The colliding spiral galaxies in Arp 272 located in the constellation of Hercules. Astronomers have studied a suite of galaxy merger simulations to conclude that our own Milky Way galaxy suffered a similar kind of merger. In particular, they found that some of the peculiar features in the Galaxy's halo structure can be best explained by a head-on collision with a dwarf galaxy six to ten billion years ago. Credit: NASA, ESA, the Hubble Heritage -STScI/AURA)-ESA/Hubble Collaboration, and K. Noll, STScI
Our Milky Way galaxy has probably collided or otherwise interacted with other galaxies during its lifetime; such interactions are common cosmic occurrences. Astronomers can deduce the history of mass accretion onto the Milky Way from a study of debris in the halo of the galaxy left as the tidal residue of such episodes. The approach has worked particularly well for studies of the most recent events like the infall of the Sagittarius dwarf galaxy a few billion years ago that left tidal streamers of stars visible in galaxy maps. The damaging effects these encounters can cause to the Milky Way have, however, not been as well studied, and events even further in the past are even less obvious as they become blurred by the galaxy's natural motions and evolution.
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