Coma Berenices

Coma Berenices or Com is a dwarf spheroidal galaxy located at a distance of about 44 kpc. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elliptical (ratio of axes ~ 5:3) shape with the half-light radius of about 70 pc.

Com is one of the smallest and faintest satellites of the Milky Way — its absolute visible magnitude of about −4.1 is much lower than the luminosity of the majority of globular clusters. However, its mass is about 1.2 million stellar masses, which means that galaxy's mass to light ratio is around 450. A high mass to light ratio implies that Com is dominated by the dark matter.

The stellar population of Com consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at [Fe/H] ≈ −2.53 ± 0.45. The stars of Com were probably among the first stars to form in the Universe. Currently there is no star formation in Com. The measurements have so far failed to detect any neutral hydrogen in it.

Coma Dwarf is located near the Sagittarius Stream, which is made of stars stripped from the Sagittarius Dwarf Elliptical Galaxy. This association may indicate that Com is a former satellite of star cluster from that galaxy.