Extended Stellar Populations in Ultrafaint Dwarf Galaxies

Abstract The possible existence of stellar halos in low-mass galaxies is being intensely discussed nowadays after some recent discoveries of stars located in the outskirts of dwarf galaxies of the Local Group. RR Lyrae stars can be used to identify the extent of these structures, taking advantage of the minimization of foreground contamination they provide. In this work we use RR Lyrae stars obtained from Gaia DR3, the Dark Energy Survey, the Zwicky Transient Facility, and Pan-STARRS1 to explore the outskirts of 45 ultrafaint dwarf galaxies. We associate the stars with a host galaxy based on their angular separations, magnitudes, and proper motions. We find a total of 120 RR Lyrae stars that belong to 21 different galaxies in our sample. We report seven new RR Lyrae stars in six ultrafaint dwarf galaxies (Hydrus I, Ursa Major I, Ursa Major II, Grus II, Eridanus II, and Tucana II). We found a large number of new possible members in Bootes I and Bootes III as well, but some of them may actually belong to the nearby Sagittarius stream. Adding to our list of 120 RR Lyrae stars the observations of other ultrafaint dwarf galaxies that were beyond the reach of our search, we find that at least 10 of these galaxies have RR Lyrae stars located at distances greater than four times their respective half-light radius, which implies that at least 33% of the 30 ultrafaint dwarfs with a population of RR Lyrae stars have extended stellar populations.