New analysis puts dark matter back into elliptical galaxies (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 3 Oct 2005 23:57:05 -0400 (EDT)
University of California-Santa Cruz
Contact:
Tim Stephens, 831-459-4352
September 28, 2005
New analysis puts dark matter back into elliptical galaxies
SANTA CRUZ, CA -- 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. 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," said Joel Primack, professor of physics at
the University of California, Santa Cruz, and a coauthor 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," added Avishai Dekel,
professor of physics at the Hebrew University of Jerusalem and first
author of the Nature paper.
"Massive dark matter halos are clearly detected in disk galaxies, so where
did they disappear to during the mergers?" said Dekel, currently a
visiting researcher at UCSC.
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 center 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 center. 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 center 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," Primack
explained.
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 center 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 center.
"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
center," Primack said.
To an observer outside the galaxy, a star on such an elongated orbit would
only appear to be far from the galactic center 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 center of the galaxy from the
perspective of the observer.
"If we see a star at a large distance from the center of the galaxy, that
star is going to be mostly moving either away from the center or back
toward the center. Almost certainly, most of its motion is perpendicular
to our line of sight," Primack said.
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, Primack said.
"Our conclusion is that what they saw is exactly what the cold dark matter
model would predict," he said. "Their data are great, and this actually
gives us more insight into how elliptical galaxies form."
"We predict that other velocity tracers in the same elliptical galaxies
will show higher velocities if they are less concentrated toward the
galaxy center or if they move on more circular orbits," Dekel said. "This
is likely to be the case for compact star clusters, which are also
observable in the outskirts of elliptical galaxies."
.
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