dwarf spheroidal galaxy
A low-luminosity (less than absolute visual magnitude -14), low-surface-brightness
dwarf elliptical galaxy of near-spherical
shape that lacks a nucleus. Although often thought in the past to be merely
large, low-density globular clusters,
recent studies have shown that dwarf spheroidal (dSph) galaxies have a more
complex stellar population than that found in globulars. Most importantly,
dwarf spheroidals show evidence for star
formation over extended periods, even though they show no sign of current
or recent star formation and have no detectable interstellar matter.
|Leo I: a dwarf spheroidal galaxy that is a remote
satellite of the Milky Way. Image: Sloan Digital Sky Survey
The stellar populations of dwarf spheroidals consists of two basic components:
The mass-to-light ratios of dSph's is also higher than that of globular
clusters, indicating that these galaxies contain a significant amount of
dark matter. In the Local
Group, 22 dwarf spheroidals are known including 12 that are satellites
of the Milky Way Galaxy and a further 7 that
are satellites of the Andromeda Galaxy.
Some recent estimates suggest that these nearby dSph's have a dark matter
content within the sphere of visible stars of about 4–5 × 107
[see refs 1, 2]. They may also have a large dark halo that extends well
beyond the luminous volume. Their luminous mass content (i.e. total stellar
mass) appears to be typically in the range 107–108
- an old metal-poor population similar to that of globular clusters
- an intermediate-age population, whose ages range from one to 10 billion
- Wilkinson, Mark I., Kleyna, Jan, T., et al. "Probing the dark matter
content of Local group dwarf spheroidal galaxies with FLAMES." Jun 2006.
paper in PDF (European Southern Observatory).
- Biermann, Peter, L. and Munyaneza, Faustin. "The Nature of Dark Matter."
To be published in Proceedings of the International School of Astrophysics
at Ultra-high Energies, 20-27 June 2006, Erice, Sicily, Italy.