the Universe ever taken at low radio frequencies, using the International Low Frequency Array (LOFAR). By observing the same regions of sky over and over again and combining the data to make a single very-long exposure image, the team has detected the faint radio glow of stars exploding as supernovae, in tens of thousands of galaxies out to the most distant parts of the Universe. A special issue of the scientific journal Astronomy & Astrophysics is dedicated to fourteen research papers describing these images and the first scientific results.

Cosmic star formation

Philip Best, University of Edinburgh (UK), who led the deep survey, explained: “When we look at the sky with a radio telescope, the brightest objects we see are produced by massive black holes at the centre of galaxies. However, our images are so deep that most of the objects in it are galaxies like our own Milky Way, which emit faint radio waves that trace their on-going star-formation.”

“The combination of the high sensitivity of LOFAR and the wide area of sky covered by our survey – about 300 times the size of the full moon – has enabled us to detect tens of thousands of galaxies like the Milky Way, far out into the distant Universe. The light from these galaxies has been travelling for billions of years to reach the Earth; this means that we see the galaxies as they were billions of years ago, back when they were forming most of their stars.”

Isabella Prandoni, INAF Bologna (Italy), added: “Star formation is usually enshrouded in dust, which obscures our view when we look with optical telescopes. But radio waves penetrate the dust, so with LOFAR we obtain a complete picture of their star-formation.” The deep LOFAR images have led to a new relation between a galaxy’s radio emission and the rate at which it is forming stars, and a more accurate measurement of the number of new stars being formed in the young Universe.

Exotic objects

The remarkable dataset has enabled a wide range of additional scientific studies, ranging from the nature of the spectacular jets of radio emission produced by massive black holes, to that arising from collisions of huge clusters of galaxies. It has also thrown up unexpected results. For example, by comparing the repeated observations, the researchers searched for objects that change in radio brightness. This resulted in the detection of the red dwarf star CR Draconis. Joe Callingham, Leiden University and ASTRON (NL), noted that “CR Draconis shows bursts of radio emission that strongly resemble those from Jupiter, and may be driven by the interaction of the star with a previously unknown planet, or because the star is rotating extremely quickly.”

Huge computational challenge

LOFAR does not directly produce maps of the sky; instead the signals from more than 70,000 antennas must be combined. To produce these deep pictures, more than 4 petabytes of raw data - equivalent to about a million DVDs – were taken and processed. “The deep radio images of our Universe are diffusely hidden, deep inside the vast amount of data that LOFAR has observed.” said Cyril Tasse from Paris Observatory, University PSL (France). “Recent mathematical advances made it possible to extract these, using large clusters of computers.”

Multi-wavelength data

Just as important in extracting the science has been a comparison of these radio images with data obtained at other wavelengths. “The parts of the sky we chose are the best-studied in the Northern sky” explained Philip Best. This has allowed the team to assemble optical, near-infrared, far-infrared and sub-millimetre data for the LOFAR-detected galaxies, which has been crucial in interpreting the LOFAR results.

LOFAR

LOFAR is the world’s leading telescope of its type. It is operated by ASTRON, the Netherlands Institute for Radio Astronomy, and coordinated by a partnership of 9 European countries: France, Germany, Ireland, Italy, Latvia, the Netherlands, Poland, Sweden and the UK. In its ‘high-band’ configuration, LOFAR observes at frequencies of around 150 MHz – between the FM and DAB radio bands. “LOFAR is unique in its ability to make high-quality images of the sky at metre-wavelengths”, said Huub Röttgering, Leiden University, who is leading the overall suite of LOFAR surveys. “These deep field images are a testament to its capabilities and a treasure trove for future discoveries”.

Additional images and videos are available at https://www.lofar-surveys.org/deepfields.html, which also hosts the papers, deep images and catalogues released today. A direct link to the special issue can be found here. 

 

Source:  ASTRON-Netherlands Institute for Radio Astronomy/News

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Help to find the location of newly discovered black holes in the LOFAR Radio Galaxy Zoo project

As an example, take the case of the famous radio source 3C236. The upper image is the radio source, the middle one an optical image showing many stars and galaxies and the lower image an overlay of the radio and the optical image. In this case, for the human eye the origin of the radio emission is clear, it is the bright point-like radio source at the center of the radio image. This is the location of the massive black hole that is driving all the radio activity. From the overlay with the optical images the galaxy that hosts the black hole can then be identified. Image credit: Aleksandar Shulevski, Erik Osinga & The LOFAR surveys team. Hi-res image

Scientists are asking for the public’s help to find the origin of hundreds of thousands of galaxies that have been discovered by the largest radio telescope ever built: LOFAR. Where do these mysterious objects that extend for thousands of light-years come from? A new citizen science project, LOFAR Radio Galaxy Zoo, gives anyone with a computer the exciting possibility to join the quest to find out where the black holes at the centre of these galaxies are located.

Astronomers use radio telescopes to make images of the radio sky, much like optical telescopes like the Hubble space telescope make maps of stars and galaxies. The difference is that the images made with a radio telescope show a sky that is very different from the sky that an optical telescope sees. In the radio sky, stars and galaxies are not directly seen but instead an abundance of complex structures linked to massive black holes at the centres of galaxies are detected. Most dust and gas surrounding a supermassive black hole gets consumed by the black hole, but part of the material will escape and gets ejected into deep space. This material forms large plumes of extremely hot gas, it is this gas that forms large structures that is observed by radio telescopes.

The Low Frequency Array (LOFAR) telescope, operated by the Netherlands Institute for Radio Astronomy (ASTRON), is continuing its huge survey of the radio sky and 4 million radio sources have now been discovered. A few hundred thousand of these have very complicated structures. So complicated that it is difficult to determine which galaxies belong to which radio source, or in other words, which black hole belongs to which galaxy?

While the international LOFAR team consists of more than 200 astronomers from 18 countries, it is simply too small to take on this daunting task of identifying which radio structures belong to which host galaxy. Therefore, LOFAR astronomers are asking the public to help. In the context of the citizen science project ‘LOFAR Radio Galaxy Zoo’, the public is asked to look at images from LOFAR and images of galaxies and then associate radio sources with galaxies.

“LOFAR’s new survey has revealed millions of previously undetected radio sources. With the help of the public we can investigate the nature of these sources: Where are their black holes? In what kind of galaxies are the black holes located?’’ says Huub Röttgering from Leiden University (The Netherlands).

Tim Shimwell, ASTRON and Leiden University, explains why this is significant: “Your task is to match the radio sources with the right galaxy. This will help researchers understand how radio sources are formed, how black holes evolve, and how vast quantities of material can be ejected into deep space with such unprecedented amounts of energy”, he says.

Radio Galaxy Zoo: LOFAR is part of the Zooniverse project, the world’s largest and most popular platform for people-powered research. This research is made possible by volunteers — more than a million people around the world who come together to assist professional researchers.

Images

• More images can be found here
• The Radio Galaxy Zoo: LOFAR page is accessible here
• The tutorial video can be viewed here

Source: ASTRON-Netherlands Institute for Radio Astronomy/News

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Star formation may be halted by cold ionised hydrogen

A composite image showing our Galaxy, the Milky Way, rising above the Engineering Development Array at the Murchison Radio-astronomy Observatory in Western Australia. The location of the centre of our Galaxy is highlighted alongside the ionized hydrogen (H+) signal detected from this region of sky. The white-blueish light shows the stars making up the Milky Way and the dark patches obscuring this light shows the cold gas that is interspersed between them. Credit: Engineering Development Array image courtesy of ICRAR. Milky Way image courtesy of Sandino Pusta.

The Engineering Development Array from ICRAR on Vimeo.

For the first time ionised hydrogen has been detected at the lowest frequency ever towards the centre of our Galaxy. The findings originate from a cloud that is both very cold (around -230 degrees Celsius) and also ionised, something that has never been detected before. This discovery may help to explain why stars don’t form as quickly as they theoretically could.

Dr. Raymond Oonk (ASTRON/Leiden Observatory/SURFsara) led this study which is published today in MNRAS. He said: "The possible existence of cold ionised gas had been hinted at in previous work, but this is the first time we clearly see it."

Ionisation is an energetic process that strips electrons away from atoms. The atom will become electrically charged and can then be called an ion. This typically happens in gas that is very hot (10000 degrees Celsius) and where atoms can easily lose their electrons. It was therefore puzzling to discover the ionised hydrogen from very cold gas in this cloud. Normal energy sources, such as photons from massive stars, would not cause this. More exotic energy forms, such as high energy particles created in supernova shockwaves and near black holes, are more likely to be responsible.

Dr. Oonk continues: "This discovery shows that the energy needed to ionise hydrogen atoms can penetrate deep into cold clouds. Such cold clouds are believed to be the fuel from which new stars are born. However, in our Galaxy we know that the stellar birth rate is very low, much lower than naively expected. Perhaps the energy observed here acts as a stabiliser for cold clouds, thereby preventing them from collapsing on to themselves and forming new stars."

The observation was made with the Engineering Development Array (EDA), a prototype station of the Square Kilometre Array (SKA), the worlds’ largest radio telescope. A/Prof. Randall Wayth (Curtin University/ICRAR) says: "This detection was made possible by the wide bandwidth of the EDA and the extremely radio quiet location of the Murchison Radio-astronomy Observatory. The low frequency portion of the Square Kilometre Array will be built at this location in the coming years, so this excellent result gives us a glimpse of what the SKA will be capable of once it's built."

The data reduction was led by Emma Alexander (University of Manchester) as part of her summer student internship at ASTRON: "It’s a very exciting time to be coming into radio astronomy, and it was great to work on the first high resolution spectroscopic data from this SKA prototype station. The technologies that are being developed for the SKA, and the science results that come from them, will be a driving force for my generation of radio astronomers."

This work was carried out as a collaboration between the Netherlands Institute for Radio Astronomy (ASTRON), Leiden University, the International Centre for Radio Astronomy Research (ICRAR), University of Manchester and the Square Kilometre Array.

Source: ASTRON-Netherlands Institute for Radio Astronomy/News

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Energy loss gives unexpected insights in evolution of quasar jets

 The radio jet of the quasar 4C+19.44, powered by a supermassive black hole lying in the center of its host galaxy and shining at long radio wavelengths as seen by the LOFAR radio telescope (magenta). The background image shows neighboring galaxies in the visible light highlighted thanks to the Hubble Space Telescope (cyan and orange) having the radio jet passing into the dark voids of intergalactic space (Harris et al. 2019). Image Credit: NASA/HST/LOFAR; Courtesy of J. DePasquale

An international team of astrophysicists observed for the first time that the jet of a quasar is less powerful on long radio wavelengths than earlier predicted. This discovery gives new insights in the evolution of quasar jets. They made this observation using the international Low Frequency Array (LOFAR) telescope, that produced high resolution radio images of quasar 4C+19.44 located over 5 billion light-years from Earth. 

 

Supermassive black holes, many millions of times more massive than our Sun reside in the central regions of galaxies. They grow even larger by attracting and consuming nearby gas and dust. If they consume material rapidly, the infalling matter shines brightly and the source is known as a quasar.

Some of this infalling matter is not digested, but instead is ejected in the form of so-called jets that punch through the surrounding galaxy and into intergalactic space for millions of light years. These jets, shining brightly at radio wavelengths, are composed of particles accelerated up to nearly the speed of light, but exactly how these particles achieve energies not attainable on the Earth is yet to be completely solved.

The discovery on quasar 4C+19.44 gives new insights to the balance between the energy in the field surrounding the quasar and that residing in the quasar jet. This finding indicates to an intrinsic property of the source rather than due to absorption effects. It implies that the energy budget available to accelerate particles and the balance between energy stored in particles and in the magnetic field, is less than expected.

"This is an important discovery that will be used for the years to come to improve simulations of jets. We observed for the first time a new signature of particle acceleration in the power emitted of quasar jets at long radio wavelengths. An unexpected behaviour that changes our interpretation on their evolution." Said Prof. Francesco Massaro from University of Turin. "We knew that this was already discovered in other cosmic sources but it was never before observed in quasars."

The international team of astrophysicists had observed the jet of the quasar 4C+19.44 at short radio wavelengths, in visible light, and X-ray wavelengths. The addition of the LOFAR images allowed astrophysicists to make this discovery. LOFAR is the first radio facility operating at long radio wavelengths, which produces sharp images with a resolution similar to that of the Hubble Space Telescope.

"We have been able to perform this experiment thanks to the highest resolution ever achieved at these long radio wavelengths, made possible by LOFAR." Said Dr Adam Deller, an astrophysicist of the Swinburne University of Technology who contributed to the LOFAR data analysis and imaging of 4C +19.44 while at ASTRON in the Netherlands, heart of the LOFAR collaboration.

Dr Raymond Oonk, an astronomer at ASTRON and Leiden University and Dr Javier Moldon, astronomer at the University of Manchester, explained that "We have developed new calibration techniques for LOFAR and this has allowed us to separate compact radio structures in the quasar jet known as radio knots, and measure their emitted light. This result was unexpected and demands to future deeper investigations. New insights and clues on particle acceleration will come soon thanks to the international stations of LOFAR."

The observation performed on the radio jet of 4C+19.44 was designed by Dr D. E. Harris, supervisor of Prof. Francesco Massaro, while working at the Harvard-Smithsonian Center for Astrophysics, several years ago. He performed the observation in collaboration with Dr Raffaella Morganti and his friends and colleagues at ASTRON. He only got the opportunity to see preliminary results as he passed away on 2015 December 6th. This publication, published in the first March issue of the Astrophysical Journal, is in memory of a career spanned much of the history of radio astronomy.

Source: ASTRON-Netherlands Institute for Radio Astronomy

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Super-slow pulsar challenges theory

Artist’s conception of the newly discovered 23.5-second pulsar. Radio pulses originating from a source in the constellation Cassiopeia are seen travelling towards the core of the LOFAR telescope array. This source is a highly magnetised radio pulsar, shown in the inset image. The pulses and sky image are derived from the actual LOFAR data. Credit: Danielle Futselaar and ASTRON.  Hi-res image


An international team of astronomers have discovered the slowest-spinning radio pulsar yet known. The neutron star spins around only once every 23.5 seconds and is a challenge for theory to explain. The researchers, including astronomers at the University of Manchester, ASTRON and the University of Amsterdam, carried out their observations with the LOFAR telescope, whose core is located in the Netherlands. Their findings will soon appear in the Astrophysical Journal.

Pulsars are rapidly rotating neutron stars that produce electromagnetic radiation in beams that emanate from their magnetic poles. These “cosmic lighthouses” are born when a massive star explodes in a supernova. Thereafter, a super-dense ball of material is left behind – rapidly spinning, and with a diameter of only about 20 kilometers. The fastest-spinning pulsar rotates once each 1.4 milliseconds. Until now, the slowest-spinning pulsar known had a period of 8.5 seconds. Now researchers have discovered a much slower, 23.5-second, pulsar, which is located in the constellation Cassiopeia.

“It is incredible to think that this pulsar spins more than 15.000 times more slowly than the fastest spinning pulsar known.” said Chia Min Tan a PhD Student at the University of Manchester who discovered the pulsar. “We hope that there are more to be found with LOFAR”.

The astronomers discovered this new pulsar during the LOFAR Tied-Array All-Sky Survey. This survey is searching for pulsars in the Northern sky. Each survey snapshot of the sky lasts for one hour. This is much longer compared to previous surveys, and gave the sensitivity needed to discover this surprising pulsar. Not only did the astronomers 'hear' the regular ticks of the pulsar signal, they could also 'see' the pulsar in LOFAR’s imaging survey. Co-author Cees Bassa (ASTRON): “This pulsar spins so remarkably slowly that we could see it blinking on and off in our LOFAR radio images. With faster pulsars that’s not possible.”

The pulsar is approximately 14 million years old, but still has a strong magnetic field. Co-author Jason Hessels (ASTRON and University of Amsterdam): “This pulsar was completely unexpected. We’re still a bit shocked that a pulsar can spin so slowly and still create radio pulses. Apparently radio pulsars can be slower than we expected. This challenges and informs our theories for how pulsars shine.”

Moving forward, the astronomers are continuing their LOFAR survey for new pulsars. They are also planning to observe their new find with the XMM-Newton space telescope. This telescope is designed to detect X-rays. If the super-slow pulsar is detected as a source of X-rays, then this will give important insights into its history and origin.


Reference: 

LOFAR discovery of a 23.5-second radio pulsar. By: C.M. Tan (1), C.G. Bassa (2), S. Cooper (1), T.J. Dijkema (2), P. Esposito (3,4), J.W.T. Hessels (2,3), V.I. Kondratiev (2,5), M. Kramer (6,1), D. Michilli (3,2), S. Sanidas (1), T.W. Shimwell (2), B.W. Stappers (1), J. van Leeuwen (2,3), I. Cognard (7,8), J.-M. Grießmeier (7,8), A. Karastergiou (9,10,11), E.F. Keane (12), C. Sobey (13,14), P. Weltevrede (1). (preprint)

Source: ASTRON-Netherlands Institute for Radio Astronomy

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Planet formation starts before star reaches maturity

TMC1A is a still developing star in the constellation Taurus. Red are areas with many dust particles. Green and blue are two types of carbon monoxide. The absence of green / blue carbon monoxide in the inner part indicates that dust particles in the young protoplanetary disk have grown from less than a thousandth of a millimeter to a millimeter.  (c) Jørgensen/Harsono/ESASky/ESAC [CC-BY-SA 3.0]

Artistic impression of a star with a protoplanetary disk and growing grains.

(c) Daria Dall'Olio [CC-BY-SA 3.0]

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A European team of astronomers has discovered that dust particles around a star already coagulate before the star is fully grown. Dust particle growth is the first step in the formation of planets. The researchers from the Netherlands, Sweden and Denmark publish their findings in Nature Astronomy. 

 

In recent years, astronomers have discovered numerous planetary systems around other stars. Almost every star is likely to have at least one planet orbiting it. Some of the major questions are centered around how planetary systems form and how this process leads to the observed diversity of planets in numbers and masses. The results of a European research project suggest that planet formation starts very early in the star formation process. 

The researchers used the Atacama Large Millimeter Array for their discovery. ALMA is a collection of 66 linked radio telescopes spread over 16 kilometer in the Atacama desert in Chile. The researchers pointed the telescope toward TMC1A, a still developing star in the constellation Taurus (the Bull). 

The astronomers saw a striking lack of carbon monoxide radiation in a disc-shaped area near the star. They suspected that the radiation was blocked by big dust particles. Using numerical models, they could demonstrate that indeed the dust particles in the young protoplanetary disk have probably grown from a thousandth of a millimeter to a millimeter. 

Lead researcher Daniel Harsono (Leiden University, the Netherlands) explains why this is so surprising: "The results indicate that planets already start forming while the star is still developing. The star is only half to three-quarters of its final mass. This is new." 

Per Bjerkeli (Chalmers University, Sweden) highlights the implication of early grain growth: "It can be an explanation for the formation of giant planets that are comparable to Jupiter and Saturn. Only early protoplanetary discs contain sufficient mass to form giant planets." 

Co-researcher Matthijs van der Wiel (ASTRON, Netherlands Institute for Radio Astronomy) is pleased with the clear and unambiguous observations. "This early particle growth could be an exception, of course. Maybe this young disk is very special." 

In the future, the researchers want to look for tell-tale signs of planet formation around other protostars in similar manner. "Currently, ALMA is the only observatory capable of resolving dust and gas emission at scales where new planets are forming, matching the scales in our Solar system. In the future, similarly high resolution observations will be attained with the dishes of the Square Kilometre Array (SKA) to be built in South Africa. Compared with ALMA’s millimeter wave detectors, the SKA will be sensitive to wavelengths of 2 cm and above, and will therefore help to localize centimeter-sized grains, the next step up in the journey from tiny dust particles to planets," says Van der Wiel.

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Reference:
 

"Evidence for the start of planet formation in a young circumstellar disk." By: Daniel Harsono (1), Per Bjerkeli (2), Matthijs H.D. van der Wiel (4), Jon P. Ramsey (3), Luke T. Maud (1), Lars E. Kristensen (3) & Jes K. Jørgensen (3). 1. Leiden University, the Netherlands. 2. Chalmers University of Technology, Sweden. 3. University of Copenhagen, Danmark. 4. ASTRON, Dwingeloo, the Netherlands. In: Nature Astronomy, 25 June 2018. 

 

Source: ASTRON-Netherlands Institute for Radio Astronomy

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Surprise discovery provides new insights into stellar deaths

Artist conception of a tidal disruption event (TDE) that happens when a star passes fatally close to a supermassive black hole, which reacts by launching a relativistic jet. Image credit: Sophia Dagnello, NRAO/AUI/NSF.  Hi-res image

Astronomers, working on a project to detect supernovas, made a surprise discovery when they found that one supernova explosion was actually a star being pulled apart by a supermassive black hole. ASTRON's Westerbork Synthesis Radio Telescope was involved in the observations.

This rare stellar death, known as a tidal disruption event, or TDE, occurs when the powerful gravity of a supermassive black hole rips apart a star that has wandered too close to the massive monster. 

Theorists have suggested that material pulled from the doomed star forms a rotating disk around the black hole, emitting intense X-rays and visible light, and launches jets of material outward from the poles of the disk close to the speed of light. 

"Never before have we been able to directly observe the formation and evolution of a jet from one of these events," said Miguel Perez-Torres, of the Astrophysical Institute of Andalucia in Granada, Spain. 

Originally, the researchers were monitoring a pair of colliding galaxies known as Arp 299, nearly 150 million light-years from Earth. This area of space is so rich in supernova explosions it has been dubbed the “supernova factory”. However, in January 2005 the researchers discovered a bright burst of infrared emission coming from the nucleus of one of these galaxies, and in July of the same year a new, distinct source of radio emission was witnessed from the same location. 

"As time passed, the new object stayed bright at infrared and radio wavelengths, but not in visible light and X-rays," said Seppo Mattila, of the University of Turku in Finland. "The most likely explanation is that thick interstellar gas and dust near the galaxy's centre absorbed the X-rays and visible light, then re-radiated it as infrared," he added. The researchers used the Nordic Optical Telescope on the Canary Islands and NASA's Spitzer space telescope to follow the object's infrared emission. 

Over the course of the next decade, the team continued to observe the radio emission using a technique known as Very Long Baseline Interferometry (VLBI). VLBI involves the remote coordination of multiple telescopes across the globe to focus on a single radio source at a given time. 

This technique provides extremely high resolution imaging when studying a radio source in space, providing the researchers with detailed data on the TDE. Telescopes in the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA) were used for the observations, while the data collected was correlated at the Joint Institute for VLBI ERIC (JIVE), the Netherlands, and the Very Large Array (VLA), USA, respectively. 

This extensive monitoring revealed in 2011 that the radio-emitting portion was expanding in one direction, forming an elongation called a jet, as previously predicted by theorists. The measured expansion indicated that the material in the jet moved at an average of one-fourth the speed of light.

Most galaxies have supermassive black holes at their cores with masses that are millions to billions of times greater than the Sun. This mass is so concentrated that the resulting gravitational pull does not even allow light to escape. In this instance, the black hole is actively drawing material from its surroundings and ripping apart a star that is twice the Sun’s mass. This material forms a rotating disk around the black hole, and superfast jets of particles are launched outward – a phenomenon seen in radio galaxies and quasars. 

"Much of the time, however, supermassive black holes are not actively devouring anything, so they are in a quiet state," Perez-Torres explained. "Tidal disruption events can provide us with a unique opportunity to advance our understanding of the formation and evolution of jets in the vicinities of these powerful objects," he added. 

"Because of the dust that absorbed any visible light, this particular tidal disruption event may be just the tip of the iceberg of what until now has been a hidden population," Mattila said. "By looking for these events with infrared and radio telescopes, we may be able to discover many more, and learn from them," he said. 

Such events may have been more common in the distant Universe, so studying them could help scientists to better understand the environment in which galaxies developed billions of years ago.

Mattila and Perez-Torres led a team of 36 scientists from 26 institutions around the world in the observations of Arp 299. Their findings are published in the journal Science, which can be accessed here: http://science.sciencemag.org/lookup/doi/10.1126/science.aao4669. 

More information: 

The European VLBI Network (EVN) is a network of radio telescopes located primarily in Europe and Asia, with additional antennas in South Africa and Puerto Rico, which performs very high angular resolution observations of cosmic radio sources. 

Collectively the EVN forms the most sensitive radio telescope array at both centimetre wavelengths and millarcsecond resolution. The data collected at each of the individual stations is collated centrally at the correlator – a data processor housed at the Joint Institute for VLBI ERIC (JIVE) in Dwingeloo, the Netherlands.

 

The following EVN antennas observed at one or more epochs: Kunming, Seshan, Urumqi (China), Effelsberg, Wettzell (Germany), Medicina, Noto (Italy), Irbene (Latvia), Torun (Poland), Badary, Svetloe, Zelenchukskaya (Russia), Robledo, Yebes (Spain), Onsala (Sweden), Westerbork (The Netherlands), Cambridge and Jodrell Bank (The United Kingdom). 

Article: Mattila, S., Pérez-Torres, M., et al. 2018. A dust enshrouded tidal disruption event with a resolved radio jet in a galaxy merger. Science. DOI: 10.1126/science.aao4669 

Source:  ASTRON-Netherlands Institute for Radio Astronomy

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Habitable planets around pulsars theoretically possible

Artistic impression of a habitable planet (centre) near a pulsar (right). Such a planet must have an enormous atmosphere that convert the deadly X-rays and high energy particles of the pulsar into heat. (c) Institute of Astronomy, University of Cambridge.  Hi-res image

It is theoretically possible that habitable planets exist around pulsars. Such planets must have an enormous atmosphere that convert the deadly X-rays and high energy particles of the pulsar into heat. That is stated in a scientific paper by astronomers Alessandro Patruno and Mihkel Kama, working in the Netherlands and the United Kingdom. The paper appears today in the journal Astronomy & Astrophysics.

Pulsars are known for their extreme conditions. They are neutron stars of only 10 to 30 kilometers in diameter. They have enormous magnetic fields, they accrete matter and they regularly burst out large amounts of X-rays and other energetic particles. Nevertheless, Alessandro Patruno (Leiden University and ASTRON) and Mihkel Kama (Leiden University and Cambridge University) suggest that there could be life in the vicinity of these stars. 

It is the first time that astronomers try to calculate so-called habitable zones near neutron stars. The calculations show that the habitable zone around a neutron star can be as large as the distance from our Earth to our Sun. An important premise is that the planet must be a super-Earth with a mass between one and ten times of our Earth. A smaller planet will lose its atmosphere within a few thousand years. Furthermore, the atmosphere must be a million times as thick as that of the Earth. The conditions on the pulsar planet surface might resemble those of the deep sea at Earth.

The astronomers studied the pulsar PSR B1257+12 about 2300 light-years away in the constellation Virgo. They used the Chandra space telescope that is specially made to observe X-rays. Three planets orbit the pulsar. Two of them are super-Earths with a mass of four to five times our Earth. The planets orbit close enough around the pulsar to warm up. Patruno: "According to our calculations, the temperature of the planets might be suitable for the presence of liquid water on their surface. Though, we don't know yet if the two super-Earths have the right, extremely dense atmosphere." 

In the future, the astronomers would love to observe the pulsar in more detail and compare it with other pulsars. The ALMA telescope of the European Southern Observatory would be able to show dust discs around neutron stars. Such disks are good predictors of planets. 

Probably our Milky Way contains about 1 billion neutron stars of which about 200,000 pulsars. So far, 3000 pulsars have been studied and only 5 pulsar planets have been found. PSR B1257+12 is a much-studied pulsar. In 1992, the first exoplanets ever were discovered around this object. 

Article:

Neutron Star Planets: Atmospheric processes and irradiation  
By: A. Patruno & M. Kama. In Astronomy & Astrophysics (free preprint) 

Source: ASTRON-Netherlands Institute for Radio Astronomy

Astronomical airplane trails do not evade but lighten up

The image shows a galaxy in orange that moves to the left and leaves a gas trail. The trail seems to extinguish slowly, but lightens up again near the second, white-yellow galaxy. Most white dots in the image are complete galaxies. (c) Francesco de Gasperin (Leiden University).  Source

An international team of astronomers led by Francesco de Gasperin (Leiden University, the Netherlands) has witnessed an unexpected phenomenon in a merger of a two clusters of galaxies. The astronomers discovered a gas trail that slowly extinguished, but then lit up again. It is unclear where the energy for the rejuvenation of this trail comes from. The researchers publish their findings in Science Advances.

The astronomers investigated Abell 1033. This is a cluster of galaxies consisting of two smaller clusters that are in the process of merging. Abell 1033 is located in the northern constellation of Leo Minor (near Ursa Major). Clusters of galaxies are the largest structures in the universe. They can contain hundreds to thousands of galaxies similar to our Milky Way. Smaller clusters can merge together to form a larger cluster.

The astronomers observed that an individual galaxy in Abell 1033 leaves a trail of gas as it traveled through the cluster. On astronomical scale, such a trail resembles the trail of colored smoke behind a stunt plane.

The astronomers had expected that the gas trail, like the ones behind a stunt plane, would slowly fade and eventually disappear. To their astonishment they saw that the end of the gas trail was brighter than the middle.

"This was totally unexpected," says Francesco de Gasperin, the first author of the research paper that is published in Science Advances. "As these clouds of electrons radiate away their energy over time, they should become fainter and disappear. Instead, in this case, after more than a hundred million years, the trail of electrons is glowing brightly."

There is no precise explanation for this phenomenon, yet. It seems that the trail brightens near the center of the cluster of galaxies. De Gasperin: "Part of the energy released in the merger event must have been transferred to rejuvenate the cloud of electrons." The research on merging clusters of galaxies is complicated because astronomers only see a snapshot of a process that takes billions of years to complete. In addition to that, the telescopes that are needed for the investigation must receive signals at extremely low frequencies. The astronomers combined data from the Indian Giant Metrewave Radio Telescope and LOFAR, the Low Frequency Array. LOFAR was designed and built by the Dutch research institute ASTRON. The telescope consists of thousands of antennas spread across eight countries. The heart of LOFAR is in Drenthe in the north-east of the Netherlands.

"It’s like being among the last explorers. As soon as we move in uncharted territories, or in this case at unexplored frequencies, our universe is still full of surprises," says De Gasperin. "And this is just a first step. Much is still to be done to understand the complexity of galaxy clusters, and find what is lurking at low radio frequencies"

Reference

Gentle re-energisation of electrons in merging galaxy clusters. By: F. de Gasperin, H.T. Intema, T.W. Shimwell, G. Brunetti, M. Brüggen, T.A. Enßlin, R.J. van Weeren, A. Bonafede, H.J.A. Röttgering. Accepted for publication in Science Advances. (open access)

Source: ASTRON-Netherlands Institute for Radio Astronomy

Mystery solved: rare cosmic high energie particles come from outside our galaxy

Mystery solved: rare cosmic high energie particles come from outside our galaxy

The Pierre Auger Collaboration, in which ASTRON is a partner, reports observational evidence demonstrating that cosmic rays with energies a million times greater than that of the protons accelerated in the Large Hadron Collider come from much further away than from our own Galaxy. These findings

Ever since the existence of cosmic rays with individual energies of several Joules was established in the 1960s, speculation has raged as to whether such particles are created there or in distant extragalactic objects. The 50 year-old mystery has been solved using cosmic particles of mean energy of 2 Joules recorded with the largest cosmic-ray observatory ever built, the Pierre Auger Observatory in Argentina. It is found that at these energies the rate of arrival of cosmic rays is ~6% greater from one side of the sky than from the opposite direction, with the excess lying 120˚ away from the Galactic centre.

In the view of Professor Karl-Heinz Kampert (University of Wuppertal), spokesperson for the Auger Collaboration, which involves over 400 scientists from 18 countries, "We are now considerably closer to solving the mystery of where and how these extraordinary particles are created, a question of great interest to astrophysicists. Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way”. Professor Alan Watson (University of Leeds), emeritus spokesperson, considers this result to be “one of the most exciting that we have obtained and one which solves a problem targeted when the Observatory was conceived by Jim Cronin and myself over 25 years ago”.

Rare particles, gigantic detector

Cosmic rays are the nuclei of elements from hydrogen (the proton) to iron. Above 2 Joules the rate of their arrival at the top of the atmosphere is only about 1 per sq km per year, equivalent to one hitting the area of a football pitch about once per century. Such rare particles are detectable because they create showers of electrons, photons and muons through successive interactions with the nuclei in the atmosphere. These showers spread out, sweeping through the atmosphere at the speed of light in a disc-like structure, similar to a dinner-plate, several kilometres in diameter. They contain over ten billion particles and, at the Auger Observatory, are detected through the Cherenkov light they produce in a few of 1600 detectors, each containing 12 tonnes of water, spread over 3000 km2 of Western Argentina, an area comparable to that of Rhode Island. The times of arrival of the particles at the detectors, measured with GPS receivers, are used to find the arrival directions of events to within ~1˚.

An extragalactic origin

By studying the distribution of the arrival directions of more than 30000 cosmic particles the Auger Collaboration has discovered an anisotropy, significant at 5.2 standard deviations (a chance of about two in ten million), in a direction where the distribution of galaxies is relatively high. Although this discovery clearly indicates an extragalactic origin for the particles, the actual sources have yet to be pinned down. The direction of the excess points to a broad area of sky rather than to specific sources as even particles as energetic as these are deflected by a few 10s of degrees in the magnetic field of our Galaxy. The direction, however, cannot be associated with putative sources in the plane or centre of our Galaxy for any realistic configuration of the Galactic magnetic field.

Cosmic rays of even higher energy than the bulk of those used in this study exist, some even with the kinetic energy of well-struck tennis ball. As the deflections of such particles are expected to be smaller, the arrival directions should point closer to their birthplaces. These cosmic rays are even rarer and further studies are underway using them to try to pin down which extragalactic objects are the sources. Knowledge of the nature of the particles will aid this identification and work on this problem is targeted in the upgrade of the Auger Observatory to be completed in 2018. Source: Radboud University

Source:  ASTRON-Netherlands Institute for Radio Astronomy