Precision Cosmography with Strong Lensing Galaxy Clusters

Project Information

ROSEAU

Grant agreement ID: 101024195

DOI

10.3030/101024195

Project terminated on 30 November 2023

EC signature date 30 March 2021

Start date 1 March 2022

End date 29 February 2024

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 171 473,28

EU contribution

€ 171 473,28

Coordinated by

UNIVERSITA DEGLI STUDI DI MILANO
Italy

Periodic Reporting for period 1 - ROSEAU (Precision Cosmography with Strong Lensing Galaxy Clusters)

Reporting period: 2022-03-01 to 2024-02-29

Summary of the context and overall objectives of the project

A wide variety of cosmological probes allows us to measure the values of the cosmological parameters that define fundamental properties of the Universe, such as its age, size, its geometry, or its expansion rate. As cutting-edge experiments have become more precise, some discrepancy between the measurements of the expansion rate (defined by the Hubble constant) of the Universe from different methods has arisen, pointing to possible unknown biases or a call for new and interesting physics. In this context, the exploration of alternative and complementary techniques is crucial.

Gravitational lensing occurs when the light rays from a background source are deflected by a galaxy or a galaxy cluster in the foreground, which acts as a lens. In the strong-lensing regime, multiple images of background sources are formed. The observed positions of these multiple images do not only depend on the total mass distribution of the foreground lens, but also on the underlying geometry of the Universe. In addition, if the luminosity of a multiply lensed source is intrinsically time varying, such as that of supernovae or quasars, the differences in light arrival times between the multiple images (or time delays) can be measured, enabling us to measure the expansion rate of the Universe.

The main challenge associated with strong lensing cosmography relies on the fact that the characterisation of the cluster total mass distribution and the determination of the cosmological parameter values are closely intertwined. To exploit strong lensing galaxy clusters as high-precision cosmological probes, the total mass distribution of the lens clusters must be determined very accurately, driving this project to develop improved and accurate strong lensing models of galaxy clusters with a proper assessment of the leading sources of systematics. The ultimate goal of the project is to exploit these extraordinary lens systems to measure the cosmic expansion rate and geometry.

This project is timely and strategically important given i) the current, intriguing tension in the measurements of the value of the cosmic expansion rate from different probes, and ii) the advent of large-volume, high-quality observations of lens galaxy clusters from state-of-the-art facilities. Developing new data analysis techniques will thus set a benchmark to exploit the exquisite, cutting-edge data from future facilities.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

To reach our goals, we have implemented new modelling techniques and developed novel ideas that go beyond the state of the art in the cluster strong lensing field, such as:
- the improvement of cluster strong lensing mass models by directly including as observational constraints the extended surface brightness distribution of strongly-lensed sources.
- the demonstration of the powerful combination of measured time delays in galaxy clusters with SNe Ia exploding in the member galaxies to obtain robust measurements of the expansion rate and geometry of Universe.
- the assessment of possible leading sources of systematic errors in parametric cluster lens models by exploiting realistic simulated lens clusters.

Expected results by the end of the project include:
- overcoming both computational and modelling challenges, this project will provide the first extended strong lensing model of a multiply-lensed source in a lens cluster, which will be used to infer the value of the Hubble constant.
- new and independent measurements of the cosmic expansion rate and geometry with a small sample of lens galaxy clusters with measured time delays.
- as part of an on-going pilot observational program, the potential finding of a SN Ia exploding in a member galaxy of MACS J1149.5+2223 and the combination of the derived luminosity distance with the measured time delays between the multiple images of SN 'Refsdal'.
- the definition of an efficient observing strategy by selecting lens clusters that yield the most precise and accurate measurements of the values of the parameters defining the expansion rate and geometry of Universe.

The results from ROSEAU will pave the way for a new generation of cluster strong lensing models, where all available lensing observables are incorporated as model constraints, allowing to fully exploit the observations from current and upcoming state-of-the-art facilities such as the LSST or from European missions such as Euclid.

Colour-composite image of the central region of the unique lens cluster SDSS J1029+2623.