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 (40)

• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• On the variation of the Initial Mass Function
• MESA Isochrones and Stellar Tracks (MIST). I: Solar-Scaled Models
• 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
• The M-sigma and M-L Relations in Galactic Bulges and Determinations of their Intrinsic Scatter
• Young massive star clusters
• The Atlas3D project - XV. Benchmark for early-type galaxies scaling relations from 260 dynamical models: mass-to-light ratio, dark matter, Fundamental Plane and Mass Plane
• The Stellar Population Histories of Local Early-Type Galaxies. I. Population Parameters
• A Substantial Population of Low Mass Stars in Luminous Elliptical Galaxies
• The Stellar Initial Mass Function in Early-Type Galaxies From Absorption Line Spectroscopy. II. Results
• The initial mass function of early-type galaxies
• The Big Problems in Star Formation: the Star Formation Rate, Stellar Clustering, and the Initial Mass Function
• Evidence for top-heavy stellar initial mass functions with increasing density and decreasing metallicity
• SPIDER VIII - Constraints on the Stellar Initial Mass Function of Early-type Galaxies from a Variety of Spectral Features
• The stellar and sub-stellar IMF of simple and composite populations
• Systematic variation of the stellar Initial Mass Function with velocity dispersion in early-type galaxies
• The Stellar Initial Mass Function in Early-Type Galaxies from Absorption Line Spectroscopy. III. Radial Gradients
• Near-IR line-strengths in elliptical galaxies: evidence for IMF variations?
• Measuring Ages and Elemental Abundances from Unresolved Stellar Populations: Fe, Mg, C, N, and Ca
• Estimating Black Hole Masses in Triaxial Galaxies
• Star Clusters in M31: V. Internal Dynamical Trends: Some Troublesome, Some Reassuring
• Variations of the stellar initial mass function in the progenitors of massive early-type galaxies and in extreme starburst environments
• The Extended IRTF Spectral Library: Expanded coverage in metallicity, temperature, and surface gravity
• Variations in the initial mass function in early-type galaxies: A critical comparison between dynamical and spectroscopic results
• On central black holes in ultra-compact dwarf galaxies
• The X-Shooter Lens Survey - I. Dark-Matter Domination and a Salpeter-type IMF in a Massive Early-type Galaxy
• Low-mass X-ray binaries indicate a top-heavy stellar initial mass function in ultra compact dwarf galaxies
• HST/WFPC2 Color-Magnitude Diagrams for Globular Clusters in M31
• 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 AIMSS Project III: The Stellar Populations of Compact Stellar Systems
• The Detailed Chemical Properties of M31 Star Clusters I. Fe, Alpha and Light Elements
• Radial Trends in IMF-Sensitive Absorption Features in Two Early-Type Galaxies: Evidence for Abundance-Driven Gradients
• The Star Formation History of M32
• Radial gradients in initial mass function sensitive absorption features in the Coma brightest cluster galaxies
• Revisiting the classics: Is [Mg/Fe] a good proxy for galaxy formation time-scales?
• A Possible Solution for the M/L-[Fe/H] Relation of Globular Clusters in M31: A metallicity and density dependent top-heavy IMF
• Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey
• A Comprehensive Archival Chandra Search for X-ray Emission from Ultracompact Dwarf Galaxies

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• A 3.5-million Solar Masses Black Hole in the Centre of the Ultracompact Dwarf Galaxy Fornax UCD3
• IMF radial gradients in most massive early-type galaxies
• The Fornax 3D project: a two-dimensional view of the stellar initial mass function in the massive lenticular galaxy FCC 167
• SDSS-IV MaNGA: Stellar initial mass function variation inferred from Bayesian analysis of the integral field spectroscopy of early type galaxies
• The origin of the 'blue tilt' of globular cluster populations in the E-MOSAICS simulations
• The impact of stripped Nuclei on the Super-Massive Black Hole number density in the local Universe
• On the black hole content and initial mass function of 47 Tuc
• MAGAZ3NE: High Stellar Velocity Dispersions for Ultra-Massive Quiescent Galaxies at $z\gtrsim 3$
• Is it possible to reconcile extragalactic IMF variations with a universal Milky Way IMF?
• sMILES SSPs: A Library of Semi-Empirical MILES Stellar Population Models with Variable [$\alpha$/Fe] Abundances
• sMILES: A Library of Semi-Empirical MILES Stellar Spectra with Variable [$\alpha$/Fe] Abundances
• New Constraints on Early-Type Galaxy Assembly from Spectroscopic Metallicities of Globular Clusters in M87
• Ultra-compact dwarfs beyond the centre of the Fornax galaxy cluster: Hints of UCD formation in low-density environments
• Efficiency of black hole formation via collisions in stellar systems: An analysis of data from simulations and observations
• Calibrated, cosmological hydrodynamical simulations with variable IMFs II: Correlations between the IMF and global galaxy properties
• Rotational Variation Allows for Narrow Age Spread in the Extended Main Sequence Turnoff of Massive Cluster NGC 1846
• The Fornax3D project: Intrinsic Correlations between Orbital Properties and the Stellar Initial Mass Function
• Initial Mass Function Variation in two Elliptical Galaxies using Near-Infrared Tracers
• Downsizing revised: Star formation timescales for elliptical galaxies with an environment-dependent IMF and number of SNIa
• Fornax 3D project: assessing the diversity of IMF and stellar population maps within the Fornax Cluster