Dwarf Spheroidal Galaxies
1. IntroductionThe term dwarf spheroidal (dSph) is applied to the twelve low luminosity dwarf elliptical galaxies that are companions to the Milky Way and to the similar systems that are companions to M31 (The Andromeda Galaxy). Although often thought in the past as merely large, low density globular clusters, detailed studies over the last 20 years or so have revealed that the dSph galaxies possess a more diverse set of properties and contain more complex stellar populations than the globular cluster analogy would predict. Indeed an alternative definition of a dSph might now be a low luminosity M(V)> -14, non-nucleated dwarf elliptical galaxy with low surface brightness (fainter than 22 V magnitude per square arcsecond). Since the individual stars in dSph galaxies can be resolved, their study will contribute to the understanding of the origin and evolution of dwarf galaxies in general.
2. Stellar PopulationsAn interesting result from the more recent studies of the dSph:s is the indication that they, to a greater or lesser extent, show evidence for star formation over extended periods. This result was unexpected given that dSph:s show no sign of current or recent star formation and have almost no detectable hydrogen (HI). The stellar populations of dwarf spheroidals consists of two basic components. These are:
3. Dark MatterThe dSph galaxies have larger mass-to-light(M/L) ratios than globular clusters, indicating that dark matter makes up large fractions of their masses and also unlike globulars there is a relationship between M/L and luminosity giving higher M/L for lower luminosity. There are also indications that the dSph:s closer to the Galactic center have the highest M/L and lowest luminosities. The central issue here is the calculation of masses from the measured velocity dispersions using the virial theorem, which demands that dSph:s are in dynamical equilibrium. If they are equilibrium systems, then a dark-matter component is required.
4. Luminosity - Abundance RelationAmong the most important properties for understanding the evolution of the dSph galaxies is the increase in their mean metallicity [Fe/H] with increasing luminosity. An approximate expression for this relation given by Zinn 1993 is [Fe/H]=-3.50-0.15M(V). There is also a luminosity - surface brightness relation where the most luminous galaxies have the highest surface brightness.
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