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From: wls@astrovax.UUCP (William L. Sebok)
Newsgroups: net.astro
Subject: Re: StarDate: October 12 The Halo of the Milky Way
Message-ID: <674@astrovax.UUCP>
Date: Sat, 19-Oct-85 15:21:04 EDT
Article-I.D.: astrovax.674
Posted: Sat Oct 19 15:21:04 1985
Date-Received: Sun, 20-Oct-85 06:09:24 EDT
References: <38@utastro.UUCP> <2140@brl-tgr.ARPA>
Reply-To: wls@astrovax.UUCP (William L. Sebok)
Organization: Princeton Univ. Astrophysics
Lines: 26

In article <2140@brl-tgr.ARPA> wmartin@brl-bmd.UUCP writes:
>... So this would disagree with the points above, wherin the "halo" is not
>made up of gas and dust, and also where "most" of the galaxy is defined
>as the inside (visible) matter.
>
>So, which is right? Or is this a currently-debated topic in astronomy,
>and things are not as clearly agreed-upon as the impression I got from
>the Disney book leads me to think?

Yes it is a currently debated topic, although I am getting the impression
that a consensus may be forming about the factor of 10 to 20 of
invisible / invisible matter.  A  meeting of the International Astronomical
Union (IAU) was held last June here in Princeton to discuss this subject.

Note that the amount of dark matter around a galaxy or inside a cluster of
galaxies may be a different issue from the amount of dark matter in
inter-galactic space.  There are some theories which have it that the density
inside the "voids" never got high enough to condense galaxies.  These voids
would then be full of ordinary matter that never condensed to galaxies.

To muddy the waters some more, my own feeling is that it may be likely that
there is more than one type of dark matter out there, each type possibly
with its own properties.
-- 
Bill Sebok			Princeton University, Astrophysics
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