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Appearances can be deceiving: clear signs of accretion in the seemingly ordinary Sextans dSph

Author/s: L. Cicuéndez, G. Battaglia

Reference: 2018 MNRAS 480 251 | Link

Spatial distributions of the two stellar populations analyzed in Sextans. Left panel: blue stars (metal poor); Right panel: red stars (metal rich). Credits: Cicuéndez, L. and Battaglia, G.
Spatial distributions of the two stellar populations analyzed in Sextans. Left panel: blue stars (metal poor); Right panel: red stars (metal rich). Credits: Cicuéndez, L. and Battaglia, G.

We report the discovery of clear observational signs of past accretion/merger events on the smallest scale so far known, that is on the Sextans dwarf spheroidal, one of the Milky Way satellite galaxies. Using data from the Victor M. Blanco Telescope (4m diameter) at the Cerro Tololo Interamerican Observatory and the Landon Clay 6 m telescope, also known as Magellan 2, at the Las Campanas Observatory, both in Chile, we observed clear signs that Sextans had absorbed a smaller stellar system. The spatial distribution of the blue, metal-poor stars is round and regular, while that of the red, metal-rich stars is much more elliptical and irregular, with an overdensity of stars on the north-eastern side. The most reasonable explanation of this phenomenon is that two galaxies merged, and had different metallicities. The analysis of the velocities and of indicators of the chemical composition of the stars also reveals the presence of a spatial substructure in the shape of a ring. This substructure shows a much higher velocity and a different chemical composition than the rest of the stars in the galaxy. Thus, with a stellar mass of just ∼ 5 × 105 solar masses, Sextans becomes the smallest galaxy presenting clear observational signs of accretion to date, showing that the hierarchical theory of galaxy formation, in which small galaxies merge to form larger ones, can explain the formation of even the smallest known galaxies, the dwarf galaxies.

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