From light to mass: accessing the initial and present-day Galactic globular cluster mass functions

TL;DR

This study models the initial and present-day mass functions of Galactic globular clusters by integrating observational data, stellar evolution, and dynamical processes, revealing their shared origins with young clusters and molecular clouds.

Contribution

It introduces a method to derive the initial and present-day mass functions of GCs using a Schechter-like ICMF and evolving it over a Hubble time, incorporating a luminosity-dependent MLR.

Findings

The present-day mass function is approximately lognormal with a turnover at 7×10^4 solar masses.

The initial mass function resembles the scale-free distributions of young clusters and molecular clouds.

The total stellar mass in GCs is about 4×10^7 solar masses, or 15% of the initial cluster mass.

Abstract

The initial and present-day mass functions (ICMF and PDMF, respectively) of the Galactic globular clusters (GCs) are constructed based on their observed luminosities, the stellar evolution and dynamical mass-loss processes, and the mass-to-light ratio (MLR). Under these conditions, a Schechter-like ICMF is evolved for approximately a Hubble time and converted into the luminosity function (LF), which requires finding the values of 5 free parameters: the mean GC age (\tA), the dissolution timescale of a $10^5 \ms$ cluster ($\tau_5$), the exponential truncation mass (\mc) and 2 MLR parametrising constants. This is achieved by minimising the residuals between the evolved and observed LFs, with the minimum residuals and realistic parameters obtained with MLRs that increase with luminosity (or mass). The optimum PMDFs indicate a total stellar mass of $\sim4\times10^7$ \ms\ still bound to GCs, representing $\sim15%$ of the mass in clusters at the beginning of the gas-free evolution. The corresponding ICMFs resemble the scale-free MFs of young clusters and molecular clouds observed in the local Universe, while the PDMFs follow closely a lognormal distribution with a turnover at $\mto\sim7\times10^4$\,\ms. For most of the GC mass range, we find an MLR lower than usually adopted, which explains the somewhat low \mto. Our results confirm that the MLR increases with cluster mass (or luminosity), and suggest that GCs and young clusters share a common origin in terms of physical processes related to formation.