To achieve the goal of performing pRecision cOsmography with Strong lEnsing gAlaxy clUsters (ROSEAU), high-precision strong lensing models of lens clusters represent a first, crucial step. Leveraging the recent avenue of high-resolution, multi-band imaging, together with extensive follow-up spectroscopic data, this project has exploited high-quality data from the HST, JWST, and VLT/MUSE of several lens clusters. In this context, we have presented a new strong lensing model of the extraordinary galaxy cluster SDSS J2222+2745, one of the few currently known lens clusters with six multiple images of a background quasar with measured time delays. We have shown that, in lens clusters with a limited number of secure multiple images, the predicted magnification and time delay values can be prone to systematic uncertainties and model degeneracies. To avoid possible biases in the derived values of the cosmological parameters, it is then crucial to include additional information, such as the observed surface brightness distribution of lensed sources and the measured time delays (Acebron, Grillo, Bergamini, et al., 2022).
ROSEAU is then extending typical lensing analyses in clusters beyond the state of the art with a novel extended surface brightness lensing modelling of the unique lens cluster SDSS J1029+2623, which shows a quasar host galaxy lensed into a long tangential arc (see the attached Figure), over ~78000 HST pixels. Overcoming modelling and computational challenges, we have shown that the extended lens model reproduces remarkably well the observed intensity and morphology of the host galaxy in the near-infrared HST F160W band.
The ideal scenario consists of multiple cosmological probes in one, where a single system would provide independent distance measurements. We have shown how Type Ia supernovae (SNe Ia) exploding in cluster member galaxies can provide an independent luminosity-distance measurement to the lens cluster, highly complementing the use of time delays. In particular, the joint method enables a gain in precision on the estimate of the value of the Hubble constant by a factor of up to ∼1.2 compared to the results from exploiting time delays alone (Acebron, Schuldt, Grillo, et al., 2023). Thanks to the Marie Curie Individual Fellowship, additional funding from the University of Milan was awarded to the project. The funding was used to secure telescope time at the Nordic Optical Telescope to carry out a monthly, wide-field monitoring of the lens galaxy cluster MACS J1149.5+2223 to implement this novel idea. The programme is still on-going.
Beyond the exploration of alternative cosmological probes, it is also important to further understand and quantify the systematic uncertainties within each technique. To define which lens clusters are best suited for cosmological applications and how many will be necessary to reach a ~1% precision, we have used a highly-realistic mock lens cluster to explore the effects of modelling constraints on the resulting precision and accuracy of the derived values of relevant cosmological parameters.
