Variations in the initial mass function in early-type galaxies: A critical comparison between dynamical and spectroscopic results

Abstract

I present a comparison between published dynamical (ATLAS3D) and spectroscopic (Conroy & van Dokkum) constraints on the stellar initial mass function (IMF) in early-type galaxies, using the 34 galaxies in common between the two works. Both studies infer an average IMF mass factor $\alpha$ (the stellar mass relative to a Kroupa-IMF population of similar age and metallicity) greater than unity, i.e. both methods favour an IMF which is heavier than that of the Milky Way, on average over the sample. However, on a galaxy-by-galaxy basis, there is no correlation between $\alpha$ inferred from the two approaches. I investigate how the two estimates of $\alpha$ are correlated systematically with the galaxy velocity dispersion, $\sigma$, and with the Mg/Fe abundance ratio. The spectroscopic method, based on the strengths of metal absorption lines, yields a correlation only with metal abundance ratios: at fixed Mg/Fe, there is no residual correlation with $\sigma$. The dynamical method, applied to exactly the same galaxy sample, yields the opposite result: the IMF variation correlates only with dynamics, with no residual correlation with Mg/Fe after controlling for $\sigma$. Hence although both methods indicate a heavy IMF on average in ellipticals, they lead to incompatible results for the systematic trends, when applied to the same set of galaxies. The sense of the disagreement could suggest that one (or both) of the methods has not accounted fully for the main confounding factors, i.e. element abundance ratios or dark matter contributions. Alternatively, the poor agreement might indicate additional variation in the detailed shape of the IMF, beyond what can currently be inferred from the spectroscopic features.

MNRAS in press

References in corpus (4)

• A systematic variation of the stellar initial mass function in early-type galaxies
• Radial variations in the stellar initial mass function of early-type galaxies
• The SAURON Project - XIV. No escape from V${}_{\mathrm{esc}}$: a global and local parameter in early-type galaxy evolution
• The SLUGGS survey: Exploring the metallicity gradients of nearby early-type galaxies to large radii

Cited by in corpus (18)

• The MASSIVE Survey - I. A Volume-Limited Integral-Field Spectroscopic Study of the Most Massive Early-Type Galaxies within 108 Mpc
• Radial variations in the stellar initial mass function of early-type galaxies
• The SINFONI Nearby Elliptical Lens Locator Survey: Discovery of two new low-redshift strong lenses and implications for the initial mass function in giant early-type galaxies
• The stellar initial mass function of early type galaxies from low to high stellar velocity dispersion: homogeneous analysis of ATLAS${}^{3D}$ and Sloan Lens ACS galaxies
• A calibration of the stellar mass fundamental plane at z ~ 0.5 using the micro-lensing induced flux ratio anomalies of macro-lensed quasars
• The SLUGGS survey: Exploring the metallicity gradients of nearby early-type galaxies to large radii
• The initial mass function of a massive relic galaxy
• Further evidence for a time-dependent initial mass function in massive early-type galaxies
• Connection between dynamically derived IMF normalisation and stellar population parameters
• The Initial Mass Function of Early-type Galaxies: no correlation with [Mg/Fe]
• The initial mass functions of M31 and M32 through far red stellar absorption features
• Variations of the Initial Mass Function in Semi-Analytical models
• Testing the Universality of the Stellar IMF with Chandra and HST
• An assessment of the evidence from ATLAS3D for a variable initial mass function
• A counter-image to the gravitational arc in Abell 1201: Evidence for IMF variations, or a $10^{10}$M${}_{\odot }$ black hole?
• Lower mass normalization of the stellar initial mass function for dense massive early-type galaxies at z ~ 1.4
• Simple Stellar Population Modeling of Low S/N Galaxy Spectra and Quasar Host Galaxy Applications
• Connection between dynamically derived IMF normalisation and stellar populations