Possible smoking-gun evidence for initial mass segregation in re-virialized post-gas expulsion globular clusters

TL;DR

This study uses N-body simulations to show that residual gas expulsion from primordial, mass-segregated globular clusters can cause a flattening of the stellar mass function, which may be observable today and linked to their formation environment.

Contribution

It demonstrates that residual gas expulsion impacts the present-day stellar mass function, especially in mass-segregated clusters, revealing clues about their initial conditions and galactic environment.

Findings

Gas expulsion leads to a shallower low- and intermediate-mass stellar MF.

External tidal fields influence the degree of MF flattening.

The MF shape correlates with cluster concentration and formation environment.

Abstract

We perform a series of direct $N$-body calculations to investigate the effect of residual gas expulsion from the gas-embedded progenitors of present-day globular clusters (GCs) on the stellar mass function (MF). Our models start either tidally filling or underfilling, and either with or without primordial mass segregation. We cover 100 Myr of the evolution of modeled clusters and show that the expulsion of residual gas from initially mass-segregated clusters leads to a significantly shallower slope of the stellar MF in the low- ($m\leq 0.50 M_\odot$) and intermediate-mass ($\simeq 0.50-0.85 M_\odot$) regime. Therefore, the imprint of residual gas expulsion and primordial mass segregation might be visible in the present-day MF. We find that the strength of the external tidal field, as an essential parameter, influences the degree of flattening, such that a primordially mass-segregated tidally-filling cluster with $r_h/r_t\geq 0.1$ shows a strongly depleted MF in the intermediate stellar mass range. Therefore, the shape of the present-day stellar MF in this mass range probes the birth place of clusters in the Galactic environment. We furthermore find that this flattening agrees with the observed correlation between the concentration of a cluster and its MF slope, as found by de Marchi et al.. We show that if the expansion through the residual gas expulsion in primordial mass segregated clusters is the reason for this correlation then GCs most probably formed in strongly fluctuating local tidal fields in the early proto-Milky Way potential, supporting the recent conclusion by Marks \& Kroupa.