On the nature of ultra-faint dwarf galaxy candidates I: DES1, Eridanus III and Tucana V

TL;DR

This study investigates three ultra-faint dwarf galaxy candidates using deep photometry, finding they are likely star clusters or SMC satellites, with diverse properties and possible origins related to the Magellanic Clouds.

Contribution

It provides detailed photometric analysis of DES1, Eri III, and Tucana V, clarifying their nature and possible association with the SMC, which was not previously well understood.

Findings

DES1 and Eri III are consistent with ultra-faint dwarf galaxies but may be star clusters.

DES1 and Eri III are likely SMC satellites based on their positions and properties.

Tucana V appears to be a star grouping related to the SMC halo or a dissolving star cluster.

Abstract

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.