HST measurements of stellar proper motions in M31 reveal that the Andromeda galaxy is in radial (head-on collision) orbit towards the Milky Way. The huge strike will happen 4 billion years from now according to the simulations. It will likely affect also the environment of the Sun and the location of the solar system.
When new evidence disagrees with our previous understanding, we must reinterpret everything.
We think many galaxies we see today had mergers and interactions in their past, but how can we know for sure? Bonfini et al. look to evidence from a subtle pattern in the distribution of globular clusters in NGC 4261.
Hubble observations of a gravitationally lensed high redshift galaxy harboring an old stellar population.