Deep Multi-telescope Photometry of NGC 5466. II. The Radial Behavior of the Mass Function Slope

We use a combination of data acquired with the Advanced Camera for Survey on board the Hubble Space Telescope and the Large Binocular Camera (LBC-blue) mounted on the Large Binocular Telescope to sample the main sequence (MS) stars of the globular cluster (GC) NGC 5466 in the mass range 0.3 < M/M⊙ < 0.8. We derive the cluster's Luminosity Function (LF) in several radial regions, from the center of the cluster out to the tidal radius. After corrections for incompleteness and field contamination, this was compared to theoretical LFs, obtained by multiplying a simple power-law mass function in the form dN/dm \propto {m}α by the derivative of the mass-luminosity relationship of the best-fit isochrone. We find that α varies from -0.6 in the core region to -1.9 in the outer region. This fact allows us to prove by observation that the stars in NGC 5466 have experienced the effects of mass segregation. We compare the radial variation of α from the center out to 5 core radii (rc) in NGC 5466 and the GC M10, finding that the gradient of α in the first 5rc is more than a factor of 2 shallower in NGC 5466 than in M10, in line with the differences in the clusters’ relaxation timescales. NGC 5466 is dynamically younger than M10, with two-body relaxation processes only recently starting to shape the distribution of MS stars. This result fully agrees with the conclusion obtained in our previous works on the radial distribution of blue straggler stars, further confirming that this can be used as an efficient clock to measure the dynamical age of stellar systems.