Strong Lensing, Stellar Dynamics and the IMF in Elliptical Galaxies

The stellar initial mass function (IMF) is the distribution of stellar masses formed in a population of stars, i.e. the relative number of high-mass and low-mass stars. Since the crucial properties of stars (their luminosities, lifetimes, end-points and release of heavy
elements) are controlled to a great degree by their masses, the evolution of galaxies (their mass-to-light ratios, chemical enrichment) depends heavily on the form of the IMF.

The IMF can only be determined directly, through counting individual stars of different masses in the Milky Way and in a few of its satellite dwarf galaxies. In the absence of better information, astronomers generally assume that the Milky Way IMF applies equally to all other types of galaxy throughout the universe, regardless of galaxy mass, formation redshift, etc; but is this assumption valid? The question of whether the IMF is truly "universal" has been hotly debated in recent years, with a particular focus on massive elliptical galaxies, which formed most of their stars in violent bursts at early cosmic epochs.
Some observational studies suggest that such galaxies harbour an excess of very low-mass stars, i.e. a so-called "bottom-heavy" IMF (van Dokkum & Conroy 2010; Treu et al. 2010; Cappellari et al. 2012).

For galaxies other than the Milky Way we rely on indirect methods to probe the IMF, either measuring the subtle effects of low-mass stars in the integrated-light spectra of galaxies, or detecting their gravitational influence through stellar dynamics or gravitational lensing. Using a novel integral-field spectroscopy approach, implemented on the ESO Very Large Telescope, we have begun the "SNELLS" project, a systematic survey to discover strong-lensing ellipticals. Mass estimates for nearby lens-galaxies are relatively insensitive to assumptions about dark matter and allow more detailed study of the lenses using complementary methods. Our results for the first three such lenses were recently published (Smith, Lucey & Conroy 2015). In contrast to the evidence for bottom-heavy IMFs found in other studies, we found mass-to-light ratios consistent with a normal Milky-Way-like IMF for these galaxies.

Clearly more than three galaxies are required to resolve this controversy definitively, so we are now extending our lens search programme to enlarge the sample. The project will involve the analysis spectroscopic data from VLT instruments for the newly-discovered systems, measure spatially-resolved kinematics, and fit dynamical models to recover the mass profile. The project will also use new Hubble Space Telescope observations to improve the lens modelling of the galaxies.
Comparison between the masses derived from lensing and from stellar dynamics will test the assumptions involved in each method, and lead to improved mass-to-light ratio estimates. In parallel with this work, the project will include the ongoing lens-discovery and follow-up campaigns, with the ultimate goal of deriving robust and reliable IMF constraints from a large and uniquely-powerful sample of lensing ellipticals.