ASTRONOMY: ON DARK MATTER IN GALAXIES
ScienceWeek http://scienceweek.com
The following points are made by Ken C. Freeman (Science 2003
302:1902):
1) According to the current paradigm for galaxy formation, the
early Universe contained a mixture of baryons (ordinary matter
made up from protons and neutrons) and cold dark matter (CDM) in
a ratio of about 1:6 by mass. Small fluctuations in this mixture
grew gravitationally into the galaxies, clusters of galaxies,
superclusters, and filamentary large-scale structure of today's
Universe. The CDM model can reproduce most of the large-scale
structure observed in the Universe, but on galactic scales, there
are some important discrepancies (1). New advances in theory and
observations are helping to resolve some of these discrepancies.
2) Galactic dark matter lies in dark halos that envelop the
luminous parts of their parent galaxies. The motions of small
satellite galaxies show that the halo of our own galaxy, the
Milky Way, extends beyond about 300,000 light-years -- much
further than the Galactic disk that contains most of the visible
mass. The dark halo's mass is about 20 times that of all stars
and gas in the Galaxy.
3) According to the CDM model, large galaxies like the Milky Way
should have large numbers of satellite galaxies; the Milky Way is
expected to have about 500 small satellites. But only 20 or so
are observed (2). The model also predicts that the density
distribution should rise rapidly toward the center of the halo,
following a steeply cusped power law (3). In contrast, most
observations indicate that dark halos have a central core of
nearly constant density (4). Despite these differences, both
theory and observation may be correct -- for example, if the
halos first form with many satellites and steeply cusped central
regions, but star formation and expulsion of gas later help to
unbind some of the satellites and flatten the central cores.
4) In most galaxies, the stars and gas rotate. The origin of
their angular momentum, which is presumably shared by the dark
matter, has long been thought to come from the tidal interactions
of fluctuations in the early Universe. Numerical simulations of
this process do produce rotating systems, but the predicted
internal distribution of angular momentum does not match the
observations. Other sources of angular momentum may thus be
needed. One such source may be the acquisition of angular
momentum through accretion of smaller galaxies by larger ones.
5) Spiral galaxies do not usually rotate like rigid bodies. Their
rotation curves (the variation of rotational velocity with
radius) can be used to derive the typical densities and core
sizes for dark halos, but first the gravitational field resulting
from the dark halo alone must be estimated. At large distances
from the galactic center, the gravitational fields are almost
always dominated by the dark halo, but closer to the center, the
stars (and gas) as well as the dark matter should contribute to
the gravitational field.(5)
References (abridged):
1. Dark Matter in Galaxies, International Astronomical Union
Symposium 220, Sydney, Australia, 21 to 25 July 2003 (Meeting)
2. B. Moore et al., Astrophys J. 524, L19 (1999)
3. J. Navarro, C. Frenk, S. White, Astrophys J. 462, 563 (1996)
4. W. de Blok, A. Bosma, Astron. Astrophys. 385, 816 (2002)
5.See antwrp.gsfc.nasa.gov/apod/ap030212.html.
Science http://www.sciencemag.org
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ON THE MERGING OF GALAXIES
The following points are made by Rosemary Wyse (Science 2003
301:1055):
1) According to cold dark matter (CDM) models, large galaxies
like the Milky Way resulted from merging and assimilation of many
smaller ones. These models have successfully predicted the large-
scale structure of the universe, particularly when they include
the effects of dark energy (CDM). However, on the smaller scale
of galaxies, the models perform less well. For example, they
predict too many dark-matter haloes around large galaxies
compared with observations in the Local Group and too much late
merging for disk galaxies to maintain old thin disks plus old
thick disks.
2) Proposed solutions to these problems maintain the cold nature
of the dark matter but change the predicted properties of stars
and gas in the dark-matter haloes. The resulting model galaxies
can then be tested against observations, particularly the mass-
assembly and star-formation histories of galaxies in the Local
Group.
3) The Local Group contains various types of galaxies, including
large spiral galaxies like our Milky Way and the Andromeda
nebula; a smaller, almost pure-disk spiral galaxy (M33);
irregular galaxies like the Large Magellanic Clouds; and dwarf
spheroidal galaxies with extremely low surface brightness. The
Milky Way also contains a supermassive black hole, which may
regulate star formation and serves as a template for
understanding black holes in more distant galaxies. The galaxies
in the Local Group differ widely in their gas content and rates
of star formation, shedding light on the physics of star
formation.
4) The merging inherent in CDM models has lasting consequences
for stars in galaxies. When two galaxies merge, their orbital
energies and angular momenta are absorbed by the composite
system. Gas can radiate away this energy and cool, but stellar
systems do not dissipate heat and therefore remain hot.
5) The most prominent feature of a spiral galaxy is the cold,
thin disk in which on-going star formation occurs, often in
spiral arms. Mergers heat this disk. The amount of heating
depends on the nature of the merger: Mergers between galaxies
with similar masses destroy the disks, whereas mergers between
galaxies with unequal masses merely thicken the disks. Low-
density small systems may be "tidally disrupted" before they can
impart much damage. In the case of galaxies, the tides can be so
strong that material is captured from the smaller system by the
larger one. The satellite galaxy may even be fully assimilated,
that is, it ceases to exist.
Science http://www.sciencemag.org
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ON GALAXIES AND DARK MATTER
The following points are made by Henning Genz (citation below):
1) We know that the curvature of space is logically intertwined
with the energy it contains. The general theory of relativity
tells us that the gravitational force, which has one energy act
on another, is nothing but a different name for the curvature of
space. Space altogether can be flat only if it contains just the
right amount of energy -- or if there is a conspiratory
cancellation of different effects... But this condition is not
met as long as we consider only the energy of all matter that, to
our knowledge, is present in the universe: The visible matter --
that is, that matter that is observable through its light
emission or reflection -- contains only about 1 percent of the
"critical" mass that is needed for an overall flat space. "Light"
in this connotation stands for electromagnetic radiation of any
wavelength that reaches Earth -- from optical to gamma rays.
2) It is not only the theory of inflation that tells us we don't
know of all of the masses in the universe. In addition, there are
observed facts that tell us the same. One of these is the chaotic
motion of galaxies inside galaxy clusters. Galaxies don't appear
singly, but rather inside such larger groupings. They are kept
from escaping by the gravitational pull of the mass of the
cluster. Individual galaxies move inside a cluster with
velocities that are measurable by means of the Doppler effect.
That's how we know these velocities to be so large that the
gravity of the visible mass of the cluster does not suffice to
hold the galaxies it contains together: Were there only the
cluster's visible mass, its galaxies would have had to fly apart
long ago. To keep all of them within the cluster, there must be
about one hundred times more mass present than what is noticeable
to us as visible matter.
3) Even the most remote stars close to the outer rim of a galaxy
like ours, the Milky Way, orbit the center of the galaxy at a
velocity that would make them take off into intergalactic space
if that galaxy contained only the matter that is visible to the
astronomer. Just to hold the stars within a galaxy, we need the
gravitational pull of at least ten times more mass than is
visible.
4) There are arguments in favor of the assumption that the
invisible matter that holds the galaxies together consists of the
same atoms as visible matter. We call it baryonic matter. Baryons
(after the Greek word for heavy) are the massive constituents of
nuclear matter, mostly the protons and neutrons... The planet
Jupiter could serve as an example for a fairly large
concentration of dark baryonic matter that becomes noticeable
mainly through its gravitational pull. Burnt-out stars and
nonluminous intergalactic clouds of dust or gas may also
contribute to dark matter. There are a number of good reasons for
astrophysicists to assume that all luminous galaxies are embedded
in large spherical regions filled with a ten-times-larger mass of
such composition.
5) The process has to be repeated on the next larger scale: To
keep galactic clusters from flying apart, it takes more dark
matter. The larger the regions we take under consideration, the
more dark matter is needed for a flat universe. All in all, we
come to the conclusion that no more than 1 percent of all matter
in the universe falls in the visible category.
Adapted from: Henning Genz: Nothingness: The Science of Empty
Space. Perseus Publishing 1998, p.298. More information at:
http://www.amazon.com/exec/obidos/ASIN/0738206105/scienceweek
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ON THE MYSTERY OF GALAXY FORMATION
The following points are made by Marco Scodeggio (Science 2001
294:537):
1) There are two basic models for galaxy formation. In the
monolithic collapse scenario, all galaxies were formed in a
single event, through the gravitational collapse of a cloud of
primordial gas, very early in the history of the universe (1,2).
In the hierarchical merging scenario, galaxies are gradually
assembled through multiple mergers of smaller subgalactic units,
a process that continues from the early universe to the current
epoch (3,4).
2) These differences extend to ideas about galaxy evolution. In
the monolithic collapse scenario, galaxies of different
morphological types (spirals and ellipticals) are born
intrinsically different, whereas in the hierarchical merging
scenario, galaxies end up as spirals or ellipticals depending on
the details of their merger history. As a result, the first model
predicts that the number of galaxies of a given type should be
approximately constant at all redshifts (that is, throughout the
history of the universe), whereas the second predicts that there
number should decrease with increasing redshift (that is,
decreasing age).
3) Attempts to discriminate between the two models focus mostly
on elliptical galaxies, which are easier to study than spiral
ones. Present-epoch ellipticals form a very homogeneous family
with very similar intrinsic properties. Compared with the
heterogeneous family of spiral galaxies, ellipticals in the local
universe have little or no dust, gas, and star formation activity
(5). Furthermore, they are mostly if not exclusively composed of
an old stellar population, about as old as the universe, with
very similar relative ages. This fact is responsible for the most
distinctive property of ellipticals: their color. Ellipticals are
the reddest galaxies in the local universe (5).
4) Neither galaxy formation model can be discarded convincingly,
although, until recently, the monolithic collapse scenario had to
contend with one important, albeit indirect, piece of evidence
against it. If elliptical galaxies all formed at high redshift in
a single event, for a short period they must have had very strong
star formation activity. Simple model calculations indicate that
galaxies with so many young and bright stars should be luminous
enough to be observable with current telescopes, despite their
large distances. But they were never observed.
References (abridged):
1. O. J. Eggen, D. Lynden-Bell, A. Sandage, Astrophys. J. 136,
748 (1962)
2. R. B. Larson, Mon. Not. R. Astron. Soc. 166, 585 (1974)
3. S. D. M. White, M. J. Rees, Mon. Not. R. Astron. Soc. 183, 341
(1978)
4. S. Cole et al., Mon. Not. R. Astron. Soc. 271, 781 (1994)
5. M. Roberts, M. P. Haynes, Annu. Rev. Astron. Astrophys. 32,
115 (1994)
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