Boötes II

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 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 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. "NAME Boötes II". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2010-02-13.
2. 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. arXiv:0805.2945. Bibcode:2008ApJ...684.1075M. doi:10.1086/590336. S2CID 17838966.
3. Walsh, S.M.; Willman, B.; Sand, D.; et al. (2008). "Boötes II ReBoöted: An MMT/MegaCam Study of an Ultrafaint Milky Way Satellite". The Astrophysical Journal. 688 (1): 245–253. arXiv:0712.3054. Bibcode:2008ApJ...688..245W. doi:10.1086/592076. S2CID 244191.
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. arXiv:0705.1378. Bibcode:2007ApJ...662L..83W. doi:10.1086/519684. S2CID 17830926.
5. Koch, Andreas; Wilkinson, Mark I.; Kleyna, Jan T.; et al. (January 2009). "A Spectroscopic Confirmation of the Boötes II Dwarf Spheroidal". The Astrophysical Journal. 690 (1): 453–462. arXiv:0809.0700. Bibcode:2009ApJ...690..453K. doi:10.1088/0004-637X/690/1/453. S2CID 14365307.
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. arXiv:0901.4975. Bibcode:2009ApJ...696..385G. doi:10.1088/0004-637X/696/1/385.