The Globular Clusters in M87: A Bimodal Colour Distribution

We present $V$ and $I$ photometry for $\sim 150$ globular clusters with $20 < V < 27$ $\sim 2.5$ arcmin from the centre of M87, the cD galaxy in the Virgo cluster. The data were acquired with the Hubble Space Telescope which, with effective resolution 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]$\approx -1.5$ and $-0.5$. We also find that the brightest clusters are predominantly metal poor, while the fainter clusters are divided roughly eaually between metal poor and metal rich. Our sample is essentially complete to $V\approx 25$, 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 $\sigma = 1.4$.