Researchers provide new insight into dark matter halos

Research from the University of Pennsylvania could shed light on the distribution of one of the most mysterious substances in the universe.

An image of a simulated galaxy cluster showing evidence for a boundary, or "edge" 
[Credit: Surhud More, Benedikt Diemer and Andre Kravtsov, 
Astrophysical Journal 2015]

In the 1970s, scientists noticed something strange about the motion of galaxies. All the matter at the edge of spiral galaxies was rotating just as fast as material in the inner part of the galaxy. But according to the laws of gravity, objects on the outskirts should be moving slower.

The explanation: A form of matter called dark matter that does not directly interact with light.

Many scientists now believe that more than 80 percent of the matter of the universe is locked away in mysterious, as yet undetected, particles of dark matter, which affect everything from how objects move within a galaxy to how galaxies and galaxy clusters clump together in the first place.

This dark matter extends far beyond the reach of the furthest stars in the galaxy, forming what scientists call a dark matter halo. While stars within the galaxy all rotate in a neat, organized disk, these dark matter particles are like a swarm of bees, moving chaotically in random directions, which keeps them puffed up to balance the inward pull of gravity.

Bhuvnesh Jain, a physics professor in Penn's School of Arts & Sciences, and postdoc Eric Baxter are conducting research that could give new insights into the structure of these halos.

The researchers wanted to investigate whether these dark matter halos have an edge or boundary.

"People have generally imagined a pretty smooth transition from the matter bound to the galaxy to the matter between galaxies, which is also gravitationally attracted to the galaxies and clusters," Jain said. "But theoretically, using computer simulations a few years ago, researchers at the University of Chicago showed that for galaxy clusters a sharp boundary is expected, providing a distinct transition that we should be able to see through a careful analysis of the data."

Scientists believe that this region, or "edge" is due to the "splashback effect."

"You have this big dark matter halo sitting there," Baxter said, "and it's been accreting matter gravitationally over its entire history. As that matter gets pulled in, it gets faster and faster. When it finally falls into the halo, it turns around and starts to orbit. That turnaround is what people have started calling splashback, because stuff is splashing back in some sense."

As the matter "splashes back," it slows down. Because this effect is happening in many different directions, it leads to a buildup of matter right at the edge of the halo and a steep fall-off in the amount of matter right outside of that position. This is what the Penn researchers explored in the data.

A two-dimensional comparison of two models for the density profile of a halo. 
Both of these models come from fitting to data in SDSS. Models with a splashback 
feature (an "edge") fit the data better than models that don't have an edge.  
New measurements provide evidence that this "edge" exists
[Credit: University of Pennsylvania]

Using a galaxy survey called the Sloan Digital Sky Survey, or SDSS, Baxter and Jain looked at the distribution of galaxies around clusters. They formed a team of experts at the University of Chicago and other institutions around the world to examine thousands of galaxy clusters. Using statistical tools to do a joint analysis of several million galaxies around them, they found a drop at the edge of the cluster. Baxter and collaborator Chihway Chang at the University of Chicago led a paper reporting the findings, accepted for publication in the Astrophysical Journal.

In addition to seeing this edge when they looked at galaxy distribution, the researchers also saw evidence of it in the form of galaxy colors.

When a galaxy is full of gas and forming many big, hot stars, the heat causes it to appear blue when scientists takes images of it.

"But those big stars live very short lives," Baxter said. "They blow up. What you're left with are these smaller, older stars that live for long periods of time, and those are red."

When scientists look at galaxies within clusters, they appear red because they aren't forming stars.

"Previous studies have shown that there are interactions inside of the cluster that can cause galaxies to stop forming stars," Baxter said. "You could imagine for instance that a galaxy falls into a cluster, and the gas from the galaxy gets stripped off by gas within the cluster. After losing its gas, the galaxy will be unable to form many stars."

Because of this, scientists expect that galaxies that have spent more time orbiting through a cluster will appear red, while galaxies that are just starting to fall in will appear blue.

The researchers noticed a sudden shift in the colors of galaxies right at the boundary, providing them with more evidence that dark matter halos have an edge.

"It was really interesting and surprising to see this sharp change in colors," Jain said, "because the change of galaxy colors is a very slow and complex process."

The researchers are working on another paper using a deeper survey of over a hundred million galaxies called the Dark Energy Survey, or DES.

Both the SDSS and the DES make massive maps of the sky using a huge camera that Jain said isn't very fundamentally different from the cameras in smartphones but bigger and more precise and costing millions of dollars to build.

In the DES, when the camera opens, it takes an exposure of a couple minutes, and then moves to a different part of the sky. This process is repeated during the course of several years using different filters to allow scientists to get a survey in multiple colors.

The DES allows the researchers to do expanded measurements, pushing to higher distances.

Instead of measuring the distribution of galaxies, the researchers are using an astrophysical phenomenon called gravitational lensing to probe the dark matter halos. In gravitational lensing, light coming to an observer bends as matter exerts gravitational force on it.

The researchers can analyze images of the sky to see how clusters stretch images of the galaxies behind them.

"Light is going to bend if there's mass," Baxter said. "By measuring these deflections we can measure the mass directly which is cool because most of the mass is dark matter which we can't see so it's kind of a unique way to probe the dark matter."

In terms of fundamental understanding of the universe, Baxter said, dark matter is one of the biggest mysteries there is right now.

"You look in the sky, even with the biggest optical telescopes, and you see nothing beyond the light of the galaxies," Jain said. "There's just this dark matter."

The researchers hope that their research will contribute to a better understanding of the mysterious substance that makes up about 80 percent of matter in the universe. If they can mark the edge of a dark matter halo, it would allow them to test things like Einstein's theory of gravity and the nature of dark matter.

"It's just a new way of looking at clusters," Jain said. "Once you find the boundary you can study both the standard physics of how galaxies interact with the cluster and the possible unknown physics of what the nature of dark matter and gravity is."

Author: Ali Sundermier | Source: University of Pennsylvania [April 17, 2017]

byUnknown

in Astronomy, Astrophysics, Universe

April 17, 2017

Researchers provide new insight into dark matter halos

Research from the University of Pennsylvania could shed light on the distribution of one of the most mysterious substances in the universe. ...

Read More

The lifetimes of massive star-forming regions

Astronomers can roughly estimate how long it takes for a new star to form: it is the time it takes for material in a gas cloud to collapse in free-fall, and is set by the mass, the size of the cloud, and gravity. Although an approximation, this scenario of quick, dynamic star formation is consistent with many observations, especially of sources where new material can flow into the cloud, perhaps along filaments, to sustain steady activity. But this simple picture might not apply in the largest systems with star clusters and high-mass stars. Rather than a quick collapse, the process there might be inhibited by pressure, turbulence, or other activities that slow it down.

An image of a region with both star-forming cores (seen in the red) and starless clumps (the dark regions). Astronomers 
have combined statistical studies of these infrared data with submillimeter images to estimate the typical age of a massive 
star forming clump as about one million years. The red data are from Herschel 70 micron images, the green and blue
 are from Spitzer IRAC images at 8 and 4.5 microns [Credit: Battersby et al.]

CfA astronomer Cara Battersby and two colleagues studied the formation, early evolution, and lifetimes of high-mass star-forming regions and their earliest evolutionary phases in dense, molecular regions. These clumps have densities of gas as high as ten million molecules per cubic centimeter (tens of thousands of times higher than typical in gas clouds); the dust associated with this gas blocks the external starlight, leaving the material very cold, only a few tens of degrees above absolute zero. The usual method for identifying these clumps is with submillimeter telescopes, which take images of the sky; automated algorithms can then process the images to identify and characterize cold clumps. The problem is that even a quiescent clump can contain subregions of activity that are not spotted with the relatively poor spatial resolutions of the submillimeter telescopes used to assemble catalogs of these regions.

Rather than rely on the submillimeter images of the entire clumps, the astronomers examined each of the multiple, individual pixels in each clump image and compared the results with data from infrared and far infrared. These infrared images sample hotter material, including that from small embedded sources that may have been overpowered in the larger image. The infrared signals the presence of star formation activity in the clump, and also characterizes the dust temperatures (which are slightly higher when such activity is present). The authors anchor their timeframe to sources called methanol masers, found in star forming regions, which last for about 35,000 years. These masers are seen in many of the dense clumps, and reasonable estimates of their properties constrain the ages of the clumps in which they are located. The statistics from all the submillimeter and infrared clumps then provides an estimate of the typical values of a clump lifetime. The astronomers find that clumps without any embedded stars last between about 0.2 and 1.7 million years, while those with stars last only about half that time. The times, in the star formation case, span a range from about 0.4 - 2.4 free-fall times, in good agreement with the models. The results also demonstrate that most high density gas is found in clumps lacking a high-mass star (however, there could be small, low-mass stars present).

The study is published in The Astrophysical Journal.

Source: Harvard-Smithsonian Center for Astrophysics [April 14, 2017]

byUnknown

in Astronomy, Astrophysics, Universe

April 14, 2017

The lifetimes of massive star-forming regions

Astronomers can roughly estimate how long it takes for a new star to form: it is the time it takes for material in a gas cloud to collapse i...

Read More

First 'image' of a dark matter web that connects galaxies

Researchers at the University of Waterloo have been able to capture the first composite image of a dark matter bridge that connects galaxies together.

Dark matter filaments bridge the space between galaxies in this false colour map. The locations of bright galaxies 
are shown by the white regions and the presence of a dark matter filament bridging the galaxies is shown in red 
[Credit: S. Epps & M. Hudson/University of Waterloo]

The composite image, which combines a number of individual images, confirms predictions that galaxies across the universe are tied together through a cosmic web connected by dark matter that has until now remained unobservable.

Dark matter, a mysterious substance that comprises around 25 per cent of the universe, doesn't shine, absorb or reflect light, which has traditionally made it largely undetectable, except through gravity.

"For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together," said Mike Hudson, a professor of astronomy at the University of Waterloo. "This image moves us beyond predictions to something we can see and measure."

As part of their research, Hudson and co-author Seth Epps, a master's student at the University of Waterloo at the time, used a technique called weak gravitational lensing, an effect that causes the images of distant galaxies to warp slightly under the influence of an unseen mass such as a planet, a black hole, or in this case, dark matter. The effect was measured in images from a multi-year sky survey at the Canada-France-Hawaii Telescope.

They combined lensing images from more than 23,000 galaxy pairs located 4.5 billion light-years away to create a composite image or map that shows the presence of dark matter between the two galaxies. Results show the dark matter filament bridge is strongest between systems less than 40 million light years apart.

"By using this technique, we're not only able to see that these dark matter filaments in the universe exist, we're able to see the extent to which these filaments connect galaxies together," said Epps.

The scientists publish their work in a new paper in Monthly Notices of the Royal Astronomical Society.

Source: Royal Astronomical Society [April 12, 2017]

byUnknown

in Astronomy, Astrophysics, Universe

April 12, 2017

First 'image' of a dark matter web that connects galaxies

Researchers at the University of Waterloo have been able to capture the first composite image of a dark matter bridge that connects galaxies...

Read More

Scientists make progress on unravelling the puzzle of merging black holes

Astrophysicists at the University of Birmingham have made progress in understanding a key mystery of gravitational-wave astrophysics: how two black holes can come together and merge.

This artist's concept depicts a supermassive black hole at the center of a galaxy 
[Credit: Ute Kraus]

During its first four months of taking data, Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) detected gravitational waves from two mergers of pairs of black holes, GW150914 and GW151226, along with the statistically less significant black hole merger candidate LVT151012.

The first confirmed detection of gravitational waves occurred on September 14 2015 at 5.51am Eastern Daylight Time by both of the twin LIGO detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. It confirmed a major prediction of Albert Einstein's 1915 general theory of relativity and opened an unprecedented new window onto the cosmos. However, we still do not know how such pairs of merging black holes form.

A new paper, published in Nature Communications, describes the results of an investigation into the formation of gravitational-wave sources with a newly developed toolkit named COMPAS (Compact Object Mergers: Population Astrophysics and Statistics).

In order for the black holes to merge within the age of the Universe by emitting gravitational waves, they must start out very close together by astronomical standards, no more than about a fifth of the distance between the Earth and the Sun. However, massive stars, which are the progenitors of the black holes that LIGO has observed, expand to be much larger than this in the course of their evolution. The key challenge, then, is how to fit such large stars within a very small orbit. Several possible scenarios have been proposed to address this.

The Birmingham astrophysicists, joined by collaborator Professor Selma de Mink from the University of Amsterdam, have shown that all three observed events can be formed via the same formation channel: isolated binary evolution via a common-envelope phase. In this channel, two massive progenitor stars start out at quite wide separations. The stars interact as they expand, engaging in several episodes of mass transfer. The latest of these is typically a common envelope - a very rapid, dynamically unstable mass transfer that envelops both stellar cores in a dense cloud of hydrogen gas. Ejecting this gas from the system takes energy away from the orbit. This brings the two stars sufficiently close together for gravitational-wave emission to be efficient, right at the time when they are small enough that such closeness will no longer put them into contact. The whole process takes a few million years to form two black holes, with a possible subsequent delay of billions of years before the black holes merge and form a single black hole.

The simulations have also helped the team to understand the typical properties of the stars that can go on to form such pairs of merging black holes and the environments where this can happen. For example, the team concluded that a merger of two black holes with significantly unequal masses would be a strong indication that the stars formed almost entirely from hydrogen and helium, with other elements contributing fewer than 0.1% of stellar matter (for comparison, this fraction is about 2% in the Sun).

First author Simon Stevenson, a PhD student at the University of Birmingham, explained: "The beauty of COMPAS is that it allows us to combine all of our observations and start piecing together the puzzle of how these black holes merge, sending these ripples in spacetime that we were able to observe at LIGO."

Senior author Professor Ilya Mandel added: "This work makes it possible to pursue a kind of 'palaeontology' for gravitational waves. A palaeontologist, who has never seen a living dinosaur, can figure out how the dinosaur looked and lived from its skeletal remains. In a similar way, we can analyse the mergers of black holes, and use these observations to figure out how those stars interacted during their brief but intense lives."

Source: University of Birmingham [April 05, 2017]

byUnknown

in Astronomy, Astrophysics, Universe

April 05, 2017

Scientists make progress on unravelling the puzzle of merging black holes

Astrophysicists at the University of Birmingham have made progress in understanding a key mystery of gravitational-wave astrophysics: how tw...

Read More

Portrait of a black hole

Astronomers want to record an image of the heart of our galaxy for the first time: a global collaboration of radio dishes is to take a detailed look at the black hole which is assumed to be located there. This Event Horizon Telescope links observatories all over the world to form a huge telescope, from Europe via Chile and Hawaii right down to the South Pole. IRAM's 30-metre telescope, an installation co-financed by the Max Planck Society, is the only station in Europe to be participating in the observation campaign. The Max Planck Institute for Radio Astronomy is also involved with the measurements, which are to run from 4 to 14 April initially.

Gravitational monster: This artistic impression shows the event horizon around the black hole at the centre 
of our galaxy [Credit: M. Moscibrodzka, T. Bronzwaar and H. Falcke, Radboud University]

At the end of the 18th century, the naturalists John Mitchell and Pierre Simon de Laplace were already speculating about "dark stars" whose gravity is so strong that light cannot escape from them. The ideas of the two researchers still lay within the bounds of Newtonian gravitational theory and the corpuscular theory of light. At the beginning of the 20th century, Albert Einstein revolutionized our understanding of gravitation - and thus of matter, space and time - with his General Theory of Relativity. And Einstein also described the concept of black holes.

These objects have such a large, extremely compacted mass that even light cannot escape from them. They therefore remain black – and it is impossible to observe them directly. Researchers have nevertheless proven the existence of these gravitational traps indirectly: by measuring gravitational waves from colliding black holes or by detecting the strong gravitational force they exert on their cosmic neighbourhood, for example. This force is the reason why stars moving at great speed orbit an invisible gravitational centre, as happens at the heart of our galaxy, for example.

It is also possible to observe a black hole directly, however. Scientists call the boundary around this exotic object, beyond which light and matter are inescapably sucked in, the event horizon. At the very moment when the matter passes this boundary, the theory states it emits intense radiation, a kind of "death cry" and thus a last record of its existence. This radiation can be registered as radio waves in the millimetre range, among others. Consequently, it should be possible to image the event horizon of a black hole.

Listening post into space: IRAM’s 30-metre dish is one of the most sensitive radio telescopes in the global
 collaboration known as the Event Horizon Telescope [Credit: IRAM/Nicolas Billot]

The Event Horizon Telescope (EHT) is aiming to do precisely this. One main goal of the project is the black hole at the centre of our Milky Way, which is around 26,000 light years away from Earth and has a mass roughly equivalent to 4.5 million solar masses. Since it is so far away, the object appears at an extremely small angle.

One solution to this problem is offered by interferometry. The principle behind this technique is as follows: instead of using one huge telescope, several observatories are combined together as if they were small components of a single gigantic antenna. In this way scientists can simulate a telescope which corresponds to the circumference of our Earth. They want to do this because the larger the telescope, the finer the details which can be observed; the so-called angular resolution increases.

The EHT project exploits this observational technique and in April it is to carry out observations at a frequency of 230 gigahertz, corresponding to a wavelength of 1.3 millimetres, in interferometry mode. The maximum angular resolution of this global radio telescope is around 26 micro-arcseconds. This corresponds to the size of a golf ball on the Moon or the breadth of a human hair as seen from a distance of 500 kilometres!

These measurements at the limit of what is observable are only possible under optimum conditions, i.e. at dry, high altitudes. These are offered by the IRAM observatory, partially financed by the Max Planck Society, with its 30-metre antenna on Pico Veleta, a 2800-metre-high peak in Spain's Sierra Nevada. Its sensitivity is surpassed only by the Atacama Large Millimeter Array (ALMA), which consists of 64 individual telescopes and looks into space from the Chajnantor plateau at an altitude of 5000 metres in the Chilean Andes. The plateau is also home to the antenna known as APEX, which is similarly part of the EHT project and is managed by the Max Planck Institute for Radio Astronomy.

The Max Planck Institute in Bonn is furthermore involved with the data processing for the Event Horizon Telescope. The researchers use two supercomputers (correlators) for this; one is located in Bonn, the other at the Haystack Observatory in Massachusetts in the USA. The intention is for the computers to not only evaluate data from the galactic black hole. During the observation campaign from 4 to 14 April, the astronomers want to take a close look at at least five further objects: the M 87, Centaurus A and NGC 1052 galaxies as well as the quasars known as OJ 287 and 3C279.

From 2018 onwards, a further observatory will join the EHT project: NOEMA, the second IRAM observatory on the Plateau de Bure in the French Alps. With its ten high-sensitivity antennas, NOEMA will be the most powerful telescope of the collaboration in the northern hemisphere.

Source: Max Planck Society [April 03, 2017]

byUnknown

in Astronomy, Astrophysics, Universe

April 03, 2017

Portrait of a black hole

Astronomers want to record an image of the heart of our galaxy for the first time: a global collaboration of radio dishes is to take a detai...

Read More