Properties of globular cluster systems in nearby early-type galaxies

We present a study of globular clusters (GCs) in 17 relatively nearby early-type galaxies, based on deep F555W and F814W images from the Wide Field Planetary Camera 2, on board the Hubble Space Telescope. A detailed analysis of color distributions, cluster sizes, and luminosity functions is performed and compared with GCs in the Milky Way. In nearly all cases, a KMM test returns a high confidence level for the hypothesis that a sum of two Gaussians provides a better fit to the observed color distribution than a single Gaussian, although histograms of the (V-I)0 distribution are not always obviously bimodal. The blue and red peak colors returned by the KMM test are both found to correlate with absolute host galaxy B-band magnitude and central velocity dispersion (at about the 2-3 σ level). Red GCs are generally smaller than blue GCs by about 20%. The size difference is seen at all radii at least out to 4′ and within sub-bins in (V-I)0 color, and exists also in the Milky Way and Sombrero (M104) spiral galaxies. Fitting t5 functions to the luminosity functions of blue and red GC populations separately, we find that the V-band turnover of the blue GCs is brighter than that of the red ones by about 0.3 mag on the average, as expected if the two GC populations have similar ages and mass distributions but different metallicities. Brighter than the turnover at MV ∼ -7.5, the luminosity functions (LFs) are well approximated by power laws with an exponent of about -1.75. This is similar to the LF for young star clusters, suggesting that young and old globular clusters form by the same basic mechanism. We discuss scenarios for GC formation and conclude that our data appear to favor in situ models in which all GCs in a galaxy formed after the main body of the protogalaxy had assembled into a single potential well.