Missing Milky Way Dark Matter

A composite image shows a dark matter disk in red. From images in the Two Micron All Sky Survey. Credit: Credit: J. Read & O. Agertz.

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Although dark matter is inherently difficult to observe, an understanding of its properties (even if not its nature) allows astronomers to predict where its effects should be felt. The current understanding is that dark matter helped form the first galaxies by providing gravitational scaffolding in the early universe. These galaxies were small and collapsed to form the larger galaxies we see today. As galaxies grew large enough to shred incoming satellites and their dark matter, much of the dark matter should have been deposited in a flat structure in spiral galaxies which would allow such galaxies to form dark components similar to the disk and halo. However, a new study aimed at detecting the Milky Way’s dark disk have come up empty.


The study concentrated on detecting the dark matter by studying the luminous matter embedded in it in much the same way dark matter was originally discovered. By studying the kinematics of the matter, it would allow astronomers to determine the overall mass present that would dictate the movement. That observed mass could then be compared to the amount of mass predicted of both baryonic matter as well as the dark matter component.

The team, led by C. Moni Bidin used ~300 red giant stars in the Milky Way’s thick disk to map the mass distribution of the region. To eliminate any contamination from the thin disc component, the team limited their selections to stars over 2 kiloparsecs from the galactic midplane and velocities characteristic of such stars to avoid contamination from halo stars. Once stars were selected, the team analyzed the overall velocity of the stars as a function of distance from the galactic center which would give an understanding of the mass interior to their orbits.

Using estimations on the mass from the visible stars and the interstellar medium, the team compared this visible mass to the solution for mass from the observations of the kinematics to search for a discrepancy indicative of dark matter. When the comparison was made, the team discovered that, “[t]he agreement between the visible mass and our dynamical solution is striking, and there is no need to invoke any dark component.”

While this finding doesn’t rule out the presence of dark matter, it does place constraints on it distribution and, if confirmed in other galaxies, may challenge the understanding of how dark matter serves to form galaxies. If dark matter is still present, this study has demonstrated that it is more diffuse than previously recognized or perhaps the disc component is flatter than previously expected and limited to the thin disc. Further observations and modeling will undoubtedly be necessary.

Yet while the research may show a lack of our understanding of dark matter, the team also notes that it is even more devastating for dark matter’s largest rival. While dark matter may yet hide within the error bars in this study, the findings directly contradict the predictions of Modified Newtonian Dynamics (MOND). This hypothesis predicts the apparent gain of mass due to a scaling effect on gravity itself and would have required that the supposed mass at the scales observed be 60% higher than indicated by this study. Continue reading “Missing Milky Way Dark Matter”

The Milky Way Might Be Square

The Pinwheel Galaxy has areas of straight arms. The Milky Way might, as well. Image Credit: NASA

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Just like being stuck inside and not being able to see what the outside of your house looks like, we’re trapped inside the Milky Way galaxy and aren’t able to see its complete structure. Most of us have this vision of a circular, spiral galaxy with gracefully curving spiral arms. Nope, says a group of astronomers from Brazil. The Milky Way might be square. Not like a box, but, in places, the spiral arms are straight rather than curved, giving the Milky Way a distinctly square look. And our solar system sits right on one the straightest parts of an outer arm.

It really IS hip to be square.

The map of the Milky Way has been redrawn several times since the first attempts in the 1950’s using radio telescopes to trace out the spiral arms of our home galaxy. However, the concept of our galaxy having square-ish arms is not so farfetched: we know of the Pinwheel Galaxy, above, that has areas of straight and squared off arms, and a 2008 study using the Very Long Baseline Array found that instead of arms neatly circling the galactic center, the stars mapped traced a more elliptical orbit. But most of the maps of the Milky Way have assumed that the material in our galaxy orbits the center in a circular fashion, so having arms stars that don’t follow this path come as somewhat of a surprise.

Jaques Lepine and his team from the University of Sao Paulo in Brazil wanted to obtain the equivalent of a ”face-on” map of the spiral arms of our Galaxy, so they studied the spectra produced by clouds of carbon monosulphide, a common gas in our galaxy, rather than the usual suspect of ionized hydrogen.

They were able to determine velocity information for 870 regions of the Milky Way which is a larger number than that of previous studies based on classical HII regions, so they’ve created a new map of the galaxy with detail never seen before. “One way to improve the description of the spiral arms is to increase the number of objects used to trace them,” the team writes in their paper.

The distribution of CS sources and maser sources in the galactic plane with the Cepheid stars in orange and the young open clusters in green. The Sun is represented by a yellow dot. Credit: Lepine, et al.

Not only did they find evidence for straight places in the arms, but they also found an additional third arm. A 2008 study by the Spitzer Space Telescope had demoted the number of arms from four to two, but other studies, including an earlier one by Levine have said three. So, yes, there is some uncertainty on the number of arms. The new arm is about 30,000 light years from the galactic core at a longitude of between 80 and 140 degrees. This one is rounded however, “with strong inward curvature.”

“Basically, our results confirm the main aspects of the spiral structure revealed by the studies of HII regions,” said Lepine and his team. “For instance if we move horizontally across the figure, to the right or to the left of the Galactic center, we find roughly 3 spiral arms on each side, like the previous works. There are departures from the pure logarithmic spirals, with segments of arms that are almost straight lines.”

Drawing a map of the Milky Way is a challenging task, since we only have an edge-on view of the galaxy in which we reside. To top it off, it’s full of dust and gas that muck up the view in the visible light spectrum. So, we have to rely on other spectra.

We may not ever know exactly what our galaxy would look like when viewed from other worlds, but we’ll keep trying.

Read the team’s paper: The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance, Lepine, et al.

Additional source: Technology Review Blog

What Galaxy Do We Live In?

Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA

If you are not an astronomy enthusiast you not have thought much about what galaxy do we live in. So depending on that the answer may surprise you. If you know anything about galaxies you know that they are groupings of stars that number in the hundreds of billions. The most famous is the Milky Way. It is from this galaxy that we even have the term. The simple point is that the Earth is part of the Milky Way even though if we see it in the sky it looks like we are observing it from the outside. Why is that? To understand you need to know exactly where we live in neighborhood of the Milky Way Galaxy.

As we are part of the solar system Earth pretty much follows the path of the sun as it goes through its own orbit around the galaxy. The Milky Way is a spiral galaxy type so it has arms sort of like an octopus. The Sun is located near the outward tip of the Sagittarius arm of the Milky Way. This makes Earth about 28,000 light years from the galactic core of our home galaxy.

The Solar System also has a galactic year that it follows. It takes around 200 million to 250 million years for the solar system to orbit the Sun. Another indicator of our position is where the galactic equator. While our star system is considered to be on the outskirts of the Milky Way this is only an estimate. It is believed that the Milky Way is larger than first estimated. There is also suspicion that our galaxy is in the process of absorbing other smaller galaxies. However, there is not enough empirical evidence available to support the claim.

So what would be so important about knowing what part of the galaxy we live in? One reason is space exploration. Some time in the future mankind may find a way to achieve faster than light space travel. This can provide a new set of challenges for engineers and astronomers to tackle. For example how would an astronaut keep from getting lost in space? Detailed mapping and computer programming in the future could help galactic wayfarers know where they are going and more importantly how to get home.

The other reason is that it never hurts to know our place in the scheme of things. Just thinking of the challenge of finding earth if we were so far way helps us to understand how truly vast the universe is.

We have written many articles about the Milky Way galaxy for Universe Today. Here are some facts about the Milky Way, and here’s an article about the closest galaxy to the Milky Way.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.

Sources: SEDS, Daily Galaxy

The Thick Disk: Galactic Construction Project or Galactic Rejects?

Our Milky Way Gets a Makeover
Our Milky Way Gets a Makeover

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The disk of spiral galaxies is comprised of two main components: The thin disk holds the majority of stars and gas and is the majority of what we see and picture when we think of spiral galaxies. However, hovering around that, is a thicker disk of stars that is much less populated. This thick disk is distinct from the thin disk in several regards: The stars there tend to be older, metal deficient, and orbit the center of the galaxy more slowly.

But where this population of the stars came from has been a long standing mystery since its identification in the mid 1970’s. One hypothesis is that it is the remainder of cannibalized dwarf galaxies that have never settled into a more standard orbit. Others suggest that these stars have been flung from the thin disk through gravitational slingshots or supernovae. A recent paper puts these hypothesis to the observational test.

At a first glance, both propositions seem to have a firm observational footing. The Milky Way galaxy is known to be in the process of merging with several smaller galaxies. As our galaxy pulls them in, the tidal effects shred these minor galaxies, scattering the stars. Numerous tidal streams of this sort have been discovered already. The ejection from the thin disk gains support from the many known “runaway” and “hypervelocity” stars which have sufficient velocity to escape the thin disk, and in some cases, the galaxy itself.

The new study, led by Marion Dierickx of Harvard, follows up on a 2009 study by Sales et al., which used simulations to examine the features stars would take in the thick disk should they be created via these methods. Through these simulations, Sales showed that the distribution of eccentricities of the orbits should be different and allow a method by which to discriminate between formation scenarios.

By using data from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), Dierickx’s team compared the distribution of the stars in our own galaxy to the predictions made by the various models. Ultimately, their survey included some 34,000 stars. By comparing the histogram of eccentricities to that of Sales’ predictions, the team hoped to find a suitable match that would reveal the primary mode of creation.

The comparison revealed that, should ejection from the thin disk be the norm there were too many stars in nearly circular orbits as well as highly eccentric ones. In general, the distribution was too wide. However, the match for the scenario of mergers fit well lending strong credence to this hypothesis.

While the ejection hypothesis or others can’t be ruled out completely, it suggests that, at least in our own galaxy, they play a rather minor role. In the future, additional tests will likely be employed, analyzing other aspects of this population.

Finding the Origin of Milky Way’s Ancient Stars

Simulation showing a Milky Way-like galaxy around five billion years ago, when most satellite galaxy collisions were happening. Credit: Andrew Cooper, John Helly (Durham University)

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From the Royal Astronomical Society

Many of the Milky Way’s ancient stars are remnants of other smaller galaxies torn apart by violent galactic collisions around five billion years ago, according to researchers at Durham University, who publish their results in a new paper in the journal Monthly Notices of the Royal Astronomical Society.

Scientists at Durham’s Institute for Computational Cosmology and their collaborators at the Max Planck Institute for Astrophysics, in Germany, and Groningen University, in Holland, ran huge computer simulations to recreate the beginnings of our Galaxy.

The simulations revealed that the ancient stars, found in a stellar halo of debris surrounding the Milky Way, had been ripped from smaller galaxies by the gravitational forces generated by colliding galaxies.

Cosmologists predict that the early Universe was full of small galaxies which led short and violent lives. These galaxies collided with each other leaving behind debris which eventually settled into more familiar looking galaxies like the Milky Way.

The researchers say their finding supports the theory that many of the Milky Way’s ancient stars had once belonged to other galaxies instead of being the earliest stars born inside the Galaxy when it began to form about 10 billion years ago.

Simulation showing the stellar halo of a Milky Way-like galaxy in the present day. Credit: Andrew Cooper (Durham University)

Lead author Andrew Cooper, from Durham University’s Institute for Computational Cosmology, said: “Effectively we became galactic archaeologists, hunting out the likely sites where ancient stars could be scattered around the galaxy.

“Our simulations show how different relics in the Galaxy today, like these ancient stars, are related to events in the distant past.

“Like ancient rock strata that reveal the history of Earth, the stellar halo preserves a record of a dramatic primeval period in the life of the Milky Way which ended long before the Sun was born.”

The computer simulations started from shortly after the Big Bang, around 13 billion years ago, and used the universal laws of physics to simulate the evolution of dark matter and the stars.

These simulations are the most realistic to date, capable of zooming into the very fine detail of the stellar halo structure, including star “streams” – which are stars being pulled from the smaller galaxies by the gravity of the dark matter.

One in one hundred stars in the Milky Way belong to the stellar halo, which is much larger than the Galaxy’s familiar spiral disk. These stars are almost as old as the Universe.

Professor Carlos Frenk, Director of Durham University’s Institute for Computational Cosmology, said: “The simulations are a blueprint for galaxy formation.

“They show that vital clues to the early, violent history of the Milky Way lie on our galactic doorstep.

“Our data will help observers decode the trials and tribulations of our Galaxy in a similar way to how archaeologists work out how ancient Romans lived from the artefacts they left behind.”

The research is part of the Aquarius Project, which uses the largest supercomputer simulations to study the formation of galaxies like the Milky Way and was partly funded by the UK’s Science and Technology Facilities Council (STFC).

Aquarius was carried out by the Virgo Consortium, involving scientists from the Max Planck Institute for Astrophysics in Germany, the Institute for Computational Cosmology at Durham University, UK, the University of Victoria in Canada, the University of Groningen in the Netherlands, Caltech in the USA and Trieste in Italy.

Durham’s cosmologists will present their work to the public as part of the Royal Society’s 350th anniversary ‘See Further’ exhibition, held at London’s Southbank Centre until July 4th.

What Galaxy is the Earth In?

What galaxy is Earth in? We're in the Milky Way

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Were you wondering what galaxy is the Earth in? You’ll probably recognize the answer: it’s the Milky Way Galaxy.

If you go to a dark spot, away from the bright city lights, and look up, you should be able to see the Milky Way as a cloudy band stretching across the sky. It really does look like spilt milk spread across the sky. But if you take a telescope and examine it more closely, you’ll see that the clouds are actually the collective light from thousands of stars.

Since we’re embedded inside the Milky Way, we’re seeing our home galaxy edge-on, from the inside. To get a better idea, grab a dinner plate and take a look at it edge on, so you can’t see the circular shape of the galaxy. You can only see the edge of the plate.

The Milky Way is an example of a barred spiral galaxy. It measures approximately 100,000 light years across and it’s only 1,000 light years thick; although, it’s more thick at the core where the galaxy bulges out. If you could fly out of the Milky Way in a rocket and then look back, you’d see a huge spiral shaped galaxy with a bar at the center. At the ends of this bar, there are two spiral arms which twist out forming the structure of the Milky Way.

The Earth is located in the Solar System, and the Solar System is located about 25,000 light-years away from the core of the galaxy. This also means that we’re about 25,000 light-years away from the outer edge of the Milky Way. We’re located in the Orion Spur, which is a minor arm located in between the two major galactic arms.

If you’d like more information on the Milky Way, check out NASA’s Starchild info on the Milky Way, and here’s more info from the WMAP mission.

We’ve written many articles about the Milky Way for Universe Today. Here’s an article with facts about the Milky Way, and here is a map of the Milky Way.

We’ve also recorded several episodes of Astronomy Cast about the Milky Way. Listen here, Episode 99: The Milky Way.

Newly-Discovered Stellar Nurseries in the Milky Way

The Orion Nebula, one of the most brilliant star-forming regions in our galaxy. Other, newly-discovered regions like the Orion Nebula could help astronomers determing the chemical composition of our galaxy. Image Credit: APOD/Hubble Space Telescope

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Our Milky Way churns out about seven new stars per year on average. More massive stars are formed in what’s called H II regions, so-named because the gas present in these stellar nurseries is ionized by the radiation of the young, massive stars forming there. Recently-discovered regions in the Milky Way that are nurseries for massive stars may hold important clues as to the chemical composition and structural makeup of our galaxy.

Thomas Bania, of Boston University, said in an NRAO press release, “We can clearly relate the locations of these star-forming sites to the overall structure of the Galaxy. Further studies will allow us to better understand the process of star formation and to compare the chemical composition of such sites at widely different distances from the Galaxy’s center.”

The announcement of these newly discovered regions was made in a presentation today at the American Astronomical Society meeting in Miami, Florida. The team of astronomers that collaborated on the search includes Thomas Bania of Boston University, Loren Anderson of the Astrophysical Laboratory of Marseille in France, Dana Balser of the National Radio Astronomy Observatory (NRAO), and Robert Rood of the University of Virginia.

H II regions that you may be familiar with include the Orion Nebula (M42), visible just South of Orion’s Belt with the naked eye, and the Horsehead Nebula, so famously imaged by the Hubble Space Telescope. For more information on other known regions (and lots of pictures), visit the 2Micron All-Sky Survey at IPAC.

By studying such regions in other galaxies, and our own, the chemical composition and distribution of a galaxy can be determined. H II regions form out of giant molecular clouds of hydrogen, and remain stable until a collision happens between two clouds, creating a shockwave, or the resulting shockwave from a nearby supernova collapses some of the gas to form stars. As these stars form and start to shine, their radiation strips the molecular hydrogen of its electrons.

The astronomers used both infrared and radio telescopes to see through the thick dust and gas that pervades the Milky Way. By combing surveys taken by the Spitzer Space Telescope’s infrared camera, and the Very Large Array (VLA) radio telescope, they identified “hot spots” that would be good candidates for H II regions. To further verify their findings, they used the Robert C. Byrd Green Bank Telescope (GBT), a sensitive radio telescope that allowed them to detect radio frequencies emitted by electrons as they rejoined protons to form hydrogen. This process of recombination to form hydrogen is a telltale sign of regions that contain ionized hydrogen, or H II.

The location of the regions is concentrated near the ends of the central bar of the Milky Way, and in its spiral arms. Over 25 of the regions discovered were further from the center of the galaxy than our own Sun – a more detailed study of these outlying regions could give astronomers a better understanding of the evolution and composition of our Milky Way.

“There is evidence that the abundance of heavy elements changes with increasing distance from the Galactic center,” Bania said. “We now have many more objects to study and improve our understanding of this effect.”

Source: NRAO Press Release

Second-Generation Star Supports Cannibal Theory of Milky Way

The newly discovered red giant star S1020549 supports the theory that our galaxy underwent a "cannibal" phase, growing to its current size by swallowing dwarf galaxies and other galactic building blocks. Credit: David A. Aguilar (CfA)

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A newly discovered red giant star is a relic from the early universe — a star that may have been among the second generation of stars to form after the Big Bang. Located in the dwarf galaxy Sculptor some 290,000 light-years away, the star has a remarkably similar chemical make-up to the Milky Way’s oldest stars. Its presence supports the theory that our galaxy underwent a “cannibal” phase, growing to its current size by swallowing dwarf galaxies and other galactic building blocks.

“This star likely is almost as old as the universe itself,” said astronomer Anna Frebel of the Harvard-Smithsonian Center for Astrophysics, lead author of the Nature paper reporting the finding.

Dwarf galaxies are small galaxies with just a few billion stars, compared to hundreds of billions in the Milky Way. In the “bottom-up model” of galaxy formation, large galaxies attained their size over
billions of years by absorbing their smaller neighbors.

“If you watched a time-lapse movie of our galaxy, you would see a swarm of dwarf galaxies buzzing around it like bees around a beehive,” explained Frebel. “Over time, those galaxies smashed together and mingled their stars to make one large galaxy — the Milky Way.”

If dwarf galaxies are indeed the building blocks of larger galaxies, then the same kinds of stars should be found in both kinds of galaxies, especially in the case of old, “metal-poor” stars. To astronomers, “metals” are chemical elements heavier than hydrogen or helium. Because they are products of stellar evolution, metals were rare in the early Universe, and so old stars tend to be metal-poor.

Old stars in the Milky Way’s halo can be extremely metal-poor, with metal abundances 100,000 times poorer than in the Sun, which is a typical younger, metal-rich star. Surveys over the past decade have
failed to turn up any such extremely metal-poor stars in dwarf galaxies, however.

“The Milky Way seemed to have stars that were much more primitive than any of the stars in any of the dwarf galaxies,” says co-author Josh Simon of the Observatories of the Carnegie Institution. “If dwarf
galaxies were the original components of the Milky Way, then it’s hard to understand why they wouldn’t have similar stars.”

The team suspected that the methods used to find metal-poor stars in dwarf galaxies were biased in a way that caused the surveys to miss the most metal-poor stars. Team member Evan Kirby, a Caltech
astronomer, developed a method to estimate the metal abundances of large numbers of stars at a time, making it possible to efficiently search for the most metal-poor stars in dwarf galaxies.

“This was harder than finding a needle in a haystack. We needed to find a needle in a stack of needles,” said Kirby. “We sorted through hundreds of candidates to find our target.”

Among stars he found in the Sculptor dwarf galaxy was one faint, 18th-magnitude speck designated S1020549. Spectroscopic measurements of the star’s light with Carnegie’s Magellan-Clay telescope in Las Campanas, Chile, determined it to have a metal abundance 6,000 times lower than that of the Sun; this is five times lower than any other star found so far in a dwarf galaxy.

The researchers measured S1020549’s total metal abundance from elements such as magnesium, calcium, titanium, and iron. The overall abundance pattern resembles those of old Milky Way stars, lending the first observational support to the idea that these galactic stars originally formed in dwarf galaxies.

The researchers expect that further searches will discover additional metal-poor stars in dwarf galaxies, although the distance and faintness of the stars pose a challenge for current optical telescopes. The next generation of extremely large optical telescopes, such as the proposed 24.5-meter Giant Magellan Telescope, equipped with high-resolution spectrographs, will open up a new window for studying the growth of galaxies through the chemistries of their stars.

In the meantime, says Simon, the extremely low metal abundance in S1020549 study marks a significant step towards understanding how our galaxy was assembled. “The original idea that the halo of the Milky
Way was formed by destroying a lot of dwarf galaxies does indeed appear to be correct.”

Source: Harvard-Smithsonian Center for Astrophysics

All-Sky Radio Image in 60 Seconds, No Moving Parts

First LOFAR high-band image (MPIfR)

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This image is a software-calibrated image with high signal-to-noise ratio at a frequency of 120 MHz, of the radio sky above Effelsberg, Germany, on November10, 2009. It has North at the top and East at the left, just as a person would have seen the entire sky when lying on their back on a flat field near Effelsberg late in the afternoon on November 10, if their eyes were sensitive to radio waves.

The two bright (yellow) spots are Cygnus A – a giant radio galaxy powered by a supermassive black hole – near the center of the image, and Cassiopeia A – a bright radio source created by a supernova explosion about 300 years ago – at the upper-left in the image. The plane of our Milky Way galaxy can also be seen passing by both Cassiopeia A and Cygnus A, and extending down to the bottom of the image. The North Polar Spur, a large cloud of radio emission within our own galaxy, can also be seen extending from the direction of the Galactic center in the South, toward the western horizon in this image. “We made this image with a single 60 second “exposure” at 120 MHz using our high-band LOFAR field in Effelsberg”, says James Anderson, project manager of the Effelsberg LOFAR station.

“The ability to make all-sky images in just seconds is a tremendous advancement compared to existing radio telescopes which often require weeks or months to scan the entire sky,” Anderson went on. This opens up exciting possibilities to detect and study rapid transient phenomena in the universe.

LOFAR, the LOw Frequency ARray, was designed and developed by ASTRON (Netherlands Institute for Radio Astronomy) with 36 stations centered on Exloo in the northeast of The Netherlands. It is now an international project with stations being built in Germany, France, the UK and Sweden connected to the central data processing facilities in Groningen (NL) and the ASTRON operations center in Dwingeloo (NL). The first international LOFAR station (IS-DE1) was completed on the area of the Effelsberg radio observatory next to the 100-m radio telescope of the Max-Planck-Institut für Radioastronomie (MPIfR).

Operating at relatively low radio frequencies from 10 to 240 MHz, LOFAR has essentially no moving parts to track objects in the sky; instead digital electronics are used to combine signals from many small antennas to electronically steer observations on the sky. In certain electronic modes, the signals from all of the individual antennas can be combined to make images of the entire radio sky visible above the horizon.

IS-DE1: Some of the 96 low-band dipole antennas, Effelsberg LOFAR station (foreground); high-band array (background) (Credit: James Anderson, MPIfR)

LOFAR uses two different antenna designs, to observe in two different radio bands, the so-called low-band from 10 to 80 MHz, and the high-band from 110 to 240 MHz. All-sky images using the low-band antennas at Effelsberg were made in 2007.

Following the observation for the first high-band, all-sky image, scientists at MPIfR made a series of all-sky images covering a wide frequency range using both the low-band and high-band antennas at Effelsberg.

Effelsberg sky through LOFAR eyes (Credit: James Anderson, MPIfR)

The movie of these all-sky images has been compiled and is shown above. The movie starts at a frequency of 35 MHz, and each subsequent frame is about 4 MHz higher in frequency, through 190 MHz. The resolution of the Effelsberg LOFAR telescope changes with frequency. At 35 MHz the resolution is about 10 degrees, at 110 MHz it is about 3.4 degrees, and at 190 MHz it is about 1.9 degrees. This change in resolution can be seen by the apparent size of the two bright sources Cygnus A and Cassiopeia A as the frequency changes.

Scientists at MPIfR and other institutions around Europe will use measurements such as these to study the large-sky structure of the interstellar matter of our Milky Way galaxy. The low frequencies observed by LOFAR are ideal for studying the low energy cosmic ray electrons in the Milky Way, which trace out magnetic field structures through synchrotron emission. Other large-scale features such as supernova remnants, star-formation regions, and even some other nearby galaxies will need similar measurements from individual LOFAR telescopes to provide accurate information on the large-scale emission in these objects. “We plan to search for radio transients using the all-sky imaging capabilities of the LOFAR telescopes”, says Michael Kramer, director at MPIfR, in Bonn. “The detection of rapidly variable sources using LOFAR could lead to exciting discoveries of new types of astronomical objects, similar to the discoveries of pulsars and gamma-ray bursts in the past decades.”

“The low-frequency sky is now truly open in Effelsberg and we have the capability at the observatory to observe in a wide frequency range from 10 MHz to 100 GHz”, says Anton Zensus, also director at MPIfR. “Thus we can cover four orders of magnitude in the electromagnetic spectrum.”

Source: Max-Planck-Institut für Radioastronomie

Milky Way Has a “Squashed Beachball”-Shaped Dark Matter Halo

This illustration shows the visible Milky Way galaxy surrounded by a "squashed beachball"-shaped dark matter halo. Source: UCLA

This illustration shows the visible Milky Way galaxy surrounded by a “squashed beachball”-shaped dark matter halo. Source: UCLA

Our galaxy is shaped like a flat spiral right? Not if you’re talking about dark matter. Astronomers announced today that the Milky Way’s dark matter halo, which represents about 70% of the galaxy’s mass, is actually shaped like a squashed beachball.

Dark matter is completely invisible, but it still obeys the law of gravity, so the existence of dark matter haloes, and their shape, can be inferred by monitoring the orbits of dwarf galaxies orbiting the much larger Milky Way.

Unfortunately, to determine the orbit of an object, you have to measure its position at several points in that orbit, and dwarf galaxies take about a billion years to go around the Milky Way. Astronomers just haven’t been around long enough to watch even a fraction of a complete orbit. Luckily, they don’t have to.

Dwarf galaxies, just like their full-sized counterparts, and made of billions of stars. When the tidal forces from a big galaxy like the Milky Way act on a dwarf galaxy, the result is a streamer of stars that trace out the dwarf galaxy’s orbit. By using data from huge all-sky surveys, a group of astronomers led by David Law at UCLA were able to reconstruct the orbit of the Sagittarius Dwarf Galaxy. There was just one problem: different parts of the dwarf galaxy had different orbits, which led to wildly different dark matter halo shapes.

Law and his colleagues Steven Majewski (University of Virginia) and Kathryn Johnston (Columbia University) solved this problem by allowing models of the dark matter halo to be “triaxial” – in other words, have different lengths in all three dimensions. The best model solution results in a halo shaped like a beach ball that has been squashed sideways.

“We expected some amount of flattening based on the predictions of the best dark-matter theories,” said Law, “but the extent, and particularly the orientation, of the flattening was quite unexpected. We’re pretty excited about this, because it begs the question of how our galaxy formed in its present orientation.”

Sagittarius is not the only dwarf galaxy orbiting the Milky Way, and Law and his colleagues plan to study the orbits of other dwarf galaxies to refine their model. “It will be important to see if these results hold up as precise orbits are measured for more of these galaxies. In the meantime, such a squashed dark-matter halo is one of the best explanations for the observed data.”

This illustration shows the visible Milky Way galaxy (blue spiral) and the streams of stars represent the tidally shredded Sagittarius dwarf galaxy. Click the image for a flyaround view. Source: UCLA

This illustration shows the visible Milky Way galaxy (blue spiral) and the streams of stars represent the tidally shredded Sagittarius dwarf galaxy. Source: UCLA