We present the serendipitous discovery of an L subdwarf , 2MASS J06164006 - 6407194 , in a search of the Two Micron All Sky Survey for T dwarfs .
Its spectrum exhibits features indicative of both a cool and metal poor atmosphere including a heavily pressured-broadened K i resonant doublet , Cs i and Rb i lines , molecular bands of CaH , TiO , CrH , FeH , and H _ { 2 } O , and enhanced collision induced absorption of H _ { 2 } .
We assign 2MASS J0616 - 6407 a spectral type of sdL5 based on a comparison of its red optical spectrum to that of near solar-metallicity L dwarfs .
Its high proper motion ( \mu = 1 \farcs 405 \pm 0.008 yr ^ { -1 } ) , large radial velocity ( V _ { \mathrm { rad } } = 454 \pm 15 km s ^ { -1 } ) , estimated uvw velocities ( 94 , - 573 , 125 ) km s ^ { -1 } and Galactic orbit with an apogalacticon at \sim 29 kpc are indicative of membership in the outer halo making 2MASS J0616 - 6407 the first ultracool member of this population .