Towards the First HSC-SSP Cluster Cosmology Results: Focus on Robustness against Projection Effects

2021 Fiscal Year Research-status Report

• Project/Area Number: 21K13917
• Research Institution: The University of Tokyo
• Principal Investigator: PARK YOUNGSOO 東京大学, カブリ数物連携宇宙研究機構, 特任研究員 (20835467)
• Project Period (FY): 2021-04-01 – 2023-03-31
• Keywords: Cosmology / Galaxy Clusters / Weak Lensing / Large Scale Structure

Outline of Annual Research Achievements

During FY2021, I have completed the initially planned FY2021 research goal of "modeling the aligned filaments underlying optical clusters." Based on simulation results, a fitting formula was developed for the impact of these filaments on cluster observables, and was tested for its validity across different richness and separation regimes.
After finishing the planned FY2021 goal, I further moved on to work on the planned FY2022 goals. First, the developed model from above was integrated into an end-to-end cluster cosmology pipeline. This novel pipeline was then validated in a blind cosmology challenge, where I used the pipeline to constrain cosmological parameters from a completely new set of simulations with zero prior knowledge of the underlying parameters for these simulations. The validation successfully demonstrated that the new pipeline was accurate and precise.
Finally, the validated pipeline was applied to real data, and I have obtained cosmological parameter constraints using the SDSS redMaPPer cluster catalog and the SDSS lensing measurements. The results interestingly suggested several key "holes" in our understanding of cluster observables, especially for cosmological purposes. This resulted in a publication, currently posted on arXiv (arXiv:2112.09059) and in review with MNRAS.

Current Status of Research Progress

1: Research has progressed more than it was originally planned.

Reason

The initially planned FY2021 goal was completed 100%, and in addition roughly 50% of the planned FY2022 goal was completed, already resulting in a publication. Thus, I believe that the project is progressing at an even faster pace than expected. Furthermore, the results obtained and summarized in the aforementioned publication suggests several key shortcomings in our current understanding of cluster observables, opening up a whole new avenue of research questions that would ultimately lead to a robust cluster cosmology analysis and a fundamental shift in the way we treat cluster observables for cosmological analyses. This I believe is an unexpected yet deeply exciting outcome from this year's research activities.

Strategy for Future Research Activity

The ultimate goal of this project is to implement a robust cluster cosmology analysis using HSC-SSP data. In light of the newest findings I obtained, it will be crucial to first analyze the biggest puzzle in the relevant physics, i.e., the apparent suppression of the cluster lensing signal not observed in simulations. Comparing cluster lensing measurements with different source samples will expedite this investigation, and fortunately, this can be done rather quickly for HSC-SSP with existing data and analysis pipelines. Thus, the first step in future research will be examining cluster lensing measurements using SDSS redMaPPer clusters and HSC-SSP shapes, and once this solves the suppression puzzle I will finally implement the HSC-SSP cluster cosmology analysis.

Causes of Carryover

During FY2021, I have requested for advance funding, anticipating additional travel and other research activities in line with the faster than expected pace at which the research project was progressing. However, due to COVID and other external circumstances this additional anticipated expenditure was actually not needed, so I ended up spending exactly the amount of funding that I originally planned and requested at the time of proposal submission. Thus the remaining amount is simply an advance funding drawn from FY2022 that is "going back" to where it originally came from.