April 14, 2016
Bjorn Carey, Stanford News Service
Composite image of the gravitational lens SDP.81 showing the distorted image of the more distant galaxy (red arcs) and the nearby lensing galaxy (blue center object). By analyzing the distortions in the ring, astronomers have determined that a dark dwarf galaxy (data indicated by white dot near left lower arc segment) is lurking nearly 4 billion light-years away. (Credit: Y. Hezaveh; ALMA)
The study develops a powerful tool for discovering galaxies that are otherwise too distant to observe, and could lead to advances that improve our understanding of dark matter.
New analysis of an image taken by the Atacama Large Millimeter/submillimeter Array (ALMA) reveals evidence that a dwarf dark galaxy – a tiny halo companion of a much larger galaxy – is lurking nearly 4 billion light-years away.
ESO/NRAO/NAOJ ALMA Array
This discovery, led by a Stanford astrophysicist and announced today, paves the way for ALMA to find many more such objects, which could help astronomers address important questions on the nature of dark matter.
In 2014, as part of ALMA’s Long Baseline Campaign, astronomers studied a variety of astronomical objects to test the telescope’s new high-resolution capabilities. One of these experimental images was that of an Einstein ring, which was produced by a massive foreground galaxy bending the light emitted by another galaxy nearly 12 billion light-years away.
This phenomenon, called gravitational lensing, was predicted by Einstein’s theory of general relativity, and it offers a powerful tool for studying galaxies that are otherwise too distant to observe. It also sheds light on the properties of the nearby lensing galaxy because of the way its gravity distorts and focuses light from more distant objects.
In a new paper accepted for publication in the Astrophysical Journal, astrophysicist Yashar Hezaveh at Stanford and his team explain how detailed analysis of this image of a galaxy called SDP.81 uncovered signs of a hidden dwarf dark galaxy in the halo of a the more nearby galaxy.
“We can find these invisible objects in the same way that you can see rain droplets on a window: You know they are there because they distort the image of the background objects,” explained Hezaveh. In the case of a raindrop, the image distortions are caused by refraction, but here similar distortions are generated by the gravitational influence of dark matter, according to Einstein’s theory of relativity.
Current theories suggest that dark matter, which makes up 80 percent of the mass of the universe, is made of as-yet-unidentified particles that don’t interact with visible light or other forms of electromagnetic radiation. Dark matter does, however, have appreciable mass, so it can be identified by its gravitational influence.
For their analysis, the researchers harnessed thousands of computers working in parallel for many weeks, including the National Science Foundation’s most powerful supercomputer, Blue Waters, to search for subtle anomalies that had a consistent and measurable counterpart in each “band” of radio data.
Cray Blue Waters supercomputer at the National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign
From these combined computations, the researchers were able to piece together an unprecedented understanding of the lensing galaxy’s halo, the diffuse and predominantly star-free region around the galaxy, and discovered a distinctive clump, less than one-thousandth the mass of the Milky Way.
Because of its relationship to the larger galaxy, its estimated mass and lack of an optical counterpart, the astronomers believe this gravitational anomaly may be caused by an extremely faint, dark-matter dominated satellite of the lensing galaxy. According to theoretical projections, most galaxies should be brimming with similar dwarf galaxies and other companion objects. Detecting them, however, has proven challenging. Even in our own Milky Way, astronomers can identify only about 40 of the thousands of satellite dwarfs that are predicted to be present.
Computer models of the evolution of the universe indicate that if the number of small dark matter clumps around distant galaxies, like the one detected here, is significantly lower than predictions, this would imply that the dark matter particles have a warm temperature.
Risa Wechsler, an associate professor of physics at Stanford, and graduate student Yao-Yuan Mao used these compter simulations to show that so far this detection is consistent with the predictions of the cold dark matter theoretical model. More observations are needed, however, to definitively rule out the possibility of a warm temperature for dark matter.
“This detection is very exciting – it shows that we finally have a tool to find these dwarf satellites efficiently in a way that was not possible before,” said Wechsler. “Now we need to look at other galaxies to hopefully find more of these small dark halos to have a statistically significant test of the cold dark matter predictions.”
The finding is an exciting demonstration of the power of ALMA, said astrophysicist Roger Blandford, the Luke Blossom Professor in the School of Humanities and Science at Stanford, who was involved in the research. “This discovery proves that ALMA can be used to provide valuable new insights into the physics of dark matter.”
The other Stanford authors on the paper* include Philip Marshall, Warren Morningstar and Laurence Perreault Levasseur. All Stanford authors are also members of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford.
Detection of lensing substructure using ALMA observations of the dusty galaxy SDP.81
YASHAR D. HEZAVEH1,12 , NEAL DALAL2,3,4,5 , DANIEL P. MARRONE6 , YAO-YUAN MAO1,7 , WARREN MORNINGSTAR1 , DI WEN2 ,
ROGER D. BLANDFORD1,7 , JOHN E. CARLSTROM8 , CHRISTOPHER D. FASSNACHT9 , GILBERT P. HOLDER10 , ATHOL KEMBALL2 ,
PHILIP J. MARSHALL7 , NORMAN MURRAY11,13 , LAURENCE PERREAULT LEVASSEUR1 , JOAQUIN D. VIEIRA2 , RISA H. WECHSLER1,7
1 Kavli Institute for Particle Astrophysics and Cosmology and Department
of Physics, Stanford University, 452 Lomita Mall, Stanford, CA
2 Astronomy Department, University of Illinois at Urbana-Champaign,
1002 W. Green Street, Urbana IL 61801, USA
3 School of Natural Sciences, Institute for Advanced Study, 1 Einstein
Drive, Princeton, NJ 08540, USA
4 Kavli Institute for the Physics and Mathematics of the Universe, TODIAS,
The University of Tokyo, Chiba, 277-8583, Japan
5 Department of Chemistry and Physics, University of Kwa-Zulu Natal,
University Road, Westville, KZN, South Africa
6 Steward Observatory, University of Arizona, 933 North Cherry Avenue,
Tucson, AZ 85721, USA
7 Kavli Institute for Particle Astrophysics and Cosmology and Department
of Particle Physics and Astrophysics; SLAC National Accelerator
Laboratory, Menlo Park, CA 94305, USA
8 Kavli Institute for Cosmological Physics, University of Chicago, 5640
South Ellis Avenue, Chicago, IL 60637, USA
9 Department of Physics, University of California, One Shields Avenue,
Davis, CA 95616, USA
10 Department of Physics, McGill University, 3600 Rue University,
Montreal, Quebec H3A 2T8, Canada
11 CITA, University of Toronto, 60 St. George St., Toronto ON M5S
12 Hubble Fellow
13 Canada Research Chair in Astrophysics
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