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\pm0.50$ and $M_V$(Eri III) $ = -2.07\pm0.50$, 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 1-sigma confidence band of the luminosity-metallicity relation for Milky Way satellite galaxies. However, their positions in the size-luminosity relation suggests that they are star clusters. Interestingly, DES1 and Eri III are at relatively large Galactocentric distances with DES1 located at $D_{GC} = 74\pm$4 kpc and Eri III at $D_{GC} = 91\pm$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): $\Delta D_{SMC,DES1}$ = 31.7 kpc and $\Delta D_{SMC,Eri III}$ = 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 centre. A SMC Northern Overdensity-like isochrone would be an adequate match to the Tuc V colour-magnitude diagram, and the proximity to the SMC ($12.1^\circ$; $\Delta D_{SMC,Tuc V}=13$ 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.