[Paper Review] Cosmological Constraints from Gas Mass Fractions of Massive, Relaxed Galaxy Clusters

A. Mantz, S. W. Allen+15 more|arXiv (Cornell University)|Nov 17, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy79 references59 citations

TL;DR

This study improves cosmological constraints using gas mass fractions (𝑓gas) in 44 massive, relaxed galaxy clusters, leveraging deeper Chandra X-ray observations and new high-redshift clusters (𝑧 = 0.97, 1.16) and a precise low-redshift measurement of the Perseus Cluster (𝑧 = 0.018). The enhanced redshift leverage reduces uncertainties in dark energy constraints, yielding ΩΛ = 0.865 ± 0.119 in non-flat ΛCDM and 𝑤 = −1.13+0.17−0.20 in flat constant-𝑤 models—41% and 29% tighter than previous work, respectively.

ABSTRACT

We present updated cosmological constraints from measurements of the gas mass fractions ($f_{gas}$) of massive, dynamically relaxed galaxy clusters. Our new data set has greater leverage on models of dark energy, thanks to the addition of the Perseus Cluster at low redshifts, two new clusters at redshifts $z>0.97$, and significantly longer observations of four clusters at $0.6<z<0.9$. Our low-redshift ($z<0.16$) $f_{gas}$ data, combined with the cosmic baryon fraction measured from the cosmic microwave background (CMB), imply a Hubble constant of $h = 0.722 \pm 0.067$. Combining the full $f_{gas}$ data set with priors on the cosmic baryon density and the Hubble constant, we constrain the dark energy density to be $\Omega_\Lambda = 0.865 \pm 0.119$ in non-flat $\Lambda$CDM (cosmological constant) models, and its equation of state to be $w = -1.13_{-0.20}^{+0.17}$ in flat, constant-w models, respectively 41 and 29 per cent tighter than our previous work, and comparable to the best constraints available from other probes. Combining $f_{gas}$, CMB, supernova, and baryon acoustic oscillation data, we also constrain models with global curvature and evolving dark energy. For the massive, relaxed clusters employed here, we find the scaling of $f_{gas}$ with mass to be consistent with a constant, with an intrinsic scatter that corresponds to just 3 per cent in distance.

Motivation & Objective

• . The primary objective is to improve cosmological constraints on dark energy using gas mass fractions (𝑓gas) in massive, relaxed galaxy clusters.
• The study aims to enhance the redshift leverage of 𝑓gas data by incorporating new high-redshift clusters (𝑧 = 0.97, 1.16) and deeper observations of existing clusters at 0.6 < 𝑧 < 0.9.
• It seeks to tighten constraints on the Hubble constant by combining low-redshift 𝑓gas with cosmic microwave background (CMB) data on the baryon fraction.
• The work aims to test the consistency of 𝑓gas scaling with cluster mass and assess intrinsic scatter, minimizing systematic uncertainties.
• It evaluates the viability of extending the 𝑓gas method to probe evolving dark energy and non-flat cosmological models.

Proposed method

• . The analysis uses deep Chandra X-ray observations to measure the gas mass fraction (𝑓gas = 𝑀gas/𝑀tot) in 44 massive, dynamically relaxed galaxy clusters.
• X-ray data are combined with weak gravitational lensing measurements to calibrate total cluster mass, reducing dependence on hydrostatic equilibrium assumptions.
• The study applies the SPA (Symmetry-Peakiness-Alignment) criterion to select only the most relaxed clusters, minimizing morphological systematics.
• Cosmological constraints are derived by comparing observed 𝑓gas to the cosmic baryon fraction (Ωb/Ωm) measured from the CMB, assuming minimal evolution in 𝑓gas with redshift.
• The analysis models the redshift evolution of 𝑓gas using a power-law parametrization, with a free normalization at 𝑧 = 0 and a slope parameter to test for mass or redshift dependence.
• Systematic uncertainties from mass calibration, gas temperature, and intrinsic scatter are modeled and marginalized over using a hierarchical Bayesian framework.

Experimental results

Research questions

• RQ1 . How do updated 𝑓gas measurements from deeper observations and new high-redshift clusters improve constraints on dark energy?
• RQ2 . To what extent does the inclusion of the Perseus Cluster (𝑧 = 0.018) enhance low-redshift leverage and tighten Hubble constant constraints?
• RQ3 . What is the intrinsic scatter in 𝑓gas across the cluster sample, and how does it relate to distance uncertainty?
• RQ4 . Is there a significant dependence of 𝑓gas on cluster mass in the 0.8–1.2 𝑟2500 radial range, and what does this imply for cosmological modeling?
• RQ5 . How do the updated 𝑓gas constraints compare with other cosmological probes when combined with CMB, supernova, and baryon acoustic oscillation data?

Key findings

• . The addition of the Perseus Cluster at 𝑧 = 0.018 and two new high-redshift clusters (𝑧 = 0.97, 1.16) significantly improves redshift leverage on the evolution of 𝑓gas.
• . The constraint on the dark energy density in non-flat ΛCDM is tightened to ΩΛ = 0.865 ± 0.119, a 41% improvement over previous work.
• . The constraint on the dark energy equation of state in flat, constant-𝑤 models is improved to 𝑤 = −1.13+0.17−0.20, a 29% improvement over prior results.
• . The intrinsic scatter in the 𝑓gas–mass relation is measured at 𝜎𝑓 = 0.043+0.020−0.032, equivalent to just ∼3% scatter in distance estimates.
• . The power-law slope of 𝑓gas with mass is constrained to 𝛼 = 0.025 ± 0.033, consistent with no significant mass dependence in the 0.8–1.2 𝑟2500 range.
• . When combined with CMB, supernova, and BAO data, the results remain consistent with the flat ΛCDM model, with no evidence for evolving dark energy or curvature.