Should DM-free galaxies ejected from DM haloes exist?

A BBC net article at

http://news.bbc.co.uk/2/hi/science/nature/6235751.stm

on the Cosmic Evolution Survey project, revealing a 3-D distribution
of dark matter in a sample of the universe (via gravitational lensing
effects) mentions that its team noted small anomalies:

"Conversely, the researchers saw that dark matter concentrations
sometimes seemed to have no corresponding ordinary matter.

"It's not forbidden, but you get a little uncomfortable because you
would think the two should go together," said Dr Linder.

Carlos Frenk commented: "Finding what I would call 'naked' clumps
of dark matter where there are no galaxies for me is very strange.
All dark matter clumps of sufficient size should have galaxies - if
our understanding is correct."

For the moment, no-one is talking about needing to revise cosmo-
logical models; but Professor Frenk said everything hinged on the
size of these anomalies.

"What would be an enormous puzzle would be to find big, luminous
galaxies sitting out there in the middle of nowhere with no dark matter
around them. That really would be shocking." "


This raises a question:
If normal luminous spiral and elliptical galaxies are embedded in
more massive clouds of dark matter (DM), shouldn't we expect a
very small fraction of them to be expelled from their host clusters
(and associated DM host clouds), at least temporarily?

The mechanism would be something like the inter-galactic version
of the gravitational slingshot effect which, for example, can expel
stars from a globular cluster when 3 or more stars interact closely.
This would require three or more galaxies to pass near to one
another such that two become closer, perhaps merging, and a
third receives a gravitational sling shot kick out of the cluster and
away from the host DM cloud. This would require that, during the
main interaction event, the ejected galaxy will retain enough of its
original gravitationally bound normal matter to resume recognizable
galactic form as it exits the cluster. During the time it was thus
free,
such a galaxy should register via gravitational lensing as having
no DM in or near it.

What is the probability that such ejected galaxies should have
occurred, free of their DM embedding? Should we have seen
them by now in the deep sky surveys being conducted?

"stargene" wrote in message
ups.com...

This raises a question:
If normal luminous spiral and elliptical galaxies are embedded in
more massive clouds of dark matter (DM), shouldn't we expect a
very small fraction of them to be expelled from their host clusters
(and associated DM host clouds), at least temporarily?

The mechanism would be something like the inter-galactic version
of the gravitational slingshot effect which, for example, can expel
stars from a globular cluster when 3 or more stars interact closely.
This would require three or more galaxies to pass near to one
another such that two become closer, perhaps merging, and a
third receives a gravitational sling shot kick out of the cluster and
away from the host DM cloud. This would require that, during the
main interaction event, the ejected galaxy will retain enough of its
original gravitationally bound normal matter to resume recognizable
galactic form as it exits the cluster. During the time it was thus
free,
such a galaxy should register via gravitational lensing as having
no DM in or near it.

What is the probability that such ejected galaxies should have
occurred, free of their DM embedding? Should we have seen
them by now in the deep sky surveys being conducted?


What would be the filter mechanism that would separate
the normal matter stars from the dark matter comprising
a given galaxy for your slingshot mechanism? We've recently
seen that high speed galactic collisions can separate out
the dark matter component, for example.

Greg Neill wrote:


What would be the filter mechanism that would separate
the normal matter stars from the dark matter comprising
a given galaxy for your slingshot mechanism? We've recently
seen that high speed galactic collisions can separate out
the dark matter component, for example.

Good question. When I posed my original question, I was thinking
of the embedding of a cluster of galaxies in a more massive DM
halo having a relatively smooth distribution of its own gas of WIMPs
(or whatever). If that were true, a galaxy might be expected, in prin-
ciple, to be capable of ejection without dragging a big complement
of DM with it. I'd neglected the idea that individual galaxies are
also embedded in their own smaller local DM haloes. I don't know
if a gravitational sling-shot event could, say, tidally separate a
single galaxy from its own DM halo, at least temporarily.

I've been looking for some natural mechanism which might mediate
the separation of normal galactic masses from their DM haloes. If
such a thing were observed, it would at least prove that dark matter,
whatever its nature, is intrinsically not associated with any property
of normal luminous baryonic matter, not even its end states: black
holes, neutron stars etc. Back to the drawing board.

I think you refer to the recent observation of the separation of DM
haloes of two colliding galactic clusters from their more massive HI
gas components. Eg: see Baalke's entry at

http://groups.google.com/group/sci.a...b19655dab39634

from which I quote:
"The hot gas in this collision was slowed by a drag force, similar to
air resistance. In contrast, the dark matter was not slowed by the
impact, because it does not interact directly with itself or the gas
except through gravity. This produced the separation of the dark
and normal matter seen in the data. If hot gas was the most massive
component in the clusters, as proposed by alternative gravity
theories, such a separation would not have been seen. Instead,
dark matter is required. "

This important result shows that the 'pass through' collision of the
clusters separated their DM haloes from that most massive com-
ponent of the clusters normal baryonic matter: the hot hydrogen
clouds. But the galaxies' stellar components (much smaller in
mass than the HI clouds) remained centered on the DM haloes
themselves. This, at least in principle, is compatible with DM still
being associated with some property of the stellar matter of the
galaxies.