  Massive galaxies are thought to form in two phases: an initial, early
collapse of gas and giant burst of central star formation, followed by the
later accretion of material that builds up their stellar and dark matter
haloes. The globular cluster systems of such galaxies are believed to form in a
similar manner. The initial central burst forms metal-rich (red) clusters,
while more metal-poor (blue) clusters are brought in by the later accretion of
less massive satellites. This formation process is thought to lead the creation
of the multimodal optical colour distributions seen in the globular cluster
systems of massive galaxies. Here we report HST/ACS observations of the massive
relic galaxy NGC 1277 and its globular clusters, a nearby unevolved example of
a high redshift "red nugget". The g-z cluster colour distribution shows that
the globular cluster system of the galaxy is unimodal and uniquely red. This is
in strong contrast to normal galaxies of similar and larger stellar mass, whose
cluster systems always exhibit (and are generally dominated by) blue clusters.
We argue that the globular cluster system of NGC 1277 indicates that the galaxy
has undergone little (if any) mass accretion after its initial collapse and use
analytic merger trees to show that the total stellar mass accretion is likely
less than ~ 10 %. These results confirm that NGC 1277 is a genuine relic galaxy
and show that the blue, metal-poor globular clusters constitute an accreted
population in present day massive galaxies.
