Deep VLT search for globular clusters in NGC 5128: color-magnitude diagrams and globular cluster luminosity function

At the distance of NGC 5128 (3.6 ± 0.2 Mpc) it is possible to resolve globular clusters with high resolution imaging from the ground, thus allowing the globular cluster candidate selection primarily through their morphological properties. I report the discovery of 71 globular clusters in NGC 5128 on VLT UT1+FORS1 images, including the faintest members (MV ∼ −5) known to date in this galaxy as well as in 5 previously-known clusters. Uand V -band photometry has been measured for all the candidates and the luminosity function, spanning −10.1 < MV < −4.9 and −9.3 < MU < −3.3, constructed. These are the deepest globular cluster luminosity functions in an elliptical galaxy determined so far. The Kolmogorov-Smirnov statistics show that the difference between the globular cluster luminosity functions of NGC 5128 and the MW is not larger than the difference between the ones of M 31 and the MW. The (U − V )0 color histogram shows a bimodal distribution. For 23 globular clusters I obtained K-band images with SOFI at the NTT in La Silla. Their positions in the (U − V ) vs. (V −K) color-color diagram indicate that they are indeed old globular clusters. Assuming that the globular clusters in NGC 5128 span a similar age range as the ones in the Milky Way and adopting a linear fit between the metallicity and (U−V )0 color, the metal–rich clusters peak at [Fe/H] = −0.6 dex and the metal–poor ones peak at [Fe/H] = −1.7 dex.