Leo I

Leo I is a dwarf spheroidal galaxy at a distance of about 820,000 light-years. It is a member of the Local Group of galaxies and is thought to be one of the most distant satellites of the Milky Way galaxy.

Mass

The measurement of radial velocities of some bright red giants in Leo I have made possible to measure its mass. It was found to be at least (2.0 ± 1.0) × 10⁷ M☉. The results are not conclusive, and do not deny or confirm the existence of a large dark matter halo around the galaxy. However, it seems to be certain that the galaxy does not rotate.

It has been suggested that Leo I is a tidal debris stream in the outer halo of the Milky Way. This hypothesis has not been confirmed, however.

Star formation

Typical to a dwarf galaxy, the metallicity of Leo I is very low. The galaxy experienced a major increase (accounting for 70% to 80% of its population) in its star formation rate between 6 Ga and 2 Ga (billion years ago). There is no significant evidence of any stars that are more than 10 Ga old. About 1 Ga ago, star formation in Leo I appears to have dropped suddenly to an almost negligible rate. Some low-level activity may have continued until 200-500 Ma. Therefore it may be the youngest dwarf spheroidal satellite galaxy of the Milky Way. In addition, the galaxy may be embedded in a cloud of ionized gas with a mass similar to that of the whole galaxy.

Globular clusters

No globular clusters have been found in the galaxy.