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

Abstract

(2017) 'The initial mass function in the nearest strong lenses from SNELLS : assessing the consistency of lensing, dynamical, and spectroscopic constraints.', The astrophysical journal., 845 (2). p. 157.Further information on publisher's website:https://doi.org/10.3847/1538-4357/aa816dPublisher's copyright statement: c 2017. The American Astronomical Society. All rights reserved. Additional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractWe 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-σ 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 2-3σ tension with lensing estimates, depending on the dark matter contribution, when considering IMFs that extend to 0.08 M e . Allowing a variable low-mass cutoff or a nonparametric form of the IMF reduces the tension among the IMF estimates to <2σ. There is moderate evidence for a reduced number of lowmass stars in the SNELLS spectra, but no such evidence in a composite spectrum of matched-σ ETGs drawn from the SDSS. Such variation in the form of the IMF at low stellar masses (m  0.3 M e ), 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-σ ETG. We provide the spectra used in this study to facilitate future comparisons.

Initial Mass Function Variability (or Not) among Low-velocity Dispersion, Compact Stellar Systems

“…Furthermore, there are still unexplained complications in the emerging picture of IMF variability. Newman et al (2016) demonstrated that even high-velocity dispersion ETGs can have Kroupa (2001;MW) IMFs, and, furthermore, it is not yet clear how IMF variability conforms to the expectations from chemical evolution and star formation measurements (e.g., Martín-Navarro 2016).…”

Evidence of a Non-universal Stellar Initial Mass Function. Insights from HST Optical Imaging of Six Ultra-faint Dwarf Milky Way Satellites∗

“…They find that while both methods agree in suggesting a global bottomheaviness of the IMF over their respective observational samples, a detailed comparison for the giant ellipticals in common between the samples shows large systematics and lack of agreement, however, thus casting doubts on the individual results. Furthermore, Newman et al (2017) perform a similar comparison between dynamical, spectroscopic, and, in addition, lensing-based mass estimates for a sample of three nearby giant ellipticals. They show that if the underlying IMF is parametrized as a single or broken power law, the results for the three methods disagree for two out of the three galaxies in the sample.…”

Implications of Galaxy Buildup for Putative IMF Variations in Massive Galaxies

“…Smith et al (2015b) found that for two strong-lens ETGs in the SINFONI Nearby Elliptical Lens Locator Survey (SNELLS) with σ∼300 km s −1 a bottom-heavy IMF is ruled out in favor of a Kroupa IMF, but find their 1.14 Na I μm index strengths to suggest that they have bottom-heavy IMFs (Smith et al 2015a). Recently Newman et al (2016) compared lensing, dynamical, and SPS techniques for inferring the IMF of these SNELLS galaxies, finding that the SPS stellar mass-to-light ratios exceed the total lens mass-to-light ratio, and that there is even a significant discrepancy between the lensing and dynamical masses. Newman et al (2016) discuss several possibilities for the origin of these tensions, but this study suggests that there could be systematic errors in at least one of the techniques used to probe the IMF of ETGs.…”

“…Recently Newman et al (2016) compared lensing, dynamical, and SPS techniques for inferring the IMF of these SNELLS galaxies, finding that the SPS stellar mass-to-light ratios exceed the total lens mass-to-light ratio, and that there is even a significant discrepancy between the lensing and dynamical masses. Newman et al (2016) discuss several possibilities for the origin of these tensions, but this study suggests that there could be systematic errors in at least one of the techniques used to probe the IMF of ETGs.…”