Portrait of a Dark Horse: a Photometric and Spectroscopic Study of the Ultra-faint Milky Way Satellite Pegasus III

Abstract

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 ($\gtrsim12$ Gyr) and metal-poor ([Fe/H]$\lesssim-2.0$ dex) stellar population at a heliocentric distance of $215\pm12$ kpc. The revised half-light radius $r_{h}=53\pm14$ pc, ellipticity $\epsilon=0.38^{+0.22}_{-0.38}$, and total luminosity $M_{V}=-3.4\pm0.4$ 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]=$-2.55\pm0.15$ 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 $1.4^{+3.0}_{-1.1} \times 10^6\rm{M_{\odot}}$ and the mass-to-light ratio $\rm{M/L}$$_{V} = 1470^{+5660}_{-1240}$ $\rm{M_{\odot}/L_{\odot}}$, 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 ($\Delta d_{spatial}=43\pm19$ kpc) and share similar radial velocities in the Galactic standard-of-rest frame ($\Delta v_{GSR}=12.3\pm3.7$ km s$^{-1}$). This suggests that they may share a common origin.