The remnant of a merger between two dwarf galaxies in Andromeda II

Abstract

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 Way 1 or Andromeda 2 . At similar mass-scales, around 10 11 solar masses in stars, further observational evidence of merging activity is also ample 3,4,5 . Mergers of lower-mass galaxies are expected within the hierarchical process of galaxy formation 6 , but have hitherto not been seen for galaxies with less than about 10 9 solar masses in stars 7,8 . Here, we report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal galaxy Andromeda II, which has a mass of only 10 7 solar masses in stars 9 . The properties of the stream show that we are observing the remnant of a merger between two dwarf galaxies. This had a dramatic influence on the dynamics of the remnant, which is now rotating around its projected major axis 10 . The stellar stream in Andromeda II illustrates the scale-free character of the formation of galaxies, down to the lowest galactic mass scales.Andromeda II (And II) is, in size, the second largest dwarf spheroidal galaxy known in the Local Group, with a half light radius 11 of about 1.2 kpc (second only to And XIX 12 ). With a luminosity 9 of L V = 9.4 × 10 6 L ⊙ , it currently sits 13 at a distance of 185 kpc from its host, and at an heliocentric distance of about 650 kpc. Among the satellites of M31, And II is one of the few for which a spectroscopic dataset of hundreds of stars is currently available. These observations, made using the Deep Imaging Multi-Object spectrograph (DEIMOS) on the Keck II telescope, revealed a strong and puzzling stellar rotation, so far unique among the dwarf spheroidal galaxies of the Local Group 10 . The results presented here are based on a re-analysis of the latter spectroscopic observations, kindly provided by N. Ho and M. Geha.We assign reliable probabilities of membership to all spectroscopic targets, comprising more than 700 candidate red giant branch stars, by allowing for the presence of both foreground contaminants from the Milky Way and interlopers from the halo population of M31. Each of the three coexisting components (members, Milky Way halo contaminants, M31 halo contaminants) -2 -are described by a distinct spatial and kinematical distribution, the parameters of which are measured through a maximum likelihood technique (see Methods). A Bayesian approach then allows us to estimate a probability of membership for each available star: we count 632 high probability members (p>0.85; see Extended Data Fig. 1).We then study the kinematical properties of And II also through a maximum likelihood method that fully takes into account the observational uncertainties on the line-of-sight velocity of each available giant star in the spectroscopic sample. We find that, within measurement ...

Mergers and the outside-in formation of dwarf spheroidals

“…These gradients are particularly clear in some galaxies and, when accompanied by correlated differences in kinematics and metallicity, are often interpreted as evidence for distinct stellar components. The Sculptor dSph makes a compelling example of a system with at least two distinct components (Tolstoy et al 2004;Battaglia et al 2008), but so do other dwarfs such as Sextans (Battaglia et al 2011); the And II companion to M31 (McConnachie et al 2007;Amorisco et al 2014); and Fornax (Battaglia et al 2006;Walker & Peñarrubia 2011). We note, however, that clear-cut evidence for distinct components is often weak, and that these may also be interpreted as simply cases where the radial gradients are particularly pronounced.…”

CALIFA reveals prolate rotation in massive early-type galaxies: A polar galaxy merger origin?

“…This formation scenario was suggested in order to explain the prolate rotation that was recently observed in the dwarf spheroidal galaxy Andromeda II (Ho et al 2012), the first dwarf galaxy observed to show such a feature in the form of a stellar stream rotating around its major projected axis (see also Kacharov et al 2017, for their recent discovery of prolate rotation in another dwarf galaxy). This feature in Andromeda II was interpreted as evidence of a past major merger between two dwarf galaxies (Amorisco et al 2014).…”

The MAGellanic Inter-Cloud (MAGIC) project – II. Slicing up the Bridge

“…Such haloes retain a fossil record of past mergers and interactions in the form of extended tidal debris (e.g. Searle & Zinn 1978;Martínez-Delgado et al 2001;Muñoz et al 2006;Sohn et al 2007;Amorisco, Evans & van de Ven 2014).…”