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

¹ European Southern Observatory, 3107 Alonso de Córdova, Vitacura, Santiago, Chile
e-mail: smieske@eso.org
² Department of Astronomy, Columbia University, 550 West 120th street, New York NY 10027, USA
³ Hubble Fellow  
⁴ Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, Nussallee 14-16, 53121 Bonn, Germany

Received: 16 April 2014
Accepted: 29 April 2014

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 S_N and host galaxy luminosity M_V.

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, f_s, depends linearly on the logarithm of the 3D mass density, ρ_3D, within the galaxy’s half-light radius, with ${\mathit{f}}_{\mathrm{s}}\mathrm{\propto }{\mathit{\rho }}_{\mathrm{3}\mathrm{D}}^{-0.17}$. 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 M_V = [−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 M_V = [−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 S_N and M_V. As a consequence, the U-shape of S_N vs. M_V gets erased almost entirely when correcting the S_N 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.