* Astronomy

Crypto-baryonic Dark Matter (Update -see previous post)

A new type of dark matter made up of collections of atoms that form balls up to 20 centimetres in size has been predicted by two theorists in Scotland and Denmark. The balls, which could weigh about 10^11 kilograms each, would be hard to detect but may exist inside heavy stars. The physicists think the balls might eat up the star and release enough energy to make it explode as a supernova. They believe there may be, on average, one dark-matter ball for a volume of space about the size of our solar system.

According to the standard model of cosmology, the universe is thought to contain about 5% of ordinary matter, 25% of dark matter and 70% of dark energy. The nature of this dark matter and energy is the biggest mystery in cosmology today.

Unlike some other recent theories, the new model for dark matter, which has been proposed by Colin Froggatt of Glasgow University and Holger Nielsen of the the Niels Bohr Institute in Copenhagen, does not require any new fundamental particles or interactions beyond the Standard Model of particle physics. The model also predicts a ratio of dark matter to ordinary matter that agrees with the value obtained by NASA's WMAP satellite in 2003. It does, however, assume the existence of an "alternative vacuum" that has the same energy density -- or "cosmological constant" -- as our own ordinary vacuum.

Froggatt and Nielsen calculate that the two different types of vacuum separated out into different regions of space quite early in the history of the universe by "domain walls" that formed at the high temperatures present at this time. Roughly one second after the Big Bang, the researchers say, these walls formed balls that encapsulated matter inside pieces of the alternative vacuum. All nucleons might have been captured this way, leading to the formation of the first light nuclei, such as helium, as the balls rapidly contracted.

This contraction continued until the helium nuclei fused together to form heavier nuclei and the energy released in the subsequent chain reactions expelled the nucleons from the balls. According to the new model, one sixth of the nucleons were freed this way, entering the ordinary vacuum and becoming normal matter. The rest of the nucleons remained trapped as dark matter inside the balls of the alternative vacuum.

The team believes that some of the balls might have collected inside heavy stars. At sufficiently high temperatures and densities, they could have started consuming the star, so releasing enough energy to make it explode in a supernova. The balls themselves could even implode and therefore provide a way to produce ultra-high energy cosmic rays from seemingly empty places in the universe. The researchers also say that their theory might explain why the amount of lithium in the universe is 2 to 3 times less than is predicted by the standard Big Bang nucleosynthesis model.

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-- Edited by Blobrana at 01:37, 2005-12-07

A new analysis that refutes challenges to the existence of dark matter in certain galaxies appears in an article published this week in the journal Nature. Leading author of the article is Avishai Dekel, professor of physics at the Hebrew University of Jerusalem.

Accepted cosmological theory postulates that every observable galaxy in the universe (each made up of billions of stars similar to our sun) is embedded in a massive “halo'' of dark matter. Though unseen, dark matter can be clearly detected indirectly by observing its tremendous gravitational effects on visible objects.

This common understanding faced a severe challenge when a team of astronomers, writing in Science in 2003, reported a surprising absence of dark matter in one type of galaxy – “elliptical'' (rounded) galaxies. Their theory was based on observations that stars located at great distances from the centre in such galaxies move at very slow speeds, as opposed to the great speed one would have expected from the heavy gravitational pull exerted by dark matter.

The new analysis in Nature provides a simple explanation for these observations.

“In fact, our analysis fits comfortably with the standard picture in which elliptical galaxies also reside in massive dark matter halos.
A dearth of dark matter in elliptical galaxies is especially puzzling in the context of the common theory of galaxy formation, which assumes that ellipticals originate from mergers of disk galaxies'' - Avishai Dekel.
''Massive dark-matter halos are clearly detected in disk galaxies, so where did they disappear to during the mergers?'' - Avishai Dekel.



Illustration of computer simulation showing two spiral galaxies combining to form an elliptical galaxy at right. (Image courtesy of Hebrew University Of Jerusalem)


The Nature article is based on simulations of galaxy mergers run on a supercomputer by graduate student Thomas J. Cox, supervised by Joel Primack, a professor of physics at the University of California, Santa Cruz. The simulations were analyzed by Dekel and collaborators Felix Stoehr and Gary Mamon at the Institute of Astrophysics in Paris, where Dekel is the incumbent of the Blaise Pascal International Chair of Research at the Ecole Normale Superieure.

The simulations show that the observations reported in Science are a predictable consequence of the violent collision and merger of the spiral galaxies that lead to the formation of the elliptical galaxies.

Evidence for dark matter halos around spiral galaxies comes from studying the circular motions of stars in these galaxies. Because most of the visible mass in a galaxy is concentrated in the central region, stars at great distances from the centre would be expected to move more slowly than stars closer in. Instead, observations of spiral galaxies show that the rotational speed of stars in the outskirts of the disk remains constant as far out as astronomers can measure it.

The reason for this, according to the dark matter theory, is the presence of an enormous halo of unseen dark matter in and around the galaxy, which exerts its gravitational influence on the stars. Additional support for dark matter halos has come from a variety of other observations.

In elliptical galaxies, however, it has been difficult to study the motions of stars at great distances from the centre. The scientists writing in Science found a decrease in the velocities with increasing distance from the centre of the galaxy, which is inconsistent with simple models of the gravitational effects of dark matter halos.

Part of the explanation for that phenomenon, put forth in the new Nature paper, lies in the fact that the velocities in the earlier study were measured along the line of sight.

''You cannot measure the absolute speeds of the stars, but you can measure their relative speeds along the line of sight, because if a star is moving toward us its light is shifted to shorter wave lengths, and if it is moving away from us its light is shifted to longer wave lengths'' - Joel Primack.

This limitation would not be a problem if the orbits of the observed stars were randomly oriented with respect to the line of sight, according to Cox's simulations, however, the stars in elliptical galaxies that are farthest from the centre are likely to be moving in elongated, eccentric orbits such that most of their motion is perpendicular to the line of sight. Therefore, they could be moving at high velocities without exhibiting much motion toward or away from the observers.

Why this is so is traceable to the processes whereby disk galaxies merge to form elliptical galaxies.
''In the merger process that produces these galaxies, a lot of the stars get flung out to fairly large distances, and they end up in highly elongated orbits that take them far away and then back in close to the centre'' - Avishai Dekel.

''If we see a star at a large distance from the centre of the galaxy, that star is going to be mostly moving either away from the centre or back toward the centre. Almost certainly, most of its motion is perpendicular to our line of sight'' - Avishai Dekel.

Under such circumstances, the star would appear to be moving quite slowly, when in fact this is not the case, based upon the models of simulated galaxy mergers studied by the Hebrew University-UCSC-Paris team.

''Our conclusion is that what the cosmologists described in 2003 is exactly what the dark matter model would predict,'' he said, “Our findings remove a problem which bothered them and make it possible to better understand the processes involved in creation of new galaxies in the universe” - Avishai Dekel.

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According to the prevailing "cold dark matter" theory of the evolution of the universe, every galaxy is surrounded by a halo of dark matter that can only be detected indirectly by observing its gravitational effects.

This theory faced a challenge in 2003, when a team of astronomers reported a surprising absence of dark matter in elliptical galaxies.
Until recently, observers could only guess how much dark matter elliptical galaxies contained. Unlike spiral galaxies, normal ellipticals lack disks that made it easy for astronomers to track rotation. That changed in 2003, when Michael Merrifield and Aaron Romanowsky, then at the University of Nottingham in England, managed to measure the speed of planetary nebulae — glowing gaseous shells shed by dying stars — skirting five elliptical galaxies: M105 and NGC 821, 2434, 4494, and 4697.

To their surprise, the velocities of the planetary nebulae fell off with distance from the galaxy centres, just as would be expected if elliptical galaxies contained "little if any" dark matter.

But a new analysis published in the September 29 issue of the journal Nature provides an explanation for the earlier observations that fits comfortably with the standard theory and puts the dark matter back into elliptical galaxies.

"These are very normal, nearby elliptical galaxies that they studied, and if those galaxies don't have dark matter it calls into question the whole theory of cold dark matter" - Joel Primack, professor of physics at the University of California, Santa Cruz, and a co-author of the Nature paper.

"A dearth of dark matter in elliptical galaxies is especially puzzling in the context of the standard theory of galaxy formation, which assumes that ellipticals originate from mergers of disk galaxies" - Avishai Dekel, professor of physics at the Hebrew University of Jerusalem and first author of the Nature paper.

Avishai Dekel is currently a visiting researcher at UCSC.

"Massive dark matter halos are clearly detected in disk galaxies, so where did they disappear to during the mergers?" - Avishai Dekel.

Primack, one of the originators and developers of the cold dark matter theory, uses supercomputers to run simulations of galaxy formation and the evolution of structure in the universe. The new paper used simulations of galaxy mergers run last year by Thomas J. Cox, then a graduate student working with Primack at UCSC and now a postdoctoral researcher at Harvard University.
The simulations show that the observations reported in 2003 are a predictable consequence of the violent galactic mergers that give rise to elliptical galaxies, Primack said. The simulations were analyzed by Dekel, Felix Stoehr, and Gary Mamon at the Institute of Astrophysics in Paris, where Dekel holds a Blaise Pascal International Chair. UCSC graduate student Greg Novak also contributed to the analysis.

Elliptical galaxies are thought to form when two spiral galaxies collide and merge. Whereas spiral galaxies are dominated by flattened, rotating disks of stars and gas, elliptical galaxies are round, smooth collections of stars.
Evidence for dark matter halos around spiral galaxies comes from studying the circular motions of stars in these galaxies.
Because most of the visible mass in a galaxy is concentrated in the central region, stars at great distances from the centre would be expected to move more slowly than stars closer in. Instead, careful observations of spiral galaxies show that the rotational speed of stars in the outskirts of the disk remains constant as far out as astronomers can measure it.
The reason for this, according to cold dark matter theory, is the presence of an enormous halo of unseen dark matter surrounding the galaxy and exerting its gravitational influence on the stars. Additional support for dark matter halos has come from a variety of other observations.
In elliptical galaxies, however, it has been difficult to study the motions of stars at great distances from the centre.
The 2003 study (A. J. Romanowsky et al., Science 301:1696-1698) focused on bright planetary nebulas in the outer parts of four nearby elliptical galaxies. Planetary nebulas are old stars that have blown off their outer layers and glow brightly in characteristic wavelengths of light.
The researchers were able to determine the line-of-sight velocities of large numbers of planetary nebulas in these elliptical galaxies. They found a decrease in the velocities with increasing distance from the centre of the galaxy, which is inconsistent with simple models of the gravitational effects of dark matter halos.
Part of the explanation put forth in the new Nature paper lies in the fact that the velocities were measured along the line of sight.

"You cannot measure the absolute speeds of the stars, but you can measure their relative speeds along the line of sight, because if a star is moving toward us its light is shifted to shorter wavelengths, and if it is moving away from us its light is shifted to longer wavelengths" - Joel Primack.

This limitation would not be a problem if the orbits of the observed stars were randomly oriented with respect to the line of sight, because any differences resulting from the orientations of the orbits would average out over a large number of observations.
According to Cox's simulations, however, the stars farthest from the centre of the galaxy at any given time are likely to be moving in elongated, eccentric orbits such that most of their motion is perpendicular to the line of sight. Therefore, they could be moving at high velocities without exhibiting much motion toward or away from the observers.
To understand why, it is necessary to look at what happens to the stars during galaxy mergers. As the merging galaxies interact, the stars themselves do not collide because they are separated by great distances, so the two galaxies essentially pass through one another. But the huge gravitational fields of the galaxies cause powerful tidal disturbances. Some of the stars are flung outward in extended tidal tails as the cores of the galaxies pass close by one another and spin apart. Sometimes the cores remain connected by a tidal bridge of stars and gas. Eventually, gravity pulls the cores back together, and the stars that were flung outward fall back in toward the centre.

"In the merger process that produces these galaxies, a lot of the stars get flung out to fairly large distances, and they end up in highly elongated orbits that take them far away and then back in close to the centre" - Joel Primack.

To an observer outside the galaxy, a star on such an elongated orbit would only appear to be far from the galactic centre if the long axis of its orbit is more or less perpendicular to the observer's line of sight. If the long axis of the orbit is aligned with the line of sight, the star would always appear to be in the crowded centre of the galaxy from the perspective of the observer.

"If we see a star at a large distance from the centre of the galaxy, that star is going to be mostly moving either away from the centre or back toward the centre. Almost certainly, most of its motion is perpendicular to our line of sight" - Joel Primack.

The simulated mergers involved typical spiral galaxies, each embedded in a halo of cold dark matter. The simulations followed the gravitational and hydrodynamic evolution of the merger systems, taking into account the complicated feedbacks from star formation, supernovae, and the heating and cooling of gases in the galaxies. Each simulation was then "observed" from three different directions and at two slightly different times after the merger.
From more than 200 merger simulations run by Cox on a supercomputer at UCSC, the researchers analyzed 10 mergers that yielded elliptical galaxies with masses similar to those of the galaxies observed in 2003. The results were completely consistent with the reported observations.

"Our conclusion is that what they saw is exactly what the cold dark matter model would predict. Their data are great, and this actually gives us more insight into how elliptical galaxies form" - Joel Primack.

"We predict that other velocity tracers in the same elliptical galaxies will show higher velocities if they are less concentrated toward the galaxy centre or if they move on more circular orbits. This is likely to be the case for compact star clusters, which are also observable in the outskirts of elliptical galaxies" - Avishai Dekel.

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