On The Nature of ultra-faint Dwarf Galaxy Candidates II: The case of Cetus II

TL;DR

This study used deep photometry to analyze Cetus II, revealing it is likely part of the Sagittarius tidal stream rather than an ultra-faint dwarf galaxy, challenging its classification.

Contribution

The paper provides new deep photometric data and analysis that suggest Cetus II is associated with the Sagittarius stream, not a distinct ultra-faint dwarf galaxy.

Findings

Cetus II's stellar population matches Sagittarius stream stars.

No concentrated overdensity detected in Cetus II.

Cetus II is likely part of the Sagittarius tidal stream.

Abstract

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.