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Giant spiral galaxies are assembled from smaller
& F# p* H8 j) X. }9 ksystems through a process known as hierarchical clustering. In orbit around , J/ c/ E4 K. I
these giants are dwarf galaxies, which are presumably remnants of the galactic 7 ` B* I; ^# E6 z/ J' k6 r/ ]' t
progenitors. Recent studies of the dwarf galaxies of the Milky Way have lead
9 d; l3 e, b. x* t+ ~8 F0 ysome astronomers to suspect that their orbits are not randomly distributed. This
4 O$ a; u6 f; K2 gsuspicion, which challenges current theories of galaxy formation, is now
: D N$ b; J5 v7 I; i* Jbolstered by the discovery of a plane of dwarf galaxies corotating as a coherent & g( C0 i! L$ Q7 C
pancake-like structure around the Andromeda galaxy, the Milky Way’s close : u5 }8 A( w2 }5 ?: x1 U5 r
neighbour and in many respects its ‘twin’. The structure is extremely thin yet
; b0 l7 ~" y# w3 R; Ycontains about half of the dwarf galaxies in the Andromeda system. The authors
& U7 w) T! R2 N7 {; ireport that 13 of the 15 satellites in the plane share the same sense of ; g, D q5 E3 C3 M: x5 S2 \
rotation. On the cover, a true colour image of the Andromeda galaxy, with two 3 C7 Y' e1 D" m2 ?$ G$ A. @9 T7 r
satellite galaxies visible, by Jean-Charles Cuillandre (CFHT) and Giovanni
7 i; z0 f! f7 m0 k- ?8 hAnselmi (Coelum Astronomia).. f+ O8 D+ k; N0 y% F. m+ Z' h
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