We present the results of a comprehensive Keck/DEIMOS spectroscopic survey of the ultra-faint Milky Way satellite galaxy Segue 1 .
We have obtained velocity measurements for 98.2 % of the stars within 67 pc ( 10′ , or 2.3 half-light radii ) of the center of Segue 1 that have colors and magnitudes consistent with membership , down to a magnitude limit of r = 21.7 .
Based on photometric , kinematic , and metallicity information , we identify 71 stars as probable Segue 1 members , including some as far out as 87 pc .
After correcting for the influence of binary stars using repeated velocity measurements , we determine a velocity dispersion of 3.7 ^ { +1.4 } _ { -1.1 } km s ^ { -1 } .
The mass within the half-light radius is 5.8 ^ { +8.2 } _ { -3.1 } \times 10 ^ { 5 } M _ { \odot } .
The stellar kinematics of Segue 1 require very high mass-to-light ratios unless the system is far from dynamical equilibrium , even if the period distribution of unresolved binary stars is skewed toward implausibly short periods .
With a total luminosity less than that of a single bright red giant and a V-band mass-to-light ratio of 3400 M _ { \odot } /L _ { \odot } , Segue 1 is the darkest galaxy currently known .
We critically re-examine recent claims that Segue 1 is a tidally disrupting star cluster and that kinematic samples are contaminated by the Sagittarius stream .
The extremely low metallicities ( [ Fe / H ] < -3 ) of two Segue 1 stars and the large metallicity spread among the members demonstrate conclusively that Segue 1 is a dwarf galaxy , and we find no evidence in favor of tidal effects .
We also show that contamination by the Sagittarius stream has been overestimated .
Segue 1 has the highest estimated dark matter density of any known galaxy and will therefore be a prime testing ground for dark matter physics and galaxy formation on small scales .