We obtained deep Gemini GMOS-S g,r photometry of the ultra-faint dwarf galaxy candidate Cetus II with the aim of providing stronger constraints on its size , luminosity and stellar population .
Cetus II is an important object in the size-luminosity plane as it occupies the transition zone between dwarf galaxies and star clusters .
All known objects smaller than Cetus II ( r _ { h } \sim 20 pc ) are reported to be star clusters , while most larger objects are likely dwarf galaxies .
We found a prominent excess of main-sequence stars in the colour-magnitude diagram of Cetus II , best described by a single stellar population with an age of 11.2 Gyr , metallicity of [ Fe/H ] = -1.28 dex , an [ \alpha /Fe ] = 0.0 dex at a heliocentric distance of 26.3 \pm 1.2 kpc .
As well as being spatially located within the Sagittarius dwarf tidal stream , these properties are well matched to the Sagittarius galaxy ’ s Population B stars .
Interestingly , like our recent findings on the ultra-faint dwarf galaxy candidate Tucana V , the stellar field in the direction of Cetus II shows no evidence of a concentrated overdensity despite tracing the main sequence for over six magnitudes .
These results strongly support the picture that Cetus II is not an ultra-faint stellar system in the Milky Way halo , but made up of stars from the Sagittarius tidal stream .