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We investigate the colour-magnitude relation of metal-poor globular clusters,the 'blue tilt', in the Hydra and Centaurus galaxy clusters and constrain theprimordial conditions for star cluster self-enrichment. We analyse U,Iphotometry for about 2500 globular clusters in the central regions of Hydra andCentaurus, based on FORS1@VLT data. We convert the measured colour-magnituderelations 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 GCmass-metallicity relation already sets in at present-day masses of a few 10^5solar masses and is well established in the luminosity range of massive MWclusters like omega Centauri. We compare the mass-metallicity relation withpredictions from the star cluster self-enrichment model by Bailin & Harris(2009). For this we include effects of dynamical and stellar evolution and aphysically well motivated primordial mass-radius scaling. The self-enrichmentmodel reproduces the observed relations well for average primordial half-lightradii r_h ~ 1-1.5 pc, star formation efficiencies f_* ~ 0.3-0.4, andpre-enrichment levels of [Fe/H] ~ -1.7 dex. Within the self-enrichmentscenario, the observed blue tilt implies a correlation between GC mass andwidth of the stellar metallicity distribution. We find that this impliedcorrelation matches the trend of width with GC mass measured in Galactic GCs,including extreme cases like omega Cen and M54. We conclude that 1. Aprimordial star cluster mass-radius relation provides a significant improvementto the self-enrichment model fits. 2. Broadenend metallicity distributions asfound in some massive MW globular clusters may have arisen naturally fromself-enrichment processes, without the need of a dwarf galaxy progenitor.

astronomy and astrophysics

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