ESO: Mapping Dark Matter in Galaxies

bystander · Post by **bystander** » Mon Jan 09, 2012 3:48 pm

Mapping Dark Matter in Galaxies
ESO Picture of the Week | 2012 Jan 09

A multitude of faint galaxies, small luminous dots scattered over the dark sky, was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. Images such as this one are powerful tools to understand how dark matter is distributed in galaxies.

The picture is part of the COMBO-17 survey (Classifying Objects by Medium-Band Observations in 17 Filters), a project dedicated to recording detailed images of small patches of the sky through filters of 17 different colours. The area covered in this image is only about the size of the full Moon, but thousands of galaxies can be identified just within this small region.

The image was taken with an exposure time of almost seven hours, which allowed the camera to capture the light from very faint and distant objects, as well as those that are closer to us. Galaxies with clear and regular structures, such as the spiral specimen viewed edge-on near the upper left corner, are only up to a few billion light-years away. The fainter, fuzzier objects are so far away that it has taken nine or ten billion years for their light to reach us.

The COMBO-17 survey is a powerful tool for studying the distribution of dark matter in galaxies. Dark matter is a mysterious substance that does not emit or absorb light and can only be detected by its gravitational pull on other objects. Some of the closer galaxies pictured act as lenses that distort the light coming from more distant galaxies placed along the same line of sight. By measuring this distortion, an effect known as gravitational lensing, astronomers are able to understand how dark matter is distributed in the objects that act as lenses.

The distortion is weak and, therefore, almost imperceptible to the human eye. However, because surveying the sky with 17 filters allows extremely precise distance measurements, it is possible to determine if two galaxies that appear to lie close to each other are actually at very different distances from the Earth. After identifying the galactic lensing systems, the distortion can be measured by averaging over thousands of galaxies. With more than 4000 galactic lenses identified, this COMBO-17 survey is an ideal method to help astronomers to understand the dark matter better.

This image was taken with three of the 17 filters from the project: B (blue), V (green), and R (red). Data through an additional near-infrared filter was also used.

Credit: ESO/COMBO-17 Survey

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bystander · Post by **bystander** » Mon Jan 09, 2012 11:47 pm

Astronomers reach new frontiers of dark matter
CFHTLenS | CFHT | UBC | Edinburgh | 2012 Jan 09

[i]The densest regions of the dark matter cosmic web host massive clusters of galaxies. Credit: Van Waerbeke (UBC), Heymans (Edinburgh), and CFHTLens collaboration.[/i]
For the first time, astronomers have mapped dark matter on the largest scale ever observed. The results, presented by Dr Catherine Heymans of the University of Edinburgh, Scotland, and Associate Professor Ludovic Van Waerbeke of the University of British Columbia, Vancouver, Canada, are being presented today to the American Astronomical Society meeting in Austin, Texas. Their findings reveal a Universe comprised of an intricate cosmic web of dark matter and galaxies spanning more than one billion light years.

An international team of researchers lead by Van Waerbeke and Heymans achieved their results by analysing images of about 10 million galaxies in four different regions of the sky. They studied the distortion of the light emitted from these galaxies, which is bent as it passes massive clumps of dark matter during its journey to Earth.

Their project, known as the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), uses data from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). This accumulated images over five years using the wide field imaging camera MegaCam, a 1 degree by 1 degree field-of-view, 340 Megapixel camera on the CFHT in Hawaii.

Galaxies included in the survey are typically six billion light years away. The light captured by the images used in the study was emitted when the Universe was six billion years old – roughly half the age it is today.

The team’s result has been suspected for a long time from studies based on computer simulations, but was difficult to verify owing to the invisible nature of dark matter. This is the first direct glimpse at dark matter on large scales showing the cosmic web in all directions.

Professor Ludovic Van Waerbeke, from the University of British Columbia, said: “It is fascinating to be able to ‘see’ the dark matter using space-time distortion. It gives us privileged access to this mysterious mass in the Universe which cannot be observed otherwise. Knowing how dark matter is distributed is the very first step towards understanding its nature and how it fits within our current knowledge of physics.”

Dr Catherine Heymans, a Lecturer in the University of Edinburgh’s School of Physics and Astronomy, said: “By analysing light from the distant Universe, we can learn about what it has travelled through on its journey to reach us. We hope that by mapping more dark matter than has been studied before, we are a step closer to understanding this material and its relationship with the galaxies in our Universe.”

Dr Christian Veillet, CFHT Executive Director, said “This dark matter study illustrates the strong legacy value of the CFHT Legacy Survey which is now enabling exciting results obtained by teams from many nations which use the images retrieved from the Canadian Astronomy Data Centre where they are archived and publicly available”.

Professor Lance Miller, from Oxford University said: “This result has been achieved through advances in our analysis techniques which we are now applying to data from the Very Large Telescope’s (VLT) Survey Telescope in Chile.”

Professor Koen Kuijken, from Leiden University, said: “Over the next three years we will image more than 10 times the area mapped by CFHTLenS, bringing us ever closer to our goal of understanding the mysterious dark side of the Universe.”

CFHTLenS: Improving the quality of photometric redshifts with precision photometry - H. Hildebrandt et al

arXiv.org > astro-ph > arXiv:1111.4434 > 18 Nov 2011 (v1), 30 Dec 2011 (v2)

bystander · Post by **bystander** » Mon Jan 09, 2012 11:58 pm

Mapping Dark Matter from Galactic Ripples
PhysOrg | via Florida Atlantic University | 2012 Jan 09

Sukanya Chakrabarti, Ph.D., an assistant professor of physics for the Charles E. Schmidt College of Science at Florida Atlantic University, has developed a way to discover and map dark matter in galaxies. Chakrabarti’s paper, “A New Probe of the Distribution of Dark Matter in Galaxies,” analyzes observed ripples in the outskirts of galaxies to infer the density profile of the dark matter halo. Chakrabarti is presenting her results at this week’s meeting of the American Astronomical Society in Austin, Texas.

The distribution of HI hydrogen in the Whirlpool Galaxy (M51) as determined by the THINGS VLA survey extends far beyond the visible stars in the galaxy and its satellite NGC 5195 (marked by cross), which is situated in the short arm of the spiral. Analysis of perturbations in the hydrogen distribution can be used to predict the location of such satellites, in particular, those satellites that are composed primarily of dark matter and are thus too faint to be detected easily. (Credit: Sukanya Chakrabarti/UC Berkeley)
“Most of the mass in the universe is dark,” said Chakrabarti, who specializes in the study of galaxies. “We have known for a long time that galaxies have massive dark halos. But there are very few probes that can be used to figure out how the dark matter is distributed in specific spiral galaxies.”

The extended gas disks of galaxies are very fragile and respond easily to gravitational interactions with passing satellites. Chakrabarti discovered that if the density profile of dark matter is varied in the spiral galaxy, it is reflected in the disturbances that form in the outer gas disk when the larger spiral galaxy interacts with a satellite galaxy. The ripples in outer gas disks of spiral galaxies act like a mirror of the potential depth of the dark matter halo in the primary galaxy. Even though the dark matter halo cannot be seen directly, scientists may infer the density profile of dark matter using this method.

Chakrabarti previously developed a mathematical method called “tidal analysis” to find satellite, or dwarf, galaxies by analyzing the ripples in the hydrogen gas distribution in large spiral galaxies in outer space. This method, called “tidal analysis,” allows us to infer the mass and relative position of satellites from analysis of ripples in outer gas disks without requiring knowledge of their optical light. Many dwarf galaxies are very dim, so it is useful to have a way of finding them that does not rely on their optical light. Earlier, she applied the method to the nearby Whirlpool Galaxy, which has an optically visible satellite to infer the mass and location of its companion and found these values to be observationally corroborated.

Building on her earlier results where she found that the mass and relative position of the Whirlpool Galaxy’s satellite could be derived using “tidal analysis,” she shows here that we can map the dark matter in Whirlpool Galaxy itself.

“The idea is that the ripples in outer gas disks are like a gravitational mirror that let us to see how the dark matter is distributed,” said Chakrabarti.

A New Probe of the Distribution of Dark Matter in Galaxies - Sukanya Chakrabarti

arXiv.org > astro-ph > arXiv:1112.1416 > 06 Dec 2011

Tracing Dark Matter with Ripples in the Whirlpool Galaxy
Universe Today | Vanessa D'Amico | 2012 Jan 09

bystander · Post by **bystander** » Tue Jan 10, 2012 12:02 am

Mundane dark matter may lurk in starry clusters
New Scientist | 2012 Jan 08

[i]Globular cluster M22 with low-mass star. (Credit: UZH)[/i]
DARK matter - the mysterious substance thought to make up about 80 per cent of the universe's matter - could be more mundane than thought. Inside balls of stars known as globular clusters, at least.

Unless we have misunderstood gravity, dark matter must be there - holding rotating galaxies together. But we don't know what it is. Initially, it was thought to be planets and stars too dim to be seen directly. Such objects would reveal themselves when they pass in front of bright stars, distorting the image with their gravity, but the objects turned up by such "microlensing" searches in our galaxy have not revealed nearly enough matter. So it is assumed that dark matter is something more exotic, such as novel theoretical particles.

Now, Pawel Pietrukowicz of Warsaw University in Poland and colleagues have spotted a tiny star in the globular cluster M22 acting as a lens for a background star. At just 0.18 times the sun's mass, it is the smallest star ever seen in a globular cluster. Because its effects on the larger star were seen after just 10 weeks of observations, the team says there are probably many more like it in the cluster, perhaps even enough to account for all the dark matter needed to hold the cluster together. The work will appear in The Astrophysical Journal Letters.

While exotic dark matter is still needed outside of globular clusters, knowing that it might not be needed in this one, and perhaps others like it, could give clues to the stuff's properties.

The first confirmed microlens in a globular cluster - Pawel Pietrukowicz et al

Astrophysical Journal Letters 744(2) L18 (2012 Jan 12) DOI: 10.1088/2041-8205/744/2/L18
arXiv.org > astro-ph > arXiv:1112.0562 > 02 Dec 2011 (v1), 16 Dec 2011 (v2)

First Low-Mass Star Detected in Globular Cluster
University of Zurich | 2011 Dec 15

bystander · Post by **bystander** » Tue Jan 10, 2012 12:29 am

Clearest Picture Yet of Dark Matter Points the Way to Better Understanding of Dark Energy
DOE | Berkeley Lab | Fermilab | 2012 Jan 09

Scientists at Fermilab and Berkeley Lab build the biggest map of dark matter yet, using methods that will improve ground-based surveys.

[i]Teams from Fermilab and Berkeley Lab used galaxies from wide-ranging SDSS Stripe 82, a tiny detail of which is shown here, to plot new maps of dark matter based on the largest direct measurements of cosmic shear to date. (Image credit SDSS) [/i]
Two teams of physicists at the U.S. Department of Energy’s Fermilab and Lawrence Berkeley National Laboratory (Berkeley Lab) have independently made the largest direct measurements of the invisible scaffolding of the universe, building maps of dark matter using new methods that, in turn, will remove key hurdles for understanding dark energy with ground-based telescopes.

The teams’ measurements look for tiny distortions in the images of distant galaxies, called "cosmic shear," caused by the gravitational influence of massive, invisible dark matter structures in the foreground. Accurately mapping out these dark-matter structures and their evolution over time is likely to be the most sensitive of the few tools available to physicists in their ongoing effort to understand the mysterious space-stretching effects of dark energy.

Both teams depended upon extensive databases of cosmic images collected by the Sloan Digital Sky Survey (SDSS), which were compiled in large part with the help of Berkeley Lab and Fermilab.

“These results are very encouraging for future large sky surveys. The images produced lead to a picture of the galaxies in the universe that is about six times fainter, or further back in time, than is available from single images," says Huan Lin, a Fermilab physicist and member of the SDSS and the Dark Energy Survey (DES).

Melanie Simet, a member of the SDSS collaboration from the University of Chicago, will outline the new techniques for improving maps of cosmic shear and explain how these techniques can expand the reach of upcoming international sky survey experiments during a talk at 2 p.m. CST on Monday, January 9, at the American Astronomical Society (AAS) conference in Austin, Texas. In her talk she will demonstrate a unique way to analyze dark matter’s distortion of galaxies to get a better picture of the universe’s past.

Eric Huff, an SDSS member from Berkeley Lab and the University of California at Berkeley, will present a poster describing the full cosmic shear measurement, including the new constraints on dark energy, from 9 a.m. to 2 p.m. CST Thursday, January 12, at the AAS conference.

Several large astronomical surveys, such as the Dark Energy Survey, the Large Synoptic Survey Telescope, and the HyperSuprimeCam survey, will try to measure cosmic shear in the coming years. Weak lensing distortions are so subtle, however, that the same atmospheric effects that cause stars to twinkle at night pose a formidable challenge for cosmic shear measurements. Until now, no ground-based cosmic-shear measurement has been able to completely and provably separate weak lensing effects from the atmospheric distortions.

"The community has been building towards cosmic shear measurements for a number of years now," says Huff, an astronomer at Berkeley Lab, "but there's also been some skepticism as to whether they can be done accurately enough to constrain dark energy. Showing that we can achieve the required accuracy with these pathfinding studies is important for the next generation of large surveys."

To construct dark matter maps, the Berkeley Lab and Fermilab teams used images of galaxies collected between 2000 and 2009 by SDSS surveys I and II, using the 2.5-meter SLOAN telescope at Apache Point Observatory in New Mexico. The galaxies lie within a continuous ribbon of sky known as SDSS Stripe 82, lying along the celestial equator and encompassing 275 square degrees. The galaxy images were captured in multiple passes over many years.

The two teams layered snapshots of a given area taken at different times, a process called coaddition, to remove errors caused by the atmospheric effects and to enhance very faint signals coming from distant parts of the universe. The teams used different techniques to model and control for the atmospheric variations and to measure the lensing signal, and have performed an exhaustive series of tests to prove that these models work.

Gravity tends to pull matter together into dense concentrations, but dark energy acts as a repulsive force that slows down the collapse. Thus the clumpiness of the dark matter maps provides a measurement of the amount of dark energy in the universe.

When they compared their final results before the AAS meeting, both teams found somewhat less structure than would have been expected from other measurements such as the Wilkinson Microwave Anisotropy Probe (WMAP), but, says Berkeley Lab’s Huff, “the results are not yet different enough from previous experiments to ring any alarm bells.”

Meanwhile, says Lin, “Our image-correction processes should prove a valuable tool for the next generation of weak-lensing surveys.”

Fermilab and University of Chicago

The SDSS Coadd: 275 deg^2 of Deep SDSS Imaging on Stripe 82 - James Annis et al
- arXiv.org > astro-ph > arXiv:1111.6619 > 28 Nov 2011 (v1), 19 Dec 2011 (v2)
The SDSS Coadd: A Galaxy Photometric Redshift Catalog - Ribamar R. R. Reis et al
- arXiv.org > astro-ph > arXiv:1111.6620 > 28 Nov 2011 (v1), 19 Dec 2011 (v2)
The SDSS Coadd: Cross-Correlation Weak Lensing and Tomography of Galaxy Clusters - Melanie Simet et al
- arXiv.org > astro-ph > arXiv:1111.6621 > 28 Nov 2011 (v1), 19 Dec 2011 (v2)
The SDSS Coadd: Cosmic Shear Measurement - Huan Lin et al
- arXiv.org > astro-ph > arXiv:1111.6622 > 28 Nov 2011 (v1), 19 Dec 2011 (v2)

Berkeley Lab and University of California at Berkeley

Seeing in the dark -- I. Multi-epoch alchemy - Eric M. Huff et al
- arXiv.org > astro-ph > arXiv:1111.6958 > 29 Nov 2011
Seeing in the dark -- II. Cosmic shear in the Sloan Digital Sky Survey - Eric M. Huff et al
- arXiv.org > astro-ph > arXiv:1112.3143 > 14 Dec 2011

Dark matter mysteries: a true game of shadows
New Scientist | Stuart Clark | 2012 Jan 09