323 research outputs found
Extreme scattering events and Galactic dark matter
Extreme Scattering Events (ESEs) are attributed to radio-wave refraction by a
cloud of free-electrons crossing the line-of-sight. We present a new model in
which these electrons form the photo-ionized 'skin' of an underlying cool,
self-gravitating cloud in the Galactic halo. In this way we avoid the severe
over-pressure problem which afflicts other models. The UV flux in the Galactic
halo naturally generates electron densities of the right order. We demonstrate,
for the first time, a good reproduction of the prototypical ESE in the quasar
0954+658. The neutral clouds are a few AU in radius and have masses less than
about 10^{-3} solar. The observed rate of ESEs implies that a large fraction of
the mass of the Galaxy is in this form.Comment: 5 pp incl 3 figs, LaTeX, uses aas2pp4.sty. Minor revisions. ApJ
Letters in pres
Secular Evolution of Galaxy Morphologies
Today we have numerous evidences that spirals evolve dynamically through
various secular or episodic processes, such as bar formation and destruction,
bulge growth and mergers, sometimes over much shorter periods than the standard
galaxy age of 10-15 Gyr. This, coupled to the known properties of the Hubble
sequence, leads to a unique sense of evolution: from Sm to Sa. Linking this to
the known mass components provides new indications on the nature of dark matter
in galaxies. The existence of large amounts of yet undetected dark gas appears
as the most natural option. Bounds on the amount of dark stars can be given
since their formation is mostly irreversible and requires obviously a same
amount of gas.Comment: 8 pages, Latex2e, crckapb.sty macros, 1 Postscript figure, replaced
with TeX source; To be published in the proceeedings of the "Dust-Morphology"
conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
Thermal stability of cold clouds in galaxy halos
We consider the thermal properties of cold, dense clouds of molecular
hydrogen and atomic helium. For cloud masses below 10^-1.7 Msun, the internal
pressure is sufficient to permit the existence of particles of solid or liquid
hydrogen at temperatures above the microwave background temperature. Optically
thin thermal continuum emission by these particles can balance cosmic-ray
heating of the cloud, leading to equilibria which are thermally stable even
though the heating rate is independent of cloud temperature. For the Galaxy,
the known heating rate in the disk sets a minimum mass of order 10^-6 Msun
necessary for survival. Clouds of this type may in principle comprise most of
the dark matter in the Galactic halo. However, we caution that the equilibria
do not exist at redshifts z > 1 when the temperature of the microwave
background was substantially larger than its current value; the formation and
survival of such clouds to the present epoch therefore remain open questions.Comment: 5 pp incl 2 figs, LaTeX, emulateapj.sty; ApJ Letters in press.
Significant revisions, results qualitatively unchange
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