On the Cluster Physics of Sunyaev-Zel'dovich Surveys I: The Influence of Feedback, Non-thermal Pressure and Cluster Shapes on Y-M Scaling Relations (1109.3709v1)

Abstract: The utility of large Sunyaev Zel'dovich (SZ) surveys for determining cosmological parameters from cluster abundances is limited by the theoretical uncertainties in the integrated SZ-flux-to-mass relation, Y-M. We explore how non-thermal pressure and the anisotropic shape of the gas distribution of the intracluster medium (ICM) impacts Y-M scaling using a suite of SPH simulations of the cosmic web. We contrast results for models with different treatments of entropy injection and transport, varying radiative cooling, star formation and accompanying supernova feedback, cosmic rays, and energetic feedback from active galactic nuclei (AGN). We find that the gas kinetic-to-thermal pressure ratio from internal bulk motions depends on the cluster mass, and increases in the outer-cluster due to enhanced substructure, as does the asphericity of the ICM gas. With only a ~5-10% correction to projected (observable) ellipticities, we can infer the 3D ellipticities. Our simulated Y-M-slope roughly follows the self-similar prediction, except for a steepening due to a deficit of gas in lower mass clusters at low redshift in our AGN-feedback simulations. AGN feedback enhances slightly the overall Y-M-scatter, from ~11% to ~13%, a reflection of accretion history variations due to cluster merging. If we split the cluster system into lower, middle and upper bands of both P_kin/P_th and long-to-short axis ratio, we find a ~10% effect on Y-M. Identifying observable second parameters related to internal bulk flows and anisotropy for cluster-selection to minimize Y-M scatter in a "fundamental plane" would allow tighter cosmological parameter constraints.

Overview of Sunyaev-Zel'dovich (SZ) Surveys and Cluster Physics

The paper "On the Cluster Physics of Sunyaev-Zel'dovich Surveys I: The Influence of Feedback, Non-thermal Pressure and Cluster Shapes on $Y$-$M$ Scaling Relations" addresses several pivotal factors impacting the Sunyaev-Zel'dovich effect observed in galaxy clusters, a phenomenon arising from cosmic microwave background (CMB) photons being up-scattered by high-energy electrons within the intracluster medium (ICM). Specifically, the paper explores non-thermal pressure contributions, feedback mechanisms, and cluster shape — each critical in understanding the integrated SZ-flux-to-mass relation, $Y$-$M$, essential for deriving cosmological parameters.

Key Findings and Numerical Results

1. Non-thermal Pressure:
   • Simulation results depict significant radial dependence of kinetic-to-thermal pressure ratios, $P_{\mathrm{kin}/P_{\mathrm{th}}$, which increases with cluster radius and varies based on cluster mass — more substantial in massive clusters.
   • At redshift $z = 0$, this pressure ratio reaches approximately 20% at $R_{500}$, validating its relevance in cluster virialization processes.
2. Cluster Shapes:
   • The paper measures the gas density and pressure distribution shapes in the ICM, finding that deviations from spherical symmetry are mass and redshift-dependent but less pronounced than those in the dark matter distribution.
   • Shape analysis reveals that the gas in clusters is more spherical, with axis ratios ($c/a$) consistently near 0.9 within $R_{500}$. This modest deviation is crucial for ensuring the spherical assumption prevalent in many semi-analytic models.
3. Impact of Feedback:
   • Incorporating AGN feedback leads to noticeable changes in cluster core profiles, slightly decreasing kinetic pressure contributions and increasing scatter in $Y$-$M$. Feedback mechanisms, therefore, contribute to deviating from self-similarity in the $Y$-$M$ scaling relation.
   • At $z = 0$, AGN feedback simulations show lower amplitude $Y$-$M$ relations than shock-heating-only scenarios, as anticipated due to entropy injection altering the ICM thermodynamics and mass proxies.
4. Self-similarity and Scaling Relations:
   • The simulated $Y$-$M$ relations generally follow the self-similar $Y \propto M^{5/3}$ law, recognizing slight deviations with redshift consistent with hierarchical structure formation assumptions.
   • AGN feedback enhances the scatter from approximately 11% to 13% in $Y$-$M$, indicating the energy injection's effect on observational accuracy.

Implications for Cosmology and Future Directions

The implications of these findings are significant for cosmological applications where precise measurements of cluster abundance and thermal energies are paramount. Understanding the nuances of internal pressure dynamics and ICM shapes aids in refining cosmological models through SZ surveys. The work underscores the necessity of advanced simulations to disentangle cosmological signals from astrophysical processes intricately woven into the fabric of cluster evolution.

Future research directions may focus on strengthening the correlation of SZ observables such as $Y$ with other mass proxies and refining feedback models to capture complex ICM phenomena. The pursuit of minimizing scatter in $Y$-$M$ scaling through robust cluster selection criteria — potentially revolutionizing constraints on cosmological parameters — illustrates the paper’s foundational contributions to precision cosmology in the era of SZ surveys.