Astronomers Report That Dark Matter 'Halos' May Contain Stars, Disprove Other Theories

 Could it be that dark matter "halos" -- the huge, invisible cocoons of mass that envelop entire galaxies and account for most of the matter in the universe -- aren't completely dark after all but contain a small number of stars? Astronomers from UCLA, UC Irvine and elsewhere make a case for that in the Oct. 25 issue of the journal Nature.

Astronomers have long disagreed about why they see more light in the universe than it seems they should -- that is, why the infrared light they observe exceeds the amount of light emitted from known galaxies.

When looking at the cosmos, astronomers have seen what are neither stars nor galaxies nor a uniform dark sky but mysterious, sandpaper-like smatterings of light, which UCLA's Edward L. (Ned) Wright refers to as "fluctuations." The debate has centered around what exaclty the source of those fluctuations is.

One explanation is that the fluctuations in the background are from very distant unknown galaxies. A second is that they're from unknown galaxies that are not so far away, faint galaxies whose light has been traveling to us for only 4 billion or 5 billion years (a rather short time in astronomy terms). In the Nature paper, Wright and his colleagues present evidence that both these explanations are wrong, and they propose an alternative.

The first explanation -- that the fluctuations are from very distant galaxies -- is nowhere close to being supported by the data the astronomers present from NASA's Spitzer Space Telescope, said Wright, a UCLA professor of physics and astronomy.

"The idea of not-so-far-away faint galaxies is better, but still not right," he added. "It's off by a factor of about 10; the 'distant galaxies' hypothesis is off by a factor of about 1,000."

Wright and his colleagues, including lead author Asantha Cooray, a UC Irvine professor of physics and astronomy, contend that the small number of stars that were kicked to the edges of space during violent collisions and mergers of galaxies may be the cause of the infrared light "halos" across the sky and may explain the mystery of the excess emitted infrared light.

As crashing galaxies became gravitationally tangled with one another, "orphaned" stars were tossed into space. It is these stars, the researchers say, that produce the diffuse, blotchy scatterings of light emitted from the galaxy halos that extend well beyond the outer reaches of galaxies.

"Galaxies exist in dark matter halos that are much bigger than the galaxies; when galaxies form and merge together, the dark matter halo gets larger and the stars and gas sink to the middle of the the halo," said Wright, who holds UCLA's David Saxon Presidential Chair in Physics. "What we're saying is one star in a thousand does not do that and instead gets distributed like dark matter. You can't see the dark matter very well, but we are proposing that it actually has a few stars in it -- only one-tenth of 1 percent of the number of stars in the bright part of the galaxy. One star in a thousand gets stripped out of the visible galaxy and gets distributed like the dark matter.

"The dark matter halo is not totally dark," Wright said. "A tiny fraction, one-tenth of a percent, of the stars in the central galaxy has been spread out into the halo, and this can produce the fluctuations that we see."

In large clusters of galaxies, astronomers have found much higher percentages of intra-halo light, as large as 20 percent, Wright said.

For this study, Cooray, Wright and colleagues used the Spitzer Space Telescope to produce an infrared map of a region of the sky in the constellation Boötes. The light has been travelling to us for 10 billion years.

"Presumably this light in halos occurs everywhere in the sky and just has not been measured anywhere else," said Wright, who is also principal investigator of NASA's Wide-field Infrared Survey Explorer (WISE) mission.

"If we can really understand the origin of the infrared background, we can understand when all of the light in the universe was produced and how much was produced," Wright said. "The history of all the production of light in the universe is encoded in this background. We're saying the fluctuations can be produced by the fuzzy edges of galaxies that existed at the same time that most of the stars were created, about 10 billion years ago."

The research was funded by the National Science Foundation, NASA and NASA's Jet Propulsion Laboratory.

Future research, especially with the James Webb Space Telescope, should provide further insights, Wright said.

"What we really need to be able to do is to see and identify the galaxies that are producing all the light in the infrared background," he said. "That could be done to a much greater extent once the James Webb Space Telescope is operational because it will be able to see much more distant, fainter galaxies."

Journal Reference:

1. Asantha Cooray, Joseph Smidt, Francesco De Bernardis, Yan Gong, Daniel Stern, Matthew L. N. Ashby, Peter R. Eisenhardt, Christopher C. Frazer, Anthony H. Gonzalez, Christopher S. Kochanek, Szymon Kozłowski, Edward L. Wright. Near-infrared background anisotropies from diffuse intrahalo light of galaxies. Nature, 2012; 490 (7421): 514

Enough Wind to Power Global Energy Demand: New Research Examines Limits, Climate Consequences

There is enough energy available in winds to meet all of the world's demand. Atmospheric turbines that convert steadier and faster high-altitude winds into energy could generate even more power than ground- and ocean-based units. New research from Carnegie's Ken Caldeira examines the limits of the amount of power that could be harvested from winds, as well as the effects high-altitude wind power could have on the climate as a whole.

Their work is published September 9 by Nature Climate Change.

Led by Kate Marvel of Lawrence Livermore National Laboratory, who began this research at Carnegie, the team used models to quantify the amount of power that could be generated from both surface and atmospheric winds. Surface winds were defined as those that can be accessed by turbines supported by towers on land or rising out of the sea. High-altitude winds were defined as those that can be accessed by technology merging turbines and kites. The study looked only at the geophysical limitations of these techniques, not technical or economic factors.

Turbines create drag, or resistance, which removes momentum from the winds and tends to slow them. As the number of wind turbines increase, the amount of energy that is extracted increases. But at some point, the winds would be slowed so much that adding more turbines will not generate more electricity. This study focused on finding the point at which energy extraction is highest.

Using models, the team was able to determine that more than 400 terawatts of power could be extracted from surface winds and more than 1,800 terawatts could be generated by winds extracted throughout the atmosphere.

Today, civilization uses about 18 TW of power. Near-surface winds could provide more than 20 times today's global power demand and wind turbines on kites could potentially capture 100 times the current global power demand.

At maximum levels of power extraction, there would be substantial climate effects to wind harvesting. But the study found that the climate effects of extracting wind energy at the level of current global demand would be small, as long as the turbines were spread out and not clustered in just a few regions. At the level of global energy demand, wind turbines might affect surface temperatures by about 0.1 degree Celsius and affect precipitation by about 1%. Overall, the environmental impacts would not be substantial.

"Looking at the big picture, it is more likely that economic, technological or political factors will determine the growth of wind power around the world, rather than geophysical limitations," Caldeira said.