Lenses for the Unresolved: Probing the Stellar Initial Mass Function of Nearby Massive Early-Type Galaxies

Abstract

The stellar initial mass function (IMF) is a key component of understanding galaxy formation and evolution, and to interpret observed properties (e.g. estimating stellar masses). In the different star-forming environments of the Milky Way (MW), the IMF is well constrained and approximately invariant. However, stellar populations within massive elliptical galaxies were likely formed by short, intense star formation events at early times, and recent observations suggest their cores deviate from a universal IMF. Studies using spectral analysis, stellar dynamics, and gravitational lensing report that for the most massive galaxies (velocity dispersion sigma > 300 km/s), the measured stellar mass-to-light ratios (M/L) are a factor of two larger than implied by a MW-like IMF. However, a subset of three low-z early-type galaxy (ETG) strong-gravitational lenses (SNL-0, SNL-1, and SNL-2) with sigma ~ 300 km/s appear to contradict these results. The M/L measured from lensing analyses are consistent with a MW-like IMF, i.e. Kroupa with <alpha> = 1.06 +/- 0.08. The mass-excess parameter, alpha, is relative to the MW, defining 1 as a MW-like IMF, and 1.64 as a Salpeter IMF. These lenses are situated at low-z where, due to the geometry of the lens, the contribution from dark matter within the Einstein radius is lower than for a more distant lens. In this thesis, I investigate the IMF within low-z massive strong-lensing ETGs. The analysis comprises two main parts. First, a re-analysis of two low-z lenses, SNL-1 and SNL-2, using high-resolution Hubble Space Telescope data. For SNL-1, I break the mass-shear degeneracy and measure alpha relative to the MW as alpha = 1.17 +/- 0.09. For SNL-2, the mass of the similar brightness companion galaxy is constrained. The derived alpha is 0.96 +/- 0.10. Both are consistent with a MW-like IMF and inconsistent with Salpeter or ‘heavier’ IMFs. The second part of this thesis is a new lens search, using the ESO/VLT Multi-Unit Spectroscopic Explorer (MUSE). The search consists of a targeted survey with new observations specific to the search (MNELLS), and an archival search of data observed for other science. The search yielded one new galaxy-scale lens, J0403-0239, three cluster-scale lenses, and nine galaxy-scale systems in which a single background source is detected within 6 arcsec. J0403-0239 lies at z = 0.0665 and has two extremely bright and extended lensed images at z = 0.1965. Although the lens is at a slightly higher redshift than the other low-z lenses, the Einstein radius probes just one-quarter of the effective radius. The measured alpha is consistent with a MW-like IMF, alpha = 1.16 +/- 0.09, with a robustly determined old stellar population. Each galaxy-scale lens or singly imaged system is used to constrain the parameters for the ensemble population of the IMF within ETGs. The population has <alpha> = 1.06 +/- 0.08 and an upper limit on the intrinsic scatter of nu = 0.24, at 90 per cent confidence. These constraints are consistent with Salpeter only at the 2.4 sigma level.