New analysis puts dark matter back into elliptical galaxies (Forwarded)

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."

Andrew Yee wrote:

New analysis puts dark matter back into elliptical galaxies

Instead of adding another "epicycle", why not re-examine the
basic premise. Epicycles were designed for Ptolemaics and
elliptical orbits are the crutches for the heliocentric
universe.
In the galactic frame, ellipses are no longer adequate to
explain galactic motion. To illustrate: A planet in an
elliptical orbit is required to spend half the time in
retrograde motion, that is opposite to the direction of the
Sun or away from the Galactic Center. John Curtis

"John Curtis" wrote in message
ups.com...
Andrew Yee wrote:

New analysis puts dark matter back into elliptical galaxies

Instead of adding another "epicycle", why not re-examine the
basic premise. Epicycles were designed for Ptolemaics and
elliptical orbits are the crutches for the heliocentric
universe.
In the galactic frame, ellipses are no longer adequate to
explain galactic motion. To illustrate: A planet in an
elliptical orbit is required to spend half the time in
retrograde motion, that is opposite to the direction of the
Sun or away from the Galactic Center. John Curtis

So what? There's nothing sacred about any given
direction.

A car doing 50 on the highway is passed by one
doing 60. Does that mean the slower car is
doing something contrary to nature?

Greg Neill wrote:
"John Curtis" wrote in message
ups.com...
Andrew Yee wrote:

New analysis puts dark matter back into elliptical galaxies

Instead of adding another "epicycle", why not re-examine the
basic premise. Epicycles were designed for Ptolemaics and
elliptical orbits are the crutches for the heliocentric
universe.
In the galactic frame, ellipses are no longer adequate to
explain galactic motion. To illustrate: A planet in an
elliptical orbit is required to spend half the time in
retrograde motion, that is opposite to the direction of the
Sun or away from the Galactic Center. John Curtis

So what? There's nothing sacred about any given
direction

Galactic gravity well?

A car doing 50 on the highway is passed by one
doing 60. Does that mean the slower car is
doing something contrary to nature?

a. The slower car illustrates the fallacy of retrograde motion.
b. If the Sun is doing 60 and Earth 50, it may indicate that
Sun's gravity is slowing the Earth and allowing the Sun to
pass the Earth, thus illustrating the fallacy of elliptical
orbits, or the section of an orbit at perihelion.
John Curtis

"John Curtis" wrote in message
oups.com...
Greg Neill wrote:
"John Curtis" wrote in message
ups.com...
Andrew Yee wrote:

New analysis puts dark matter back into elliptical galaxies

Instead of adding another "epicycle", why not re-examine the
basic premise. Epicycles were designed for Ptolemaics and
elliptical orbits are the crutches for the heliocentric
universe.
In the galactic frame, ellipses are no longer adequate to
explain galactic motion. To illustrate: A planet in an
elliptical orbit is required to spend half the time in
retrograde motion, that is opposite to the direction of the
Sun or away from the Galactic Center. John Curtis

So what? There's nothing sacred about any given
direction

Galactic gravity well?

A car doing 50 on the highway is passed by one
doing 60. Does that mean the slower car is
doing something contrary to nature?

a. The slower car illustrates the fallacy of retrograde motion.
b. If the Sun is doing 60 and Earth 50, it may indicate that
Sun's gravity is slowing the Earth and allowing the Sun to
pass the Earth, thus illustrating the fallacy of elliptical
orbits, or the section of an orbit at perihelion.
John Curtis


There is no reason why a body can't orbit another within
the larger gravity well of a third. All of the planetary
moons, including our own, demonstrate this. Further,
the galactic gravitational field is so large and the relative
size of the solar system so tiny by comparison, that the
field is essentially flat across the expanse of the solar
system. Thus, for example, it is not necessary to correct
planetary orbital calculations for the field of the galaxy
in order to obtain sub arcsecond accuracy in positional
predictions.

One can always choose an arbitrary reference frame from which
an underlying simple motion appears complex. There is no
"fallacy of retrograde motion", it is simply an artefact of
the chosen reference frame.

The fact remains that, given an inverse square force law
between two isolated bodies, an elliptical orbit (in fact
any conic section) is the result. Superimposing other
motions or forces on top of that basic result does not
change the underlying principle.

What's your beef against elliptical orbits?

Greg Neill wrote:

What's your beef against elliptical orbits?

Elliptical orbits, just like the retrograde
motion, are products (visual effects) of the
reference frames and should remain limited
to the respective reference frames.
John Curtis

"John Curtis" wrote in message
oups.com...

Greg Neill wrote:

What's your beef against elliptical orbits?

Elliptical orbits, just like the retrograde
motion, are products (visual effects) of the
reference frames and should remain limited
to the respective reference frames.
John Curtis

One reference frame is as valid as another.

It just so happens that in the reference frame of
a body in a two-body gravitational system, the
orbit that obtains is elliptical. One could
argue that this is one of more natural (simpler)
frames one could choose to describe the motion;
the orbit is a closed, simple conic section
amenable to straightforward mathematical analysis.

Can you think of a simpler/better one?

In message . com, John
Curtis writes
Andrew Yee wrote:

New analysis puts dark matter back into elliptical galaxies

Instead of adding another "epicycle", why not re-examine the
basic premise. Epicycles were designed for Ptolemaics and
elliptical orbits are the crutches for the heliocentric
universe.
In the galactic frame, ellipses are no longer adequate to
explain galactic motion. To illustrate: A planet in an
elliptical orbit is required to spend half the time in
retrograde motion, that is opposite to the direction of the
Sun or away from the Galactic Center. John Curtis

You've been looking at too many of Gerald Kelleher's posts, or you're
trolling - I hope :-)
Disregarding the "not even wrong" comment about the Galactic Centre,
retrograde motion has absolutely nothing to do with an elliptical orbit,
but is an illusion caused by observation from an inferior planet in a
heliocentric system.
--
Boycott Yahoo!
Remove spam and invalid from address to reply.

In message , Jonathan Silverlight
writes

retrograde motion has absolutely nothing to do with an elliptical
orbit, but is an illusion caused by observation from an inferior planet
in a heliocentric system.

Sorry to follow up my own post, but my old encyclopedia of astronomy
suggests this should be called "apparent retrogression", as opposed to
the true retrograde motion of moons like Triton. I hope John Curtis
isn't saying that "true retrograde motion" is impossible :-)
What's the "correct" terminology?

Jonathan Silverlight wrote:
In message , Jonathan Silverlight
writes

retrograde motion has absolutely nothing to do with an elliptical
orbit, but is an illusion caused by observation from an inferior planet
in a heliocentric system.

Sorry to follow up my own post, but my old encyclopedia of astronomy
suggests this should be called "apparent retrogression", as opposed to
the true retrograde motion of moons like Triton. I hope John Curtis
isn't saying that "true retrograde motion" is impossible :-)
What's the "correct" terminology?

"Correct termninology" depends on the frame of reference.
"Apparent retrogression" is a heliocentric term which was totally
incomprehensible to ptolemaics.
Ptolemaic term is "retrograde motion" which requires fudge factors
(epicycles) to approximate reality. Now that you taught me the proper
terminology, let me restate the theorem:
Apparent retrogression does not an ellipse make. John Curtis