Pegasus III ( Peg III ) is one of the few known ultra-faint stellar systems in the outer halo of the Milky Way .
We present the results from a follow-up campaign with Magellan/IMACS and Keck/DEIMOS .
Deep stellar photometry down to r _ { 0 } \approx 25 mag at 50 % completeness level has allowed accurate measurements of its photometric and structural properties .
The color-magnitude diagram of Peg III confirms that the stellar system is well described by an old ( \gtrsim 12 Gyr ) and metal-poor ( [ Fe/H ] \lesssim - 2.0 dex ) stellar population at a heliocentric distance of 215 \pm 12 kpc .
The revised half-light radius r _ { h } = 53 \pm 14 pc , ellipticity \epsilon = 0.38 ^ { +0.22 } _ { -0.38 } , and total luminosity M _ { V } = -3.4 \pm 0.4 are in good agreement with the values quoted in our previous paper .
We further report on the spectroscopic identification of seven , possibly eight member stars of Peg III .
The Ca II triplet lines of the brightest member stars indicate that Peg III contains stars with metallicity as low as [ Fe/H ] = -2.55 \pm 0.15 dex .
Peg III has a systemic velocity of -222.9 \pm 2.6 km s ^ { -1 } and a velocity dispersion of 5.4 ^ { +3.0 } _ { -2.5 } km s ^ { -1 } .
The inferred dynamical mass within the half-light radius is 1.4 ^ { +3.0 } _ { -1.1 } \times 10 ^ { 6 } { M _ { \odot } } and the mass-to-light ratio \textup { M / L } _ { V } = 1470 ^ { +5660 } _ { -1240 } { M _ { \odot } / L _ { \odot } } , providing further evidence that Peg III is a dwarf galaxy satellite .
We find that Peg III and another distant dwarf satellite Pisces II lie relatively close to each other ( \Delta d _ { spatial } = 43 \pm 19 kpc ) and share similar radial velocities in the Galactic standard-of-rest frame ( \Delta v _ { GSR } = 12.3 \pm 3.7 km s ^ { -1 } ) .
This suggests that they may share a common origin .