The early evolution of the star cluster mass function

TL;DR

This paper models the evolution of the star cluster mass function, showing it remains approximately a power law with index -2 over the first billion years despite disruption processes, and applies the model to M51 galaxy data.

Contribution

It introduces a model of the evolving star cluster mass function using a Schechter function, accounting for disruption effects and observational biases, with application to M51 galaxy clusters.

Findings

Mass function remains approximately a power law with index -2 over ~Gyr.

Disruption shifts the observable mass range to higher masses over time.

Application to M51 shows a truncation mass of ~2x10^5 solar masses.

Abstract

Several recent studies have shown that the star cluster initial mass function (CIMF) can be well approximated by a power law, with indications for a steepening or truncation at high masses. This contribution considers the evolution of such a mass function due to cluster disruption, with emphasis on the part of the mass function that is observable in the first ~Gyr. A Schechter type function is used for the CIMF, with a power law index of -2 at low masses and an exponential truncation at M*. Cluster disruption due to the tidal field of the host galaxy and encounters with giant molecular clouds flattens the low-mass end of the mass function, but there is always a part of the `evolved Schechter function' that can be approximated by a power law with index -2. The mass range for which this holds depends on age, t, and shifts to higher masses roughly as t^0.6. Mean cluster masses derived from luminosity limited samples increase with age very similarly due to the evolutionary fading of clusters. Empirical mass functions are, therefore, approximately power laws with index -2, or slightly steeper, at all ages. The results are illustrated by an application to the star cluster population of the interacting galaxy M51, which can be well described by a model with M*=(1.9+/-0.5)x10^5 M_sun and a short (mass-dependent) disruption time destroying M* clusters in roughly a Gyr.