Leo IV (dwarf galaxy)

{{Short description|Galaxy in the constellation Leo}}

{{Infobox Galaxy

| image = Leo IV dwarf galaxy.jpeg

| image_size = 300px

| caption = View of the sparse stars of Leo IV, hardly distinguishable from the background.{{cite news|title=Hubble Unmasks Ghost Galaxies|url=http://www.spacetelescope.org/news/heic1211/|access-date=11 July 2012|newspaper=ESA/Hubble Press Release}}

| name = Leo IV Dwarf Galaxy{{cite simbad|title=NAME Leo IV Dwarf Galaxy|accessdate=2010-01-31}}

| epoch = J2000

| type = dSph

| ra = {{RA|11|32|57}}

| dec = {{DEC|−00|32|00}}

| dist_ly = {{val|520|+49|-45|ul=kly}} ({{val|160|+15|-14|ul=kpc}})
{{val|154|4|u=kpc}}

| z =

| appmag_v = {{val|15.9|0.5}}

| size_v = 5.7{{prime}}

| constellation name = Leo

| names = Leo IV, PGC 4713561

}}

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.{{Cite journal | last1 = Belokurov | first1 = V. | last2 = Zucker | first2 = D. B. | last3 = Evans | first3 = N. W. | last4 = Kleyna | first4 = J. T. | last5 = Koposov | first5 = S. | last6 = Hodgkin | first6 = S. T. | last7 = Irwin | first7 = M. J. | last8 = Gilmore | first8 = G. |author-link8 = Gerard F. Gilmore| last9 = Wilkinson | first9 = M. I. | last10 = Fellhauer | doi = 10.1086/509718 | first10 = M. | last11 = Bramich | first11 = D. M. | last12 = Hewett | first12 = P. C. | last13 = Vidrih | first13 = S. | last14 = De Jong | first14 = J. T. A. | last15 = Smith | first15 = J. A. | last16 = Rix | first16 = H. -W. | last17 = Bell | first17 = E. F. | last18 = Wyse | first18 = R. F. G. | last19 = Newberg | first19 = H. J. | last20 = Mayeur | first20 = P. A. | last21 = Yanny | first21 = B. | last22 = Rockosi | first22 = C. M. |author-link22=Constance M. Rockosi| last23 = Gnedin | first23 = O. Y. | last24 = Schneider | first24 = D. P. | last25 = Beers | first25 = T. C. | last26 = Barentine | first26 = J. C. | last27 = Brewington | first27 = H. | last28 = Brinkmann | first28 = J. | last29 = Harvanek | first29 = M. | last30 = Kleinman | first30 = S. J. | title = Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions | journal = The Astrophysical Journal | volume = 654 | issue = 2 | pages = 897–906 | year = 2007 |arxiv = astro-ph/0608448 |bibcode = 2007ApJ...654..897B | s2cid = 18617277 }} 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. 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.{{#tag:ref|From other sources the half-radius is around 160 pc.|group=note}}

Leo IV is one of the smallest and faintest satellites of the Milky Way; its integrated luminosity is about {{val|15000}} times that of the Sun (absolute visible magnitude of {{val|-5.5|0.3}}), which is much lower than the luminosity of a typical globular cluster. 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.{{Cite journal | last1 = Simon | first1 = J. D. | last2 = Geha | first2 = M. |author2-link= Marla Geha | doi = 10.1086/521816 | title = The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem | journal = The Astrophysical Journal | volume = 670 | pages = 313–331 | year = 2007 | issue = 1 |arxiv = 0706.0516 |bibcode = 2007ApJ...670..313S | s2cid = 9715950 }}

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 {{nowrap|[Fe/H] ≈ −2.58 ± 0.75}}, which means that they contain 400 times less heavy elements than the Sun.{{Cite journal | last1 = Kirby | first1 = E. N. | last2 = Simon | first2 = J. D. | last3 = Geha | first3 = M.|author3-link= Marla Geha | last4 = Guhathakurta | first4 = P. | last5 = Frebel | first5 = A. | title = Uncovering Extremely Metal-Poor Stars in the Milky Way's Ultrafaint Dwarf Spheroidal Satellite Galaxies | doi = 10.1086/592432 | journal = The Astrophysical Journal | volume = 685 | pages = L43–L46 | year = 2008 | issue = 1 |arxiv = 0807.1925 |bibcode = 2008ApJ...685L..43K | s2cid = 3185311 }} The observed stars were primarily red giants, although a number of Horizontal branch stars including three RR Lyrae variable stars were also discovered.{{#tag:ref|The distance to Leo IV measured using RR Lyrae stars is {{val|154|4|u=kpc}}.|group=note}} 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.{{cite journal|last1=Sand|first1=David J. |last2=Seth|first2=Anil |last3=Olszewski|first3=Edward W. |display-authors=etal |title=A Deeper Look at Leo IV: Star Formation History and Extended Structure|date=2010|journal=The Astrophysical Journal|volume=718|issue=1 |pages=530–42 |bibcode=2010ApJ...718..530S|doi=10.1088/0004-637X/718/1/530|arxiv=0911.5352|s2cid=32122744 }} 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.{{Cite journal | last1 = Grcevich | first1 = J. | last2 = Putman | first2 = M. E. | doi = 10.1088/0004-637X/696/1/385 | title = H I in Local Group Dwarf Galaxies and Stripping by the Galactic Halo | journal = The Astrophysical Journal | volume = 696 | pages = 385–395 | year = 2009 | issue = 1 |arxiv = 0901.4975 |bibcode = 2009ApJ...696..385G }}

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.{{cite journal|last1=Belokurov|first1=V. |last2=Walker|first2=M. G. |last3=Evans|first3=N. W. |display-authors=etal |title=Leo V: A companion of a companion of the Milky Way galaxy|date=2008|journal=The Astrophysical Journal|volume=686|issue=2|pages=L83–L86|doi=10.1086/592962|bibcode=2008ApJ...686L..83B|arxiv=0807.2831|s2cid=11090182 }}