The globular clusters in M87: A bimodal, colour distribution

We present V and I photometry for similar to 150 globular clusters with 20 < V < 27 about 2.5 arcmin from the centre of M87, the cD galaxy in the Virgo cluster. The data were acquired with the Hubble Space Telescope which, with an effective resolution of 0.1 arcsec, allows us for the first time to distinguish between globular clusters and most background galaxies on the basis of morphology, and to obtain accurate photometry even at faint magnitudes. The (V-I) distribution of the clusters is clearly bimodal, implying a corresponding bimodality in the metallicity distribution, with peaks at [Fe/H] approximate to - 1.5 and - 0.5. We also find that the brightest clusters are predominantly metal-poor, while the fainter clusters are divided roughly equally between metal-poor and metal-rich. Our sample is essentially complete to V approximate to 25, a full magnitude past the expected peak of a 'universal' luminosity function, and the observed luminosity function is well represented by the standard Gaussian distribution with [M(V)]=-7.2 and sigma=1.4.