Leo IV (dwarf galaxy)

Leo IV is a dwarf spheroidal galaxy situated in the Leo constellation, discovered in 2006 in the data obtained by the Sloan Digital Sky Survey.^[3] The galaxy is located at the distance of about 160 kpc from the Sun and moves away from the Sun with the velocity of about 130 km/s.^[3][5] 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.^{[4][note 1]}

Leo IV Dwarf Galaxy[1]
View of the sparse stars of Leo IV, hardly distinguishable from the background.[2]
Observation data (J2000 epoch)
Constellation	Leo
Right ascension	11h 32m 57s[1]
Declination	−00° 32′ 00″[1]
Distance	520+49 −45 kly (160+15 −14 kpc)[3] 154±4 kpc[4]
Apparent magnitude (V)	15.9±0.5[3]
Characteristics
Type	dSph[3]
Apparent size (V)	5.7′[4]
Other designations
Leo IV,[1] PGC 4713561

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

1. From other sources the half-radius is around 160 pc.^[3]
2. The distance to Leo IV measured using RR Lyrae stars is 154±4 kpc.^[4]

References

1. "NAME Leo IV Dwarf Galaxy". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2010-01-31.
2. "Hubble Unmasks Ghost Galaxies". ESA/Hubble Press Release. Retrieved 11 July 2012.
3. Belokurov, 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. S2CID 18617277.
4. Sand, David J.; Seth, Anil; Olszewski, Edward W.; et al. (2010). "A Deeper Look at Leo IV: Star Formation History and Extended Structure". The Astrophysical Journal. 718 (1): 530–42. arXiv:0911.5352. Bibcode:2010ApJ...718..530S. doi:10.1088/0004-637X/718/1/530. S2CID 32122744.
5. Simon, J. D.; Geha, M. (2007). "The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem". The Astrophysical Journal. 670 (1): 313–331. arXiv:0706.0516. Bibcode:2007ApJ...670..313S. doi:10.1086/521816. S2CID 9715950.
6. Kirby, E. N.; Simon, J. D.; Geha, M.; Guhathakurta, P.; Frebel, A. (2008). "Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies". The Astrophysical Journal. 685 (1): L43–L46. arXiv:0807.1925. Bibcode:2008ApJ...685L..43K. doi:10.1086/592432. S2CID 3185311.
7. Grcevich, J.; Putman, M. E. (2009). "H I in Local Group Dwarf Galaxies and Stripping by the Galactic Halo". The Astrophysical Journal. 696 (1): 385–395. arXiv:0901.4975. Bibcode:2009ApJ...696..385G. doi:10.1088/0004-637X/696/1/385.
8. Belokurov, V.; Walker, M. G.; Evans, N. W.; et al. (2008). "Leo V: A companion of a companion of the Milky Way galaxy". The Astrophysical Journal. 686 (2): L83–L86. arXiv:0807.2831. Bibcode:2008ApJ...686L..83B. doi:10.1086/592962. S2CID 11090182.