How tidal erosion has shaped the relation between globular cluster specific frequency and galaxy luminosity

Abstract

Aims. We quantify to what extent tidal erosion of globular clusters (GCs) has contributed to the observed U-shaped relation between GC specific frequencies SN and host galaxy luminosity MV. Methods. We used our Muesli code to calculate GC survival rates for typical early-type galaxy potentials covering a wide range of observed galaxy properties. We do this for isotropic and radially anisotropic GC velocity distributions. Results. We find that the calculated GC survival fraction, fs, depends linearly on the logarithm of the 3D mass density, ρ3D, within the galaxy's half-light radius, with fs ∝ ρ3D-0.17. For a given galaxy, survival rates are lower for radially anisotropic configurations than for the isotropic GC cases. We apply these relations to a literature sample of 219 early-type galaxies from Harris et al. (2013, ApJ, 772, 82) in the range MV = [-24.5:-15.5] mag. The expected GC survival fraction ranges from 50% for the most massive galaxies with the largest radii to 10% for the most compact galaxies. We find that intermediate luminosity galaxies MV = [-20.5:-17.5] mag have the strongest expected GC erosion. Within the literature sample considered, the predicted GC survival fraction therefore defines a U-shaped relation with M V, similar to the relation between specific frequency SN and MV. As a consequence, the U-shape of SN vs. M V gets erased almost entirely when correcting the SN values for the effect of GC erosion. Conclusions. Tidal erosion is an important contributor to the U-shaped relation between GC specific frequency and host galaxy luminosity. It must be taken into account when inferring primordial star cluster formation efficiencies from observations of GC systems in the nearby universe. © 2014 ESO.