On the Nature of Ultra-faint Dwarf Galaxy Candidates. II. The Case of Cetus II

Conn, Blair C.; Jerjen, Helmut; Kim, Dongwon; Schirmer, Mischa

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On the Nature of Ultra-faint Dwarf Galaxy Candidates. II. The Case of Cetus II

MPS-Authors

Conn, Blair C.
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Jerjen, Helmut
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Kim, Dongwon
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

Schirmer, Mischa
Max Planck Institute for Astronomy, Max Planck Society and Cooperation Partners;

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https://ui.adsabs.harvard.edu/abs/2018ApJ...857...70C
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Citation

Conn, B. C., Jerjen, H., Kim, D., & Schirmer, M. (2018). On the Nature of Ultra-faint Dwarf Galaxy Candidates. II. The Case of Cetus II. The Astrophysical Journal, 857.

Cite as: https://hdl.handle.net/21.11116/0000-0005-CE46-5

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 ̃ 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 color- 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 [α/Fe] = 0.0 dex at a heliocentric distance of 26.3 ± 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.