AR_final file_2018-19

Figure 7: . together with strong lensing and stellar kinematics. The analysis was based on two galaxy samples: 45 strong lenses from the SLACS survey and 1700 mas- sive quiescent galaxies from the Sloan Digital Sky Survey main spectroscopic sample with weak lens- ing measurements from the Hyper Suprime-Cam survey. A Bayesian hierarchical approach was used to jointly model all three observables. The data was fit with models of varying complexity, and it was shown that a model with a radial gradient in the stellar M/L is required to simultaneously de- scribe both galaxy samples. This result is driven by a subset of strong lenses with very steep total density profile that cannot be fitted by models with no gradient. The measurements are unable to de- termine whether M*/L gradients are due to varia- tions in stellar population parameters at fixed IMF, or to gradients in the IMF itself. The inclusion of such gradients decreases dramatically the inferred IMF normalization, compared to previous lensing- based studies, with the exact value depending on the assumed dark matter profile. The main effect of strong lensing selection is to shift the stellar mass distribution towards the high-mass end, while the halo mass and stellar IMF distribution at fixed stel- lar mass are not significantly affected. Constraining the stellar IMF with gravitational lenses The determination of the stellar initial mass func- tion (IMF) of massive galaxies is one of the open problems in cosmology. Strong gravitational lens- ing is one of the few methods that allow us to constrain the IMF at cosmological distances. The goal of this study was to statistically constrain the distribution in the IMF mismatch parameter, de- fined as the ratio between the true stellar mass of a galaxy and that inferred assuming a reference IMF, of massive galaxies from the BOSS CMASS sample. Anupreeta More and collaborators have taken 22 gravitational lens systems drawn from the CMASS sample, measured their Einstein radii and stellar masses using multi-band photometry from HSC, then fitted a model distribution for the IMF mismatch parameter and dark matter halo mass to the whole sample. They use a prior on halo mass from weak lensing measurements and account for strong lensing selection effects. Assuming an NFW density profile for the dark matter distribu- tion, they infer a value µ IMF = 0.03 ± 0.11 for the average base-10 logarithm of the IMF mismatch pa- rameter, defined with respect to a Chabrier IMF. A Salpeter IMF is in tension with this measurement. Uniform modelling of quadruply lensed quasars Gravitational lens systems with quadruply imaged quasars (quads) are unique probes to address sev- eral fundamental problems in cosmology and astro- physics. Although, they are intrinsically very rare, ongoing and planned wide-field deep-sky surveys are set to discover thousands of such systems in the next decade. It is, thus, paramount to devise a general framework to model strong-lens systems to cope with this large influx without being limited by expert investigator time. Anupreeta More and collaborators have proposed such a general mod- elling framework (implemented with the publicly available software LENSTRONOMY), and apply it to uniformly model three-band Hubble Space Tele- scope Wide Field Camera 3 images of 13 quads. This is the largest uniformly modelled sample of ( 67 )