| Leo IV Dwarf Galaxy[1] | |
|---|---|
 View of the sparse stars of Leo IV, hardly distinguishable from the background.[2] | |
| Observation data (J2000epoch) | |
| 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 adwarf spheroidal galaxy situated in theLeoconstellation, discovered in 2006 in the data obtained by theSloan Digital Sky Survey.[3] The galaxy is located at the distance of about 160 kpc from theSun and moves away from the Sun with thevelocity of about 130 km/s.[3][5] It is classified as adwarf 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 is one of the smallest and faintest satellites of theMilky Way; its integrated luminosity is about15000 times that of the Sun (absolutevisible magnitude of−5.5±0.3), which is much lower than the luminosity of a typicalglobular cluster.[4] However, its mass is about 1.5 millionsolar masses, which means that Leo'smass to light ratio is around 150. A high mass to light ratio implies that Leo IV is dominated by thedark 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 primarilyred giants, although a number ofHorizontal branch stars including threeRR Lyrae variable stars were also discovered.[4][note 2] The stars of Leo IV were probably among the first stars to form in theUniverse. 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 complicatedstar formation history of this galaxy.[4] Currently there is no star formation in Leo IV. The measurements have so far failed to detect anyneutral hydrogen in it—the upper limit is just 600 solar masses.[7]
In 2008, another galaxy calledLeo 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]
