'Missing' Dark Matter Is Really There, Says Hebrew University Cosmologist

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

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

The new analysis in Nature provides a simple explanation for theseobservations. “In fact,” says Dekel, “our analysis fits comfortablywith the standard picture in which elliptical galaxies also reside inmassive dark matter halos.

''A dearth of dark matter in elliptical galaxies is especiallypuzzling in the context of the common theory of galaxy formation, whichassumes that ellipticals originate from mergers of disk galaxies,''added Dekel. ''Massive dark-matter halos are clearly detected in diskgalaxies, so where did they disappear to during the mergers?'' asksDekel.

The Nature article is based on simulations of galaxy mergers run ona supercomputer by graduate student Thomas J. Cox, supervised by JoelPrimack, a professor of physics at the University of California, SantaCruz. The simulations were analyzed by Dekel and collaborators FelixStoehr and Gary Mamon at the Institute of Astrophysics in Paris, whereDekel is the incumbent of the Blaise Pascal International Chair ofResearch at the Ecole Normale Superieure.

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

Evidence for dark matter halos around spiral galaxies comes fromstudying the circular motions of stars in these galaxies. Because mostof the visible mass in a galaxy is concentrated in the central region,stars at great distances from the center would be expected to move moreslowly than stars closer in. Instead, observations of spiral galaxiesshow that the rotational speed of stars in the outskirts of the diskremains constant as far out as astronomers can measure it.

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

In elliptical galaxies, however, it has been difficult to study themotions of stars at great distances from the center. The scientistswriting in Science found a decrease in the velocities with increasingdistance from the center of the galaxy, which is inconsistent withsimple models of the gravitational effects of dark matter halos.

Part of the explanation for that phenomenon, put forth in the newNature paper, lies in the fact that the velocities in the earlier studywere measured along the line of sight. ''You cannot measure theabsolute speeds of the stars, but you can measure their relative speedsalong the line of sight, because if a star is moving toward us itslight is shifted to shorter wave lengths, and if it is moving away fromus its light is shifted to longer wave lengths,'' Primack explained.

This limitation would not be a problem if the orbits of the observedstars were randomly oriented with respect to the line of sight,according to Cox's simulations, however, the stars in ellipticalgalaxies that are farthest from the center are likely to be moving inelongated, eccentric orbits such that most of their motion isperpendicular to the line of sight. Therefore, they could be moving athigh velocities without exhibiting much motion toward or away from theobservers.

Why this is so is traceable to the processes whereby disk galaxiesmerge to form elliptical galaxies. ''In the merger process thatproduces these galaxies, a lot of the stars get flung out to fairlylarge distances, and they end up in highly elongated orbits that takethem far away and then back in close to the center,'' explained Dekel.

''If we see a star at a large distance from the center of thegalaxy, that star is going to be mostly moving either away from thecenter or back toward the center. Almost certainly, most of its motionis perpendicular to our line of sight,'' Dekel said. Under suchcircumstances, the star would appear to be moving quite slowly, when infact this is not the case, based upon the models of simulated galaxymergers studied by the Hebrew University-UCSC-Paris team.

''Our conclusion is that what the cosmologists described in 2003 isexactly what the dark matter model would predict,'' he said, “Ourfindings remove a problem which bothered them and make it possible tobetter understand the processes involved in creation of new galaxies inthe universe.”