The WFPC2 ultraviolet survey: The blue straggler population in NGC 5824

We have used a combination of high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and wide-field ground-based observations, in ultraviolet and optical bands, to study the blue straggler star population of the massive outer halo globular cluster NGC 5824 over its entire radial extent. We have computed the center of the cluster and constructed the radial density profile from detailed star counts. The profile is well reproduced by a Wilson model with a small core (r_c ≅ 4.''4) and a concentration parameter c ≅ 2.74. We also present the first age determination for this cluster. From a comparison with isochrones, we find t = 13 ± 0.5 Gyr. We discuss this result in the context of the observed age-metallicity relation of Galactic globular clusters. A total of 60 bright blue stragglers has been identified. Their radial distribution is found to be bimodal, with a central peak, a well-defined minimum at r ∼ 20'', and an upturn at large radii. In the framework of the dynamical clock recently defined by Ferraro et al., this feature suggests that NGC 5824 is a cluster of intermediate dynamical age.