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Systematic variations of the IMF in early-type galaxies, and their connectionwith possible drivers such as velocity dispersion or metallicity, have beenmuch debated in recent years. Strong lensing over galaxy scales combined withphotometric and spectroscopic data provides a powerful method to constrain thestellar mass-to-light ratio and hence the functional form of the IMF. Wecombine photometric and spectroscopic constraints from the latest set ofpopulation synthesis models of Charlot & Bruzual, including a varying IMF, witha non-parametric analysis of the lens masses of 18 ETGs from the SLACS survey,with velocity dispersions in the range 200-300 km/s. We find that verybottom-heavy IMFs are excluded. However, the upper limit to the bimodal IMFslope ($\mu \lesssim 2.2$, accounting for a dark matter fraction of 20-30%,where $\mu=1.3$ corresponds to a Kroupa-like IMF) is compatible at the$1\sigma$ level with constraints imposed by gravity-sensitive line strengths. Atwo-segment power law parameterisation of the IMF (Salpeter-like for highmasses) is more constrained ($\Gamma \lesssim 1.5$, where $\Gamma$ is the powerindex at low masses) but requires a dark matter contribution of $\gtrsim 25\%$to reconcile the results with a Salpeter IMF. For a standard Milky Way-like IMFto be applicable, a significant dark matter contribution is required within$1R_e$. Our results reveal a large range of allowed IMF slopes, which, wheninterpreted as intrinsic scatter in the IMF properties of ETGs, could explainthe recent results of Smith et al., who find Milky Way-like IMF normalisationsin a few massive lensing ETGs.

Strong Gravitational Lensing and the Stellar IMF of Early-type Galaxies