dSph

Dwarf spheroidal galaxies, abbreviated dSph, are small, low-luminosity galaxies characterized by spheroidal stellar distributions, little or no gas, and a lack of recent star formation. In the Local Group, many dSphs orbit larger galaxies such as the Milky Way and Andromeda. They are among the most dark matter–dominated systems known, with high mass-to-light ratios inferred from stellar kinematics despite their faint stellar content.

Their stellar populations are generally old and metal-poor, and they show little dust. Morphologically they are diffuse and smooth in integrated light, with half-light radii of a few hundred parsecs and luminosities typically in the range 10^5 to 10^7 solar luminosities. The absence of substantial gas and ongoing star formation distinguishes dSphs from dwarf irregulars.

Kinematically, dSphs are dispersion-supported; their stars move in random orbits rather than coherent rotation. Radial velocity measurements and velocity dispersion profiles are used to infer the underlying dark matter halos and to test models of dark matter on small scales. Their proximity to massive hosts makes them susceptible to tidal effects, which can influence their structure and evolution, though many retain relatively undisturbed spheroidal morphologies.

Dwarf spheroidals are important laboratories for studying dark matter, early galaxy formation, and the evolution of the Local Group. They also serve as targets in searches for indirect dark matter signals, such as gamma-ray emission, though no definitive detections have been reported.

Notable examples in the Milky Way’s halo include Fornax, Sculptor, Draco, and Ursa Minor, with additional dwarfs known around Andromeda.