Boötes I
An ultrafaint dwarf spheroidal with two crossing tidal tails.
Boötes I (Boo I dSph) is an ultrafaint dwarf spheroidal galaxy discovered in 2006. It is notable for being one of the more luminous ultrafaint dwarf galaxies, though its entire system is fainter than the Rigel system. The galaxy appears significantly elongated and is tidally disrupted by the Milky Way, exhibiting two stellar tails that cross to form a cross—an unusual feature, as tidally disrupted galaxies typically produce only one tail.
Quick Facts
- Epoch
- J2000
- Ra
- 14 · 00 · 06
- Dec
- +14 · 30 · 00 ± 15
- Dist Ly
- 197 ± 18 kly (60 ± 6 kpc)
- Appmag V
- 13.1A · a · none
- Constellation Name
- Boötes
- Names
- Boo dSph, Boötes Satellite, Boötes Dwarf Spheroidal Galaxy, Boötes dSph galaxy, PGC 4713553
Facts from the source article.
Lore & Background
Boötes I orbits the Milky Way Galaxy and lies approximately 197,000 light-years away in the constellation Boötes. Its stellar population is predominantly very old, with two populations having ages of 13.4 billion and 13.3 billion years; most stars belong to the younger of these two groups. The galaxy is metal-poor, with a mean metallicity of −2.34, consistent with other ultrafaint dwarf spheroidals.
The galaxy appears to be tidally disrupted by the Milky Way, and it possesses two stellar tails that cross over each other to form a cross. This is unusual because tidally disrupted galaxies usually form only one tail. The galaxy is significantly elongated, with an ellipticity of ε = 0.68 ± 0.15.
Reader's Guide
Boötes I holds significance as one of the more luminous ultrafaint dwarf spheroidal galaxies, yet its entire system is fainter than the Rigel system (absolute magnitude –7.84). Its discovery in 2006 added to the growing census of Milky Way satellite galaxies. The galaxy's unusual tidal disruption—producing two crossing stellar tails rather than the typical single tail—provides a rare case study in galactic interactions and the effects of the Milky Way's gravitational field on its smaller companions. The presence of two distinct but nearly coeval stellar populations, both extremely old, suggests a star formation history that was brief and early, with little subsequent activity. Its low metallicity and faintness place it among the ultrafaint dwarfs that are thought to be relics of the early universe, offering clues to the formation of the smallest galaxies and the process of hierarchical assembly.
Did You Know?
- Boötes I has an absolute magnitude of –5.8, making it fainter than the Rigel system.
- The galaxy has two stellar tails that cross to form a cross, unlike most tidally disrupted galaxies which have only one tail.
- Its stellar population consists of two groups with ages 13.4 billion and 13.3 billion years.
- The galaxy's ellipticity is 0.68 ± 0.15, indicating significant elongation.
More in Elliptical and Irregular Galaxies, Part 5 1-24
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