Astrophysics

As observations of clusters through their SZ imprint on the CMB becomes more routine, it is now feasible to add this signal to the set of observables we use to study galaxy clusters. Using the Sunyaev-Zel'dovich Array (SZA), we are pursuing a variety of programs to investigate the correlation between cluster properties and their SZ signatures. I will present early results from these comparisons. The SZA is also a unique tool for resolved SZ imaging as part of the 23-element CARMA interferometer. I will discuss our initial experiment with heterogeneous array interferometry later this year and the future capabilities of the full array.

"There is considerable uncertainty in the theoretical predictions for the angular power spectrum from the Sunyaev-Zeldovich effect (SZe). The level of precision reached by ACT, SPT, and Planck for measurements of the normalization of the SZe power spectrum, sigma_8, will be limited by the uncertainty in the theoretical models for the angular power spectrum. The uncertainties in the predicted spectrum arise from the complicated physics of the ICM. We have explored these ICM complexities using hydrodynamical simulations in a cosmological setting with several different variants of simulated physics, including cooling and star formation, star formation feedback by galactic winds and supernovae as well as cosmic ray physics. Our statistics were compiled from two independently stacked cluster samples consisting of cosmological box simulations and individual high-resolution cluster simulations. We show that a simple parametrized fit describes averaged ICM pressure profiles sufficiently well and compare this finding to previous hydrostatic models. We find that radiative cooling and the associated star formation is the dominant physical process that modifies our fit parameters for these profiles and the angular power spectrum. "

This talk will describe the theoretical history "THEN" of CITA's semi-analytic and simulation forecasts of the "ambient" (aka blank field) SZ effect, from the beginnings in the mid-80s to the "NOW" and near future of copious ACT and SPT ambient-SZ cluster detections, Along the way, we will recall the simulation and analytic state of SZ analysis of the CBI excess power in 2002 (and 2008) and the impact of ACBAR and BIMA on the results, now punctuated by recent QuAD and SZA releases, NOW the ACT, SPT and Planck pressure of high precision imminence in SZ is re-focussing us on pressure uncertainties in SZ power and maps from energy feedback, non-equilibrium and non-thermal processes, and cluster core complications as a function of redshift with large simulations. CITA's gassy-sim theoretical approach to this problem will be described, along with a conclusion that high resolution SZ and other observations must be our guide.

Clusters of galaxies provide us the opportunity to study an "ecosystem" - a volume that is a high-density microcosm of the rest of the Universe. At the same time clusters are excellent laboratories for studying plasma physical processes as well as for studying how super-massive black holes interact with the ambient cluster plasma. Guided by high-resolution simulations of galaxy clusters that self-consistently follow dissipative gas and cosmic ray physics, I will show how non-thermal processes in clusters build up over cosmic time. This enables us to understand how the Sunyaev-Zel'dovich effect and hydrostatic masses of galaxy clusters are expected to change - a finding which is critical in calibrating clusters as high-precision cosmological probes. On small scales, the Chandra X-ray Observatory is finding a large number of cavities in the X-ray emitting intra-cluster medium which often coincide with the lobes of the central radio galaxy. These are thought to provide the key for understanding the thermal evolution of galaxy clusters. I will argue that high-resolution observations of the Sunyaev-Zel'dovich effect are uniquely suited to unveil the composition of radio plasma bubbles. Solving this enigma would yield further insight into the complex physical processes within the cooling cores of clusters as well as provide hints about the composition of relativistic outflows of radio galaxies.

Clusters of galaxies are unique probes of cosmology and astrophysics, promising to provide new insights into both the nature of dark energy and dark matter and the physics of galaxy formation. One of the key challenges facing this approach lies in our understanding of cluster physics and their impact on cluster structure and evolution. In this talk, I will present numerical simulations of galaxy clusters and their comparisons to recent Chandra X-ray observations, with focus on thermodynamics of intracluster plasma. Numerical simulations including gas cooling and star formation reproduce global properties of the intracluster medium (ICM) and observable-mass relations with an accuracy of ~10%. I will further show that non-thermal processes, such as turbulence, cosmic-rays, and ICM plasma physics, will become important for understanding the remaining systematic uncertainty in the cluster mass estimate and cosmological constraints derived using galaxy clusters.

The XMM Cluster Survey (XCS) is a serendipitous galaxy cluster survey being conducted using publicly available X-ray data from the XMM-Newton Science Archive. One of the primary aims of the XCS is to determine the form of the evolution of the cluster gas - knowledge of which is crucial for the use of X- ray or SZ selected clusters in constraining cosmological parameters - through measuring the X-ray scaling relations using a large, well characterised cluster sample spanning ~7 Gyr in lookback time. I will present an update on the status of the survey, discuss the expected cosmological constraints, and briefly describe some recent results from galaxy evolution studies conducted under the umbrella of the XCS project.

We build simple, 'top-down', models for the gas density and temperature profiles for clusters of galaxies based on current high precision XRay observations so as to 'exactly' satisfy observed XRay scaling relation between temperature and mass. The gas is assumed to be in hydrostatic equilibrium along with a component of non-thermal pressure due to dispersion and the gas fraction reaches universal value only at or beyond the virial radius. For these models, we calculate the Sunyaev-Zel'dovich Effect (SZE) scaling relations. We show that all the predicted SZE scaling relations between the integrated SZE flux and the gas temperature, the gas mass, the total mass, as well as, the gas fraction are in excellent agreement with recent SZE observations by Bonamente etal (2008). The consistency between the global properties of clusters detected in X-Ray's and in SZE hints that we are looking at the same population of clusters as a whole. Implications for SZE power spectrum, SZE flux-M200 scaling relation and number counts are discussed

"Taking gravitational potential wells from a dark matter simulation, and assuming a polytropic equation of state and hydrostatic equilibrium, one can predict the state of the hot gas in clusters of galaxies. With reasonable values for star formation efficiency, energy input, and nonthermal pressure support, these model clusters can reproduce observed X-ray trends of gas temperature and gas mass fraction with cluster mass, as well as observed entropy and pressure profiles. Normalizing to X-ray observations is a vital step in using such models to predict the SZ signal. "

The observed thermal properties of the ICM shows much greater dispersion than expected if the gas was subject only to shock-heating by mergers and during infall. This diversity can be best understood as a byproduct of AGN feedback occurring in galaxies destined to become cluster members, both before and after cluster formation. Theoretical considerations suggest that the level of preheating ought to vary from one proto-cluster region to another. The entropy profiles of roughly 50% of the clusters with long central cooling times require that the gas be "preheated" to high entropy. Gas density profiles in such systems form hot central cores. Clusters with gas that isn't preheated to sufficiently high values forms peaked density profiles. I will show how variable preheating explain the various observed X-ray/X-ray correlations and discuss some of its implications for SZ studies. I will also present optical results that shed new light on the fate of the cold gas in cooling-unstable clusters, and propose observations tests of the "AGN preheating" aspect of the picture.

Scaling relations among galaxy cluster observables, which will become available in large future samples of galaxy clusters, could be used to constrain not only cluster structure,but also cosmology. I will discuss the utility of this approach, employing a physically motivated parametric model to describe cluster structure, and applying it to the expected relation between the Sunyaev-Zel’dovich decrement (S_{\nu}) and the emission–weighted X–ray temperature (Tew). With a suitable choice of fiducial parameter values, the cluster model satisfies several existing observational constraints. A Fisher matrix is employed to estimate the joint errors on cosmological and cluster structure parameters from a measurement of S_{\nu} vs. Tew in a future survey. I will also compare the cosmology constraints from the scaling relation to those expected from the number counts (dN/dz) of the same clusters.