The XLENS Project: Do More Massive Early-Type Galaxies Have More Dark Matter or Different Stellar IMFs?

The X-shooter Lens Survey (XLENS) aims to study the interplay of dark matter (DM) and stellar content in the inner regions of massive early-type galaxies (ETGs) by combining strong gravitational lensing, dynamical models, and spectroscopic stellar population analysis. XLENS targets a sample of ETGs from the SLACS survey (The Sloan Lens ACS Survey, e.g. Bolton et al. 2006) with velocity dispersions >=250 kms^-1 using the X-Shooter spectrograph on ESO's Very Large Telescope. Recent observations indicate that the internal dark-matter fraction of ETGs increases rapidly with galaxy mass, although some hints for a varying initial mass function (IMF) have also been suggested, where the low-mass end of the stellar IMF steepens with galaxy mass. XLENS first results unambiguously confirm that DM plays an important role already within one effective radius for very massive systems (Spiniello et al. 2011). Moreover, studying equivalent widths of certain red spectral features which are indicators of low-mass stars in massive ETGs (e.g. NaI and TiO2) as a function of age and metallicity (i.e. Mgb, Fe, Hβ), and as function of stellar velocity dispersion, has shown that the IMF slope is varying mildly with galaxy mass (Spiniello et al. 2012).