Leo IV is one of the smallest and faintest satellites of the Milky Way; its integrated luminosity is about 15000 times that of the Sun (absolute visible magnitude of −5.5±0.3), which is much lower than the luminosity of a typical globular cluster.[4] However, its mass is about 1.5 million solar masses, which means that Leo's mass to light ratio is around 150. A high mass to light ratio implies that Leo IV is dominated by the dark matter.[5]
The stellar population of Leo IV consists mainly of old stars formed more than 12 billion years ago.[4] The metallicity of these old stars is also very low at [Fe/H] ≈ −2.58 ± 0.75, which means that they contain 400 times less heavy elements than the Sun.[6] The observed stars were primarily red giants, although a number of Horizontal branch stars including three RR Lyrae variable stars were also discovered.[4][note 2] 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.[4] Currently there is no star formation in Leo IV. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is just 600 solar masses.[7]
In 2008, another galaxy called Leo V was discovered in the vicinity of Leo IV. The former is located 20 kpc further from the Milky Way than the latter and 3 degrees (~ 10 kpc) away from it. These two galaxies may be physically associated with each other.[8]
Notes
^From other sources the half-radius is around 160 pc.[3]
^The distance to Leo IV measured using RR Lyrae stars is 154±4 kpc.[4]
^ abcdefBelokurov, V.; Zucker, D. B.; Evans, N. W.; Kleyna, J. T.; Koposov, S.; Hodgkin, S. T.; Irwin, M. J.; Gilmore, G.; Wilkinson, M. I.; Fellhauer, M.; Bramich, D. M.; Hewett, P. C.; Vidrih, S.; De Jong, J. T. A.; Smith, J. A.; Rix, H. -W.; Bell, E. F.; Wyse, R. F. G.; Newberg, H. J.; Mayeur, P. A.; Yanny, B.; Rockosi, C. M.; Gnedin, O. Y.; Schneider, D. P.; Beers, T. C.; Barentine, J. C.; Brewington, H.; Brinkmann, J.; Harvanek, M.; Kleinman, S. J. (2007). "Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions". The Astrophysical Journal. 654 (2): 897–906. arXiv:astro-ph/0608448. Bibcode:2007ApJ...654..897B. doi:10.1086/509718. S2CID18617277.
