Evidence for temporal evolution in the M33 disc as traced by its star
  clusters
  We present precision radial velocities and stellar population parameters for
77 star clusters in the Local Group galaxy M33. Our GTC and WHT observations
sample both young, massive clusters and known/candidate globular clusters,
spanning ages ~ 10^6 - 10^10 yr, and metallicities, [M/H] ~-1.7 to solar. The
cluster system exhibits an age-metallicity relation; the youngest clusters are
the most metal-rich. When compared to HI data, clusters with [M/H] ~ -1.0 and
younger than ~ 4 Gyr are clearly identified as a disc population. The clusters
show evidence for strong time evolution in the disc radial metallicity gradient
(d[M/H]dt / dR = 0.03 dex/kpc/Gyr). The oldest clusters have stronger, more
negative gradients than the youngest clusters in M33. The clusters also show a
clear age-velocity dispersion relation. The line of sight velocity dispersions
of the clusters increases with age similar to Milky Way open clusters and
stars. The general shape of the relation is reproduced by disc heating
simulations, and the similarity between the relations in M33 and the Milky Way
suggests that heating by substructure, and cooling of the ISM both play a role
in shaping this relation. We identify 12 "classical" GCs, six of which are
newly identified GC candidates. The GCs are more metal-rich than Milky Way halo
clusters, and show weak rotation. The inner (R < 4.5 kpc) GCs exhibit a steep
radial metallicity gradient (d[M/H]/dR = -0.29+-0.11 dex/kpc) and an
exponential-like surface density profile. We argue that these inner GCs are
thick disc rather than halo objects.
