Leo IV

Leo IV is a dwarf spheroidal galaxy located at the distance of about 160 kpc from the Milky Way. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an approximately round shape with the half-light radius of about 130 pc.

Leo IV is one of the smallest and faintest satellites of the Milky Way — its absolute visible magnitude of −5.5 ± 0.3 is much lower than the luminosity of a typical globular cluster. However, its mass is about 1.5 million stellar masses, which means that Leo IV's mass to light ratio is around 150. A high mass to light ratio implies that Leo IV is dominated by the dark matter.

The stellar population of Leo IV 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] ≈ −2.58 ± 0.75. The observed stars are primarily red giants, although a number of Horizontal branch stars including three variable RR Lyrae stars have also been discovered. The stars of Leo IV were probably among the first stars to form in the Universe. Nevertheless, the detailed study of the stellar population revealed the presence of a small number of much younger stars with the age of about 2 billion years or less. This discovery points to a complicated star formation history of this galaxy. Currently there is no star formation in Leo IV. The measurements have so far failed to detect any neutral hydrogen in it.

Another galaxy, called Leo V is in the vicinity of Leo IV. The former is located 20 kpc further from the Milky Way than the latter and ~10 kpc away from it. These two galaxies may be physically associated with each other.