Driven by gravity, massive structures like galaxies and clusters of galaxies are believed to grow continuously through hierarchical merging and accretion of smaller systems. Observational evidence of accretion events is provided by the coherent stellar streams crossing the outer haloes of massive galaxies, such as the Milky Way1 or Andromeda2. At similar mass scales, around 1011 solar masses in stars, further evidence of merging activity is also ample3,4,5. Mergers of lower-mass galaxies are expected within the hierarchical process of galaxy formation6, but have hitherto not been seen for galaxies with less than about 109 solar masses in stars7,8. Here we report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal Andromeda II, which has a mass of only 107 solar masses in stars9. The properties of the stream show that we are observing the remnant of a merger between two dwarf galaxies. This had a drastic influence on the dynamics of the remnant, which is now rotating around its projected major axis10. The stellar stream in Andromeda II illustrates the scale-free character of the formation of galaxies, down to the lowest galactic mass scales.
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We thank M. Irwin for discussions on the photometric properties of And II. The Dark Cosmology Centre is funded by the Danish National Research Foundation. This work was partly supported by Sonderforschungsbereich SFB 881 ‘The Milky Way System’ (subproject A7) of the German Research Foundation.
The authors declare no competing financial interests.
Extended data figures and tables
The spectroscopic data set in the plane (vlos, R), shaded according to the probability of each target belonging to the stellar population of And II. Non-member targets with velocities higher than the systematic velocity of And II, vsys = −191.4 ± 0.4, are foreground contaminants from the Milky Way, whereas non-member targets at lower negative velocities are interlopers from the Andromeda halo.
Andromeda II has an approximately flat velocity dispersion profile, except for a significant dip near the average projected radius of the stellar stream. Points of different sizes and shading depths refer to different circular annuli sizes (as in the key), and error bars display 68% confidence levels around the most likely central value.
The distribution of the stars belonging to the stream (blue points) and stars in the control sample (red points) in V-band magnitude versus V − I colour.
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Amorisco, N., Evans, N. & van de Ven, G. The remnant of a merger between two dwarf galaxies in Andromeda II. Nature 507, 335–337 (2014). https://doi.org/10.1038/nature12995
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Nature Astronomy (2022)