Initial Mass Function Variability (or not) Among \\ Low-Velocity Dispersion, Compact Stellar Systems

TL;DR

This study investigates whether the stellar initial mass function varies among low-velocity dispersion, compact stellar systems, finding limited variation and suggesting other factors influence IMF differences beyond metallicity.

Contribution

The paper introduces new SPS models applied to diverse compact stellar systems, revealing minimal IMF variation and highlighting the need to consider additional parameters beyond metallicity.

Findings

Globular clusters have M/L ratios consistent with Kroupa IMF.

Ultra-compact dwarf and compact elliptical show mildly elevated M/L ratios.

IMF variation among these systems is about 50%, much less than in massive ETGs.

Abstract

Analyses of strong gravitational lenses, galaxy-scale kinematics, and absorption line stellar population synthesis (SPS) have all concluded that the stellar initial mass function (IMF) varies within the massive early-type galaxy (ETG) population. However, the physical mechanism that drives variation in the IMF is an outstanding question. Here we use new SPS models to consider a diverse set of compact, low-velocity dispersion stellar systems: globular clusters (GCs), an ultra-compact dwarf (UCD), and the compact elliptical (cE) galaxy M32. We compare our results to massive ETGs and available dynamical measurements. We find that the GCs have stellar mass-to-light ratios (M/L) that are either consistent with a Kroupa IMF or are slightly bottom-light while the UCD and cE have mildly elevated M/L. The separation in derived IMFs for systems with similar metallicities and abundance patterns indicates that our SPS models can distinguish abundance and IMF effects. Variation among the sample in this paper is only $\sim 50\%$ in normalized M/L compared to the $\sim 4\times$ among the ETG sample. This suggests that metallicity is not the sole driver of IMF variability and additional parameters need to be considered.