Boötes II

Short description: Dwarf spheroidal galaxy in the constellation Boötes

Boötes II Dwarf Galaxy[1]

Observation data (J2000 epoch)
Constellation| Boötes
Right ascension|  13h 58m 00s[1]
Declination| +12° 51′ 00″[1]
Distance| 136 ± 7 kly (42 ± 2 kpc)[2][3]
Apparent magnitude (V)| 15.8 ± 0.5[4]
Characteristics
Type| dSph[4]
Apparent size (V)| 8.0+2.2−2.8′[2]
Other designations
Boo II,[2] 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.[4] 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.[4][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 51 pc.[2][note 1]

Boötes II is one of the smallest and faintest satellites[note 2] 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.[2] However the mass of the galaxy is substantial corresponding to the mass to light ratio of more than 100.[3]

The stellar population of Boötes II consists mainly of moderately old stars formed 10–12 billion years ago.[5] The metallicity of these old stars is low at [Fe/H]=−1.8, which means that they contain 80 times less heavy elements than the Sun.[3][5] 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.[6]

Boötes II is located only 1.5 degrees (~1.6 kpc) away from another dwarf galaxy—Boötes I,[4] 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.[5]

## Notes

1. ↑ Other sources give the half-light radius of about 36 pc.[3]
2. ↑ Only Segue 1, Segue 2 and Willman 1 are fainter.[2]

## References

1. ↑ 1.0 1.1 1.2 "NAME Boötes II". SIMBAD. Centre de données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=NAME+Boötes+II.
2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Martin, Nicolas F.; de Jong, Jelte T. A.; Rix, Hans-Walter (September 2008). "A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites". The Astrophysical Journal 684 (2): 1075–1092. doi:10.1086/590336. Bibcode: 2008ApJ...684.1075M.
3. ↑ 3.0 3.1 3.2 3.3 Walsh, S.M. et al. (2008). "Boötes II ReBoöted: An MMT/MegaCam Study of an Ultrafaint Milky Way Satellite". The Astrophysical Journal 688 (1): 245–253. doi:10.1086/592076. Bibcode: 2008ApJ...688..245W.
4. ↑ 4.0 4.1 4.2 4.3 4.4 Walsh, S.M.; Jerjen, H.; Willman, B. (June 2007). "A Pair of Boötes: A New Milky Way Satellite". The Astrophysical Journal 662 (2): L83–L86. doi:10.1086/519684. Bibcode: 2007ApJ...662L..83W.
5. ↑ 5.0 5.1 5.2 5.3 Koch, Andreas et al. (January 2009). "A Spectroscopic Confirmation of the Boötes II Dwarf Spheroidal". The Astrophysical Journal 690 (1): 453–462. doi:10.1088/0004-637X/690/1/453. Bibcode: 2009ApJ...690..453K.
6. ↑ Grcevich, Jana; Putman, Mary E. (May 2009). "H I in Local Group Dwarf Galaxies and Stripping by the Galactic Halo". The Astrophysical Journal 696 (1): 385–395. doi:10.1088/0004-637X/696/1/385. Bibcode: 2009ApJ...696..385G.

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|

Sagittarius Spheroidal|

* Sagittarius Stream
* Boötes II
* Coma Berenices
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* Segue 1
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* Terzan 7

Dwarfs|

* Antlia 2
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Other|

* Gaia Sausage
* Monoceros Ring
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* Koposov I
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* Segue 3
* Willman 1

Related|

* Andromeda–Milky Way collision
* Baade's Window

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