Cosmology from UNIONS weak lensing profiles of galaxy clusters

TL;DR

This work introduces a novel cosmological probe based on the mass distribution in the infall regions of galaxy clusters, accessible through UNIONS weak-lensing profiles. By comparing observed $\Delta\Sigma(R)$ in three public cluster catalogues to 19 dark-matter simulations across $\Omega_{ m m}$ and $\sigma_8$, the authors derive constraints on these parameters and measure mean splashback radii. They fit a physically motivated 3D density model (Diemer-fit) to the profiles, incorporating mis-centering and mass scatter, and also explore using the splashback and truncation radii as cosmological tracers. The results demonstrate competitive constraints and highlight the potential for significant improvements with the full UNIONS data set, while outlining current limitations from sample completeness and mass calibration systematics.

Abstract

Cosmological information is encoded in the structure of galaxy clusters. In Universes with less matter and larger initial density perturbations, clusters form earlier and have more time to accrete material, leading to a more extended infall region. Thus, measuring the mean mass distribution in the infall region provides a novel cosmological test. The infall region is largely insensitive to baryonic physics, and provides a cleaner structural test than other measures of cluster assembly time such as concentration. We consider cluster samples from three publicly available galaxy cluster catalogues: the Spectroscopic Identification of eROSITA Sources (SPIDERS) catalogue, the X-ray and Sunyaev-Zeldovich effect selected clusters in the meta-catalogue M2C, and clusters identified in the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey. Using a preliminary shape catalogue from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), we derive excess surface mass density profiles for each sample. We then compare the mean profile for the DESI Legacy sample, which is the most complete, to predictions from a suite of simulations covering a range of $Ω_{\rm m}$ and $σ_8$, obtaining constraints of $Ω_{\rm m}=0.34\pm 0.06$ and $σ_8=0.77 \pm 0.04$. We also measure mean (comoving) splashback radii for SPIDERS, M2C and DESI Legacy Imaging Survey clusters of $1.39^{+0.21}_{-0.18} {\rm cMpc}$, $1.77^{+0.20}_{-0.18} {\rm cMpc}/h$ and $1.42^{+0.11}_{-0.12} {\rm cMpc}/h$ respectively. Performing this analysis with the final UNIONS shape catalogue and the full sample of spectroscopically observed clusters in DESI, we can expect to improve on the best current constraints from cluster abundance studies by a factor of 2 or more.

Figures (12)

• Figure 1: Left: Mass distributions of the three cluster catalogues used in this work. M2C and SPIDERS clusters have masses calibrated from an X-ray luminosity -- mass relationship, and in DLIS a total stellar mass-cluster mass relationship is used. For plotting purposes only, $M_{500{\rm c}}$ values in DLIS have been converted to $M_{200{\rm c}}$ using a fiducial cosmology ($\Omega_{\rm m}=0.3$, $\sigma_8=0.8$). Also shown are all the mass distributions of dark matter halos with $0.1<z<0.5$ in $19$ cosmological simulations with different values of $\Omega_{\rm m}$ and $\sigma_8$, where darker line colours are simulations with larger values of $S_8$. Middle: Redshift distributions of clusters. The redshift of halos in the simulations are taken from 29 snapshots between $z=0.1$ and $z=0.5$. The difference in the redshift distributions explains the large discrepancy in the mass function in the left panel. Right: The mass-redshift distribution, illustrating the selection biases in the SPIDERS and M2C catalogues.
• Figure 2: Data points with error bars show mean excess mass density profiles $\Delta\Sigma(R)$ (in comoving units) for three publicly available lens catalogues, calculated with the code dsigma from a UNIONS shape catalogue. The DESI Legacy Imaging Survey (DLIS) catalogue is used with two choices for the minimum mass in the subsample (bottom panels). The value $\Omega_{\rm m}=0.3$ is used to convert angles and redshifts to distances. Also overlaid are the corresponding mean profiles from simulations with $\Omega_{\rm m}=0.3$ and $\sigma_8=0.7, 0.8, 0.9$ (solid, dashed, dash-dot respectively). The uncertainty in a simulation profile is given by a shaded band. For DLIS, the match is performed using all simulated halos above the quoted mass limit and weighting the contribution of each halo to the mean profile based on the ratio of observed clusters and simulated halos at that halo's redshift. For M2C and SPIDERS, when taking the mean we weight the contribution of each simulated halo by the relative number of halos at that mass in the observed sample. We have corrected the simulated profiles for mis-centering, based on fits to the observed data. The grey band in the bottom right panel indicates the radial range used to compare the observed profile to the simulated profiles in our cosmological analysis.
• Figure 3: Cosmological constraints from comparing simulated dark matter halos, with UNIONS weak lensing profiles of DESI Legacy Imaging Survey (DLIS) galaxy clusters with $M_{500{\rm c}}>10^{14}M_{\odot}/h$. The observed profile in the infall region is compared to the profile in each of a suite of $19$ cosmological simulations varying $\Omega_{\rm m}$ and $\sigma_8$ (black points) and a goodness-of-fit $\chi^2$ value is calculated for each. A 3D paraboloid is fit to these $\chi^2$ values. The best fit cosmology is indicated with a green cross, $68.3\%$ and $95.5\%$ confidence regions at $\Delta\chi^2=2.3,6.81$ from the minimum are overlaid. The dash-dot contour is the case when no redshift re-weighting of the simulation profiles is included.
• Figure 4: Left: Observed excess surface mass densities for each of the cluster samples. For DLIS, we show the result from two different choices for the lower mass limit. These profiles are fitted with a 3D density profile model Diemer_fit. Right: The gradient of the 3D density profile fit is used to identify the splashback radius of each cluster sample.
• Figure 5: $68.3\%$ and $95.5\%$ credible regions for parameters of the 3D density profile model of Diemer_fit, fitted to the observed $\Delta\Sigma$ profile of three publicly available cluster catalogues. Chains are created using a Preconditioned Monte Carlo algorithm pocomc2pocomc1