Andromeda X ( And X ) is a newly discovered low-luminosity M31 dwarf spheroidal galaxy ( dSph ) found by Zucker et al .
( 2007 ) in the Sloan Digital Sky Survey ( SDSS – York et al .
2000 ) .
In this paper , we present the first spectroscopic study of individual red giant branch stars in And X , as a part of the SPLASH Survey ( Spectroscopic and Photometric Landscape of Andromeda ’ s Stellar Halo ) .
Using the Keck II telescope and multiobject DEIMOS spectrograph , we target two spectroscopic masks over the face of the galaxy and measure radial velocities for \sim 100 stars with a median accuracy of \sigma _ { v } \sim 3 km s ^ { -1 } .
The velocity histogram for this field confirms three populations of stars along the sight line : foreground Milky Way dwarfs at small negative velocities , M31 halo red giants over a broad range of velocities , and a very cold velocity “ spike ” consisting of 22 stars belonging to And X with v _ { rad } = -163.8 \pm 1.2 km s ^ { -1 } .
By carefully considering both the random and systematic velocity errors of these stars ( e.g. , through duplicate star measurements ) , we derive an intrinsic velocity dispersion of just \sigma _ { v } = 3.9 \pm 1.2 km s ^ { -1 } for And X , which for its size , implies a minimum mass-to-light ratio of M / L _ { V } = 37 ^ { +26 } _ { -19 } assuming the mass traces the light .
Based on the clean sample of member stars , we measure the median metallicity of And X to be [ Fe/H ] = - 1.93 \pm 0.11 , with a slight radial metallicity gradient .
The dispersion in metallicity is large , \sigma ( [ Fe/H ] _ { phot } ) = 0.48 , possibly hinting that the galaxy retained much of its chemical enrichment products .

And X has a total integrated luminosity ( M _ { V } = - 8.1 \pm 0.5 ) that straddles the classical Local Group dSphs and the new SDSS ultra-low luminosity galaxies .
The galaxy is among the most metal-poor dSphs known , especially relative to those with M _ { V } < - 8 , and has the second lowest intrinsic velocity dispersion of the entire sample .
Our results suggest that And X is less massive by a factor of four when compared to Milky Way dSphs of comparable luminosity ( e.g. , Draco and Ursa Minor ) .
We discuss the potential for better understanding the formation and evolution mechanisms for M31 ’ s system of dSphs through ( current ) kinematic and chemical abundance studies , especially in relation to the Milky Way sample .