There's more to the cosmos than meets the eye. About 80 percent of the matter in the universe is invisible to telescopes, yet its gravitational influence is manifest in the orbital speeds of stars around galaxies and in the motions of clusters of galaxies. Yet, despite decades of effort, no one knows what this "dark matter" really is. Many scientists think it's likely that the mystery will be solved with the discovery of new kinds of subatomic particles, types necessarily different from those composing atoms of the ordinary matter all around us. The search to detect and identify these particles is underway in experiments both around the globe and above it.
Scientists working with data from NASA's Fermi Gamma-ray Space Telescope have looked for signals from some of these hypothetical particles by zeroing in on 10 small, faint galaxies that orbit our own. Although no signals have been detected, a novel analysis technique applied to two years of data from the observatory's Large Area Telescope (LAT) has essentially eliminated these particle candidates for the first time. See: Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter
04.02.12
Dwarf spheroidal galaxy (dSph) is a
term in
astronomy applied to low luminosity
galaxies that are companions to the
Milky Way and to the similar systems that are companions to the
Andromeda Galaxy M31. While similar to
dwarf elliptical galaxies in appearance and properties such as little to no
gas or dust or recent
star formation, they are approximately
spheroidal in shape, generally lower luminosity, and are only recognized as satellite galaxies in the
Local Group.
[1]
While there were nine "classical" dSph galaxies discovered up until 2005, the
Sloan Digital Sky Survey
has resulted in the discovery of 11 more dSph galaxies—this has
radically changed the understanding of these galaxies by providing a
much larger sample to study.
[2]
Recently, as growing evidence has indicated that the vast majority of
dwarf ellipticals have properties that are not at all similar to
elliptical galaxies, but are closer to irregular and late-type spiral
galaxies, this term has been used to refer to all of the galaxies that
share the properties of those above. These sorts of galaxies may in fact
be the most common type of galaxies in the universe, but are much
harder to see than other types of galaxies because they are so faint.
Because of the faintness of the lowest luminosity dwarf spheroidals
and the nature of the stars contained within them, some astronomers
suggest that dwarf spheroidals and
globular clusters may not be clearly separate and distinct types of objects.
[3]
Other recent studies, however, have found a distinction in that the
total amount of mass inferred from the motions of stars in dwarf
spheroidals is many times that which can be accounted for by the mass of
the stars themselves. In the current predominantly accepted
Cold Dark Matter cosmology, this is seen as a sure sign of
dark matter,
and the presence of dark matter is often cited as a reason to classify
dwarf spheroidals as a different class of object from globular clusters
(which show little to no signs of dark matter). Because of the extremely
large amounts of dark matter in these objects, they may deserve the
title "most dark matter-dominated galaxies"
[4]
See also
External links
References
- ^ Mashchenko,
Sergey; Sills, Alison; Couchman, H. M. (March 2006), "Constraining
Global Properties of the Draco Dwarf Spheroidal Galaxy", The Astrophysical Journal 640 (1): 252–269, arXiv:astro-ph/0511567, Bibcode 2006ApJ...640..252M, DOI:10.1086/499940
- ^ Simon, Josh; Geha, Marla (November 2007), "The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem", The Astrophysical Journal 670 (1): 313–331, Bibcode 2007ApJ...670..313S, DOI:10.1086/521816
- ^ van den Bergh, Sidney (November 2007), "Globular Clusters and Dwarf Spheroidal Galaxies", MNRAS (Letters), in press 385 (1): L20, arXiv:0711.4795, Bibcode 2008MNRAS.385L..20V, DOI:10.1111/j.1745-3933.2008.00424.x
- ^ Strigari, Louie;
Koushiappas, et al; Bullock, James S.; Kaplinghat, Manoj; Simon,
Joshua D.; Geha, Marla; Willman, Beth (September 2007), "The Most Dark
Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest
Milky Way Dwarfs", The Astrophysical Journal 678 (2): 614, arXiv:0709.1510, Bibcode 2008ApJ...678..614S, DOI:10.1086/529488
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