Baryons at the Edge of the X-ray Brightest Galaxy Cluster (1102.2429v1)

Abstract: Studies of the diffuse X-ray emitting gas in galaxy clusters have provided powerful constraints on cosmological parameters and insights into plasma astrophysics. However, measurements of the faint cluster outskirts have become possible only recently. Using data from the Suzaku X-ray telescope, we determined an accurate, spatially resolved census of the gas, metals, and dark matter out to the edge of the Perseus Cluster. Contrary to previous results, our measurements of the cluster baryon fraction are consistent with the expected universal value at half of the virial radius. The apparent baryon fraction exceeds the cosmic mean at larger radii, suggesting a clumpy distribution of the gas, which is important for understanding the ongoing growth of clusters from the surrounding cosmic web.

• The paper employs Suzaku observations to map the radial distribution of baryons in the Perseus Cluster outskirts.
• Its spectral analysis reveals detailed temperature and metallicity profiles, highlighting gas clumping beyond 0.5 r200.
• The study refines mass estimates using an NFW model, aligning the observed baryon fraction with cosmic expectations.

Analysis of Baryons in the Outskirts of the X-ray Brightest Galaxy Cluster

The paper presents a detailed investigation into the baryonic composition at the outskirts of the Perseus Cluster using observations from the Suzaku X-ray telescope. The Perseus Cluster, known for its brightness and proximity, serves as an exemplary subject for investigating the outer regions of galaxy clusters — zones that were previously less understood due to observational constraints.

Key Observations and Data Analysis

The authors utilize 260 kiloseconds of Suzaku data across a large mosaic, extending two degrees from the core of the Perseus Cluster. This strategic observational setup allows for a precise mapping of the baryon distribution, including the gas, metals, and dark matter, out to the cluster's virial radius. The deployment of Suzaku, owing to its lower instrumental background, enables the measurement of faint emissions in the cluster outskirts, providing an opportunity to move beyond prior limitations that relied heavily on extrapolation from inner cluster data.

The extraction of spectra from annuli centered on the cluster's core and the modeling of these spectra as single-temperature thermal plasmas has revealed insightful radial profiles of temperature and metallicity. These profiles showcase the cluster's outer regions' metal enrichment, reaching up to one third of solar metallicity, and highlight thermodynamic disparities, such as colder regions known as “cold fronts.”

Baryon Fraction and Gas Clumping

A significant contribution of this paper lies in the measurement of the baryon fraction within Perseus, which aligns with the universal baryon fraction up to half the virial radius. Interestingly, beyond this limit, the baryon fraction appears to exceed the cosmic mean, pointing toward the presence of gas clumping. Such clumping is of cosmological interest as it provides insight into the cluster's growth dynamics from the cosmic web.

Thermodynamic Profiles and Cosmic Implications

The thermodynamic analysis extends to electron density, entropy, and pressure profiles, revealing that outside central cold fronts, these profiles adhere well to theoretical expectations. The entropy profile notably displays a flattening beyond 0.66 times the virial radius, consistent with previous findings, while the observed pressure profile is shallower than previous expectations.

Employing the assumption of hydrostatic equilibrium, the authors fit a Navarro-Frenk-White (NFW) model to the North-West arm data, estimating the total mass distribution. The resulting virial radius ($r_{200}$) and total mass ($M_{200}$) estimations align well with theoretical models and simulations, corroborating the NFW profile's applicability to such analyses.

Addressing Gas Clumping

Contrary to earlier assumptions of baryon deficits at large radii, this paper suggests significant gas clumping outside 0.5$r_{200}$. By correcting for clumping, the profiles present a reconciled picture with theoretical models, eliminating any need for additional physics to explain the observed thermodynamic behaviors. The implications of this clumping can be profound, necessitating refined models of the intra-cluster medium (ICM) physics that consider the effects of clumping on estimates of gas properties.

Conclusions and Future Directions

The findings underscore the necessity of incorporating gas clumping considerations in the paper of galaxy cluster outskirts, potentially altering interpretations of thermodynamics in these regions. The Suzaku observations of Perseus set a precedent for future cluster studies, emphasizing the need to validate these findings across other clusters and further refine simulations to accommodate the physical processes underlying baryon and gas distributions.

Future research might focus on leveraging combined X-ray and SZ observations to independently quantify gas clumping, enhancing the robustness of cosmological inferences drawn from cluster studies. This work effectively highlights the intricate balance of observational precision and theoretical modeling required to advance our comprehension of galaxy cluster dynamics and their role as cosmological probes.