Segue 2 , discovered by Belokurov et al .
( 4 ) , is a galaxy with a luminosity of only 900 ~ { } L _ { \sun } .
We present Keck/DEIMOS spectroscopy of 25 members of Segue 2—a threefold increase in spectroscopic sample size .
The velocity dispersion is too small to be measured with our data .
The upper limit with 90 % ( 95 % ) confidence is \sigma _ { v } < 2.2 ( 2.6 ) km s ^ { -1 } , the most stringent limit for any galaxy .
The corresponding limit on the mass within the 3-D half-light radius ( 46 pc ) is M _ { 1 / 2 } < 1.5 ( 2.1 ) \times 10 ^ { 5 } ~ { } M _ { \sun } .
Segue 2 is the least massive galaxy known .
We identify Segue 2 as a galaxy rather than a star cluster based on the wide dispersion in [ Fe/H ] ( from -2.85 to -1.33 ) among the member stars .
The stars ’ [ \alpha /Fe ] ratios decline with increasing [ Fe/H ] , indicating that Segue 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100 Myr .
The mean metallicity , \langle [ Fe / H ] \rangle = -2.22 \pm 0.13 ( about the same as the Ursa Minor galaxy , 330 times more luminous than Segue 2 ) , is higher than expected from the luminosity–metallicity relation defined by more luminous dwarf galaxy satellites of the Milky Way .
Segue 2 may be the barest remnant of a tidally stripped , Ursa Minor-sized galaxy .
If so , it is the best example of an ultra-faint dwarf galaxy that came to be ultra-faint through tidal stripping .
Alternatively , Segue 2 could have been born in a very low-mass dark matter subhalo ( v _ { max } < 10 km s ^ { -1 } ) , below the atomic hydrogen cooling limit .