Are the total mass density and the low-mass end slope of the IMF anticorrelated?

Abstract

We conduct a detailed lensing, dynamics and stellar population analysis of nine massive lens early-type galaxies (ETGs) from the X-Shooter Lens Survey (XLENS). Combining gravitational lensing constraints from HST imaging with spatially-resolved kinematics and lineindices constraints fromVery Large Telescope (VLT)X-Shooter spectra,we infer the low-mass slope and the low cut-off mass of the stellar initial mass function (IMF): x250 = 2.37+0.12-0.12 and Mlow,250 = 0.131+0.023-0.026M⊙, respectively, for a reference point with σ* ≡ 250 km s-1 and Reff ≡ 10 kpc. All the XLENS systems are consistent with an IMF slope steeper than Milky Way-like. We find no significant correlations between IMF slope and any other quantity, except for an anticorrelation between total dynamical mass density and low-mass IMF slope at the 87 per cent CL [dx/d log(ρ) = -0.19+0.15-0.15]. This anticorrelation is consistent with the low-redshift lenses found by Smith et al. that have high velocity dispersions and high stellar mass densities but surprisingly shallow IMF slopes.