A Hero's Dark Horse: Discovery of an Ultra-Faint Milky Way Satellite in Pegasus

Abstract

We report the discovery of an ultra-faint Milky Way satellite galaxy in the constellation of Pegasus. The concentration of stars was detected by applying our overdensity detection algorithm to the SDSS-DR 10 and confirmed with deeper photometry from the Dark Energy Camera at the 4-m Blanco telescope. Fitting model isochrones indicates that this object, Pegasus III, features an old and metal-poor stellar population ([Fe/H]$\sim -2.1$) at a heliocentric distance of $205\pm 20$ kpc. The new stellar system has an estimated half-light radius of $r_h=78_{-24}^{+30}$ pc and a total luminosity of $M_V\sim -4.1\pm 0.5$ that places it into the domain of dwarf galaxies on the size--luminosity plane. Pegasus III is spatially close to the MW satellite Pisces II. It is possible that the two might be physically associated, similar to the Leo IV and Leo V pair. Pegasus III is also well aligned with the Vast Polar Structure, which suggests a possible physical association.

5 pages, 4 figures, 1 table. Accepted for publication in ApJL

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• On the nature of ultra-faint dwarf galaxy candidates I: DES1, Eridanus III and Tucana V
• Portrait of a Dark Horse: a Photometric and Spectroscopic Study of the Ultra-faint Milky Way Satellite Pegasus III
• Deep Subaru Hyper Suprime-Cam observations of Milky Way satellites Columba I and Triangulum II
• d1005+68: A New Faint Dwarf Galaxy in the M81 Group
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• Identification of RR Lyrae stars in multiband, sparsely-sampled data from the Dark Energy Survey using template fitting and Random Forest classification
• Quantifying tidal stream disruption in a simulated Milky Way
• Stellar cluster candidates discovered in the Magellanic System
• The contribution of dissolving star clusters to the population of ultra-faint objects in the outer halo of the Milky Way
• Search for gamma-ray line features from Milky Way satellites with Fermi-LAT Pass 8 data
• A MegaCam Survey of Outer Halo Satellites. VII. A Single Sérsic Index v/s Effective Radius Relation for Milky Way Outer Halo Satellites
• Testing the dark matter subhalo hypothesis of the gamma-ray source 3FGL J2212.5+0703
• The Carnegie-Chicago Hubble Program: Discovery of the Most Distant Ultra-faint Dwarf Galaxy in the Local Universe
• Structural and Photometric Properties of the Andromeda Satellite Dwarf Galaxy Lacerta I from Deep Imaging with WIYN pODI