Boötes II

{{Short description|Dwarf spheroidal galaxy in the constellation Boötes}}

{{ Infobox Galaxy

| image =

| caption =

| name = Boötes II Dwarf Galaxy{{cite simbad

|title=NAME Boötes II

|access-date=2010-02-13}}

| epoch = J2000

| type = dSph

| ra = {{RA|13|58|00}}

| dec = {{DEC|+12|51|00}}

| dist_ly = 136 ± 7 kly {{nowrap|(42 ± 2 kpc)}}

| z =

| appmag_v = 15.8 ± 0.5

| size_v = 8.0{{±|2.2|2.8}}{{prime}}

| constellation name = Boötes

| names = Boo II, PGC 4713552

}}

Boötes II or Boo II is a dwarf spheroidal galaxy situated in the constellation Boötes and discovered in 2007 in the data obtained by Sloan Digital Sky Survey.{{cite journal|last1=Walsh|first1=S.M.|last2=Jerjen |first2=H. |last3=Willman |first3=B. |title=A Pair of Boötes: A New Milky Way Satellite|date=June 2007|journal=The Astrophysical Journal|volume=662|issue=2|pages=L83–L86|doi=10.1086/519684|bibcode=2007ApJ...662L..83W|arxiv=0705.1378 |s2cid=17830926}} The galaxy is located at the distance of about 42 kpc from the Sun and moves towards the Sun with the speed of 120 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 51 pc.{{cite journal

| last1 = Martin

| first1 = Nicolas F.

| last2 = de Jong

| first2 = Jelte T. A.

| last3 = Rix

| first3 = Hans-Walter

| date = September 2008

| title = A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites

| journal = The Astrophysical Journal

| volume = 684

| issue = 2

| pages = 1075–1092

| bibcode = 2008ApJ...684.1075M

| doi = 10.1086/590336

|arxiv = 0805.2945 | s2cid = 17838966

}}{{refn|Other sources give the half-light radius of about 36 pc.|group=note}}

Boötes II is one of the smallest and faintest satellites{{refn|Only Segue 1, Segue 2 and Willman 1 are fainter.|group=note}} of the Milky Way—its integrated luminosity is about 1,000 times that of the Sun (absolute visible magnitude of about −2.7), which is much lower than the luminosity of the majority of globular clusters. However the mass of the galaxy is substantial corresponding to the mass to light ratio of more than 100.{{cite journal|last1=Walsh|first1=S.M.|last2=Willman |first2=B. |last3=Sand |first3=D. |title=Boötes II ReBoöted: An MMT/MegaCam Study of an Ultrafaint Milky Way Satellite|date=2008|journal=The Astrophysical Journal|volume=688|issue=1|pages=245–253|doi=10.1086/592076|bibcode=2008ApJ...688..245W|arxiv=0712.3054 |s2cid=244191|display-authors=etal}}

The stellar population of Boötes II consists mainly of moderately old stars formed 10–12 billion years ago. The metallicity of these old stars is low at {{nowrap|[Fe/H]{{=}}−1.8}}, which means that they contain 80 times less heavy elements than the Sun. Currently there is no star formation in Boötes II. The measurements have so far failed to detect any neutral hydrogen in it—the upper limit is only 86 solar masses.{{cite journal

| last1 = Grcevich

| first1 = Jana

| last2 = Putman

| first2 = Mary E.

| date = May 2009

| title = H I in Local Group Dwarf Galaxies and Stripping by the Galactic Halo

| journal = The Astrophysical Journal

| volume = 696

| issue = 1

| pages = 385–395

| bibcode = 2009ApJ...696..385G

| doi = 10.1088/0004-637X/696/1/385

|arxiv = 0901.4975 }}

Boötes II is located only 1.5 degrees (~1.6 kpc) away from another dwarf galaxy—Boötes I, although they are unlikely to be physically associated because they move in opposite directions relative to the Milky Way. Their relative velocity—about 200 km/s is too high. It is more likely associated with the Sagittarius Stream and, therefore, with the Sagittarius Dwarf Elliptical Galaxy (SagDEG). Boötes II may be either a satellite galaxy of SagDEG or one of its star clusters torn from the main galaxy 4–7 billion years ago.{{cite journal |last1=Koch|first1=Andreas |last2=Wilkinson|first2=Mark I. |last3=Kleyna|first3=Jan T.|title=A Spectroscopic Confirmation of the Boötes II Dwarf Spheroidal |date=January 2009 |journal=The Astrophysical Journal |volume=690 |issue=1 |pages=453–462 |doi=10.1088/0004-637X/690/1/453 |bibcode=2009ApJ...690..453K |arxiv=0809.0700 |s2cid=14365307 |display-authors=etal}}