The colour-magnitude relation of Globular Clusters in Centaurus and
Hydra - Constraints on star cluster self-enrichment with a link to massive
Milky Way GCs
We investigate the colour-magnitude relation of metal-poor globular clusters,
the 'blue tilt', in the Hydra and Centaurus galaxy clusters and constrain the
primordial conditions for star cluster self-enrichment. We analyse U,I
photometry for about 2500 globular clusters in the central regions of Hydra and
Centaurus, based on FORS1@VLT data. We convert the measured colour-magnitude
relations into mass-metallicity space and obtain a scaling of Z \propto M^{0.27
\pm 0.05} for Centaurus GCs and Z \propto M^{0.40 \pm 0.06} for Hydra GCs,
consistent with results in other environments. We find that the GC
mass-metallicity relation already sets in at present-day masses of a few 10^5
solar masses and is well established in the luminosity range of massive MW
clusters like omega Centauri. We compare the mass-metallicity relation with
predictions from the star cluster self-enrichment model by Bailin & Harris
(2009). For this we include effects of dynamical and stellar evolution and a
physically well motivated primordial mass-radius scaling. The self-enrichment
model reproduces the observed relations well for average primordial half-light
radii r_h ~ 1-1.5 pc, star formation efficiencies f_* ~ 0.3-0.4, and
pre-enrichment levels of [Fe/H] ~ -1.7 dex. Within the self-enrichment
scenario, the observed blue tilt implies a correlation between GC mass and
width of the stellar metallicity distribution. We find that this implied
correlation matches the trend of width with GC mass measured in Galactic GCs,
including extreme cases like omega Cen and M54. We conclude that 1. A
primordial star cluster mass-radius relation provides a significant improvement
to the self-enrichment model fits. 2. Broadenend metallicity distributions as
found in some massive MW globular clusters may have arisen naturally from
self-enrichment processes, without the need of a dwarf galaxy progenitor.Comment: 15 pages, 13 figures. Language edited version of paper accepted for
publication in Astronomy & Astrophysics. Colour-composite in Figure 1 reduced
in resolutio
