THE LEAST-LUMINOUS GALAXY: SPECTROSCOPY OF THE MILKY WAY SATELLITE SEGUE 1

We present Keck/DEIMOS spectroscopy of Segue 1, an ultra-low-luminosity (M(V) = -1.5(-0.8)(+0.6)) Milky Way satellite companion. While the combined size and luminosity of Segue 1 are consistent with either a globular cluster or a dwarf galaxy, we present spectroscopic evidence that this object is a dark matter-dominated dwarf galaxy. We identify 24 stars as members of Segue 1 with a mean heliocentric recession velocity of 206 +/- 1.3 km s(-1). Although Segue 1 spatially overlaps the leading arm of the Sagittarius stream, its velocity is 100 km s(-1) different from that predicted for recent Sagittarius tidal debris at this position. We measure an internal velocity dispersion of 4.3 +/- 1.2 km s(-1). Under the assumption that these stars are gravitationally bound and in dynamical equilibrium, we infer a total mass of 4.5(2.5)(+4.7) x 10(5)M(circle dot) in the mass-follow-light case; using a two-component maximum-likelihood model, we determine a mass within 50 pc of 8.7(-5.2)(+13) x 105M(circle dot). These imply mass-to-light (M/L) ratios of ln(M/L(V)) = 7.2(-1.2)(+1.1) (M/L(V) = 1320(-940)(+2680)) and M/L(V) = 2440(-1775)(+1580), respectively. The error distribution of the M/L is nearly lognormal, thus Segue 1 is dark matter-dominated at a high significance. Although we cannot rule out the possibility that Segue 1 has been tidally disrupted, we do not find kinematic evidence supporting tidal effects. Using spectral synthesis modeling, we derive a metallicity for the single red giant branch star in our sample of [Fe/H] = -3.3 +/- 0.2 dex. Finally, we discuss the prospects for detecting gamma rays from annihilation of dark matter particles and show that Segue 1 is the most promising satellite for indirect dark matter detection. We conclude that Segue 1 is the least luminous of the ultra-faint galaxies recently discovered around the Milky Way, and is thus the least-luminous known galaxy.