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

Understand

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.