The Initial Mass Function in the Nearest Strong Lenses from SNELLS: Assessing the Consistency of Lensing, Dynamical, and Spectroscopic Constraints

TL;DR

This study compares different methods of estimating the initial mass function in nearby strong lensing galaxies, finding some tension but also potential reconciliation through variations in the low-mass end of the IMF.

Contribution

It provides a detailed comparison of lensing, dynamical, and spectroscopic IMF estimates in the nearest strong lenses, exploring IMF variations at low stellar masses.

Findings

Good agreement in one galaxy among methods

2-3σ tension in two galaxies with fixed low-mass cutoff

Reduced tension (<2σ) when allowing IMF variations

Abstract

We present new observations of the three nearest early-type galaxy (ETG) strong lenses discovered in the SINFONI Nearby Elliptical Lens Locator Survey (SNELLS). Based on their lensing masses, these ETGs were inferred to have a stellar initial mass function (IMF) consistent with that of the Milky Way, not the bottom-heavy IMF that has been reported as typical for high-$\sigma$ ETGs based on lensing, dynamical, and stellar population synthesis techniques. We use these unique systems to test the consistency of IMF estimates derived from different methods. We first estimate the stellar $M_*/L$ using lensing and stellar dynamics. We then fit high-quality optical spectra of the lenses using an updated version of the stellar population synthesis models developed by Conroy & van Dokkum. When examined individually, we find good agreement among these methods for one galaxy. The other two galaxies show 2-3$\sigma$ tension with lensing estimates, depending on the dark matter contribution, when considering IMFs that extend to 0.08 Msol. Allowing a variable low-mass cutoff or a nonparametric form of the IMF reduces the tension among the IMF estimates to $<2\sigma$. There is moderate evidence for a reduced number of low-mass stars in the SNELLS spectra, but no such evidence in a composite spectrum of matched-$\sigma$ ETGs drawn from the SDSS. Such variation in the form of the IMF at low stellar masses (m <~ 0.03 Msol), if present, could reconcile lensing/dynamical and spectroscopic IMF estimates for the SNELLS lenses and account for their lighter $M_*/L$ relative to the mean matched-$\sigma$ ETG. We provide the spectra used in this study to facilitate future comparisons.