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

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.

Accepted to The Astrophysical Journal Letters

References in corpus (9)

• The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies
• Measuring Ages and Elemental Abundances from Unresolved Stellar Populations: Fe, Mg, C, N, and Ca
• Variations of the stellar initial mass function in the progenitors of massive early-type galaxies and in extreme starburst environments
• Variations in the initial mass function in early-type galaxies: A critical comparison between dynamical and spectroscopic results
• The Extended IRTF Spectral Library: Expanded coverage in metallicity, temperature, and surface gravity
• IMF shape constraints from stellar populations and dynamics from CALIFA
• The Stellar Initial Mass Function in Early-Type Galaxies From Absorption Line Spectroscopy. IV. A Super-Salpeter IMF in the center of NGC 1407 from Non-Parametric Models
• The Detailed Chemical Properties of M31 Star Clusters I. Fe, Alpha and Light Elements
• Radial gradients in initial mass function sensitive absorption features in the Coma brightest cluster galaxies

Cited by in corpus (1)

• New Constraints on Early-Type Galaxy Assembly from Spectroscopic Metallicities of Globular Clusters in M87