On the Nature of Ultra-faint Dwarf Galaxy Candidates. I. DES1, Eridanus III, and Tucana v

We use deep Gemini/GMOS-S g, r photometry to study the three ultra-faint dwarf galaxy candidates DES1, Eridanus III (Eri III), and Tucana V (Tuc V). Their total luminosities, M _V(DES1) = -1.42 ±0.50 and M_V(Eri III) = -2.07 ±0.50, and mean metallicities, [Fe/H] = -2.38^+0.21_-0.19 and [Fe/H] = -2.40^+0.19_-0.12, are consistent with them being ultra-faint dwarf galaxies, as they fall just outside the 1σ confidence band of the luminosity-metallicity relation for Milky Way satellite galaxies. However, their positions in the size-luminosity relation suggest that they are star clusters. Interestingly, DES1 and Eri III are at relatively large Galactocentric distances, with DES1 located at D_GC = 74 ± 4 kpc and Eri III at D_GC = 91 ± 4 kpc. In projection, both objects are in the tail of gaseous filaments trailing the Magellanic Clouds and have similar 3D separations from the Small Magellanic Cloud (SMC): δD_SMC,DES1 = 31.7 kpc and δD_{SMC,Eri III} = 41.0 kpc, respectively. It is plausible that these stellar systems are metal-poor SMC satellites. Tuc V represents an interesting phenomenon in its own right. Our deep photometry at the nominal position of Tuc V reveals a low-level excess of stars at various locations across the GMOS field without a well-defined center. An SMC Northern Overdensity-like isochrone would be an adequate match to the Tuc V color-magnitude diagram, and the proximity to the SMC (12.°1; δD_{SMC,Tuc V} = 13 kpc) suggests that Tuc V is either a chance grouping of stars related to the SMC halo or a star cluster in an advanced stage of dissolution.