The Dark Energy Survey view of the Sagittarius stream : discovery of two faint stellar system candidates

Abstract

We report the discovery of two new candidate stellar systems in the constellation of Cetus using the data from the first two years of the Dark Energy Survey (DES). The objects, DES J0111−1341 and DES J0225+0304, are located at a heliocentric distance of ∼25 kpc and appear to have old and metal-poor populations. Their distances to the Sagittarius orbital plane, ∼1.73 kpc (DES J0111−1341) and ∼0.50 kpc (DES J0225+0304), indicate that they are possibly associated with the Sagittarius dwarf stream. The half-light radius (rh 4.55 pc) and luminosity (MV +0.3) of DES J0111−1341 are consistent with it being an ultrafaint stellar cluster, while the half-light radius (rh 18.55 pc) and luminosity (MV −1.1) of DES J0225+0304 place it in an ambiguous region of size–luminosity space between stellar clusters and dwarf galaxies. Determinations of the characteristic parameters of the Sagittarius stream, metallicity spread (−2.18 [Fe/H] −0.95) and distance gradient (23 kpc D 29 kpc), within the DES footprint in the Southern hemisphere, using the same DES data, also indicate a possible association between these systems. If these objects are confirmed through spectroscopic follow-up to be gravitationally bound systems and to share a Galactic trajectory with the Sagittarius stream, DES J0111−1341 and DES J0225+0304 would be the first ultrafaint stellar systems associated with the Sagittarius stream. Furthermore, DES J0225+0304 would also be the first confirmed case of an ultrafaint satellite of a satellite.