Using metallicities from the literature , combined with the Revised Bologna Catalogue of photometric data for M31 clusters and cluster candidates ( the latter of which is the most comprehensive catalogue of M31 clusters currently available , including 337 confirmed globular clusters – GCs – and 688 GC candidates ) , we determine 443 reddening values and intrinsic colours , and 209 metallicities for individual clusters without spectroscopic observations .
This , the largest sample of M31 GCs presently available , is then used to analyse the metallicity distribution of M31 GCs , which is bimodal with peaks at [ Fe / H ] \approx - 1.7 and -0.7 dex .
An exploration of metallicities as a function of radius from the M31 centre shows a metallicity gradient for the metal-poor GCs , but no such gradient for the metal-rich GCs .
Our results show that the metal-rich clusters appear as a centrally concentrated spatial distribution ; however , the metal-poor clusters tend to be less spatially concentrated .
There is no correlation between luminosity and metallicity among the M31 sample clusters , which indicates that self-enrichment is indeed unimportant for cluster formation in M31 . The reddening distribution shows that slightly more than half of the GCs are affected by a reddening of E ( B - V ) \la 0.2 mag ; the mean reddening value is E ( B - V ) = 0.28 _ { -0.14 } ^ { +0.23 } mag .
The spatial distribution of the reddening values indicates that the reddening on the northwestern side of the M31 disc is more significant than that on the southeastern side , which is consistent with the conclusion that the northwestern side in nearer to us .