High-resolution Cosmological Simulations of Galaxy Clusters


Clusters of galaxies consist of approximately 85% of dark matter, 10% of hot gas and 2-5% of stars. Detailed theoretical modeling of clusters is thus a complicated astrophysics problem involving a variety of physical phenomena from the nonlinear collapse and merging of dark matter to radiative cooling of gas, star formation, chemical enrichment of intergalactic medium by supernovae and     

Chandra X-ray Cosmology Projects


In collaboration with X-ray astronomers,  we have applied our simulations for interpreting the state-of-the-art Chandra X-ray observations of galaxy clusters.  Our simulations were one of the first to achieve the dynamic range and include a wide range of physical processes necessary for detailed modeling of the X-ray emitting ICM.  These simulations have significantly improved our understanding of the structure and evolution of galaxy clusters and their application to cosmology. Specifically, we identified and characterized major sources of systematic uncertainties associated with non-linear astrophysical processes (e.g., gas clumping, non-thermal pressure due to turbulent gas motions and energy injection from stars and black holes) and developed a novel technique (e.g., robust mass proxy Yx and universal pressure profile) to control them in the cosmological analyses based on the Chandra data.  Our work has provided independent confirmation of dark energy based on the growth of structure and significantly improved its constraints.  Our results confirmed that the dark energy is consistent with a cosmological constant as originally proposed by Albert Einstein.  Results of this work, including measurements of cosmological parameters, were reported in a series of six papers published in Astrophysical Journals.

Galaxy Formation and Evolution


Clusters of galaxies are the regions of the highest galaxy density in the Universe, where a wide range of environment-dependent processes such as dynamical friction, tidal disruption, ram-pressure stripping, and morphological transformation significantly affect the properties of galaxies.  With the advent of fast computers and sophisticated algorithms, it has now become possible to model the formation and evolution of galaxies self-consistently in a realistic cosmological setting.  This simulation-based approach is complementary to semi-analytic modeling of galaxies, which relies on various simplifying assumptions for important physical processes. These simulations therefore enable theoretical studies of galaxy formation and evolution with unprecedented sophistication and details.  By tracking the evolution of simulated galaxies and comparing the models to a suite of optical/IR observations of galaxies in ``observational plane'', we are investigating physical processes that govern the evolution of galaxies in clusters, assess their relative importance, and study how these physical processes shape observed properties of galaxies and their evolution.

Cosmology with Sunyaev-Zel’dovich Effect


Since its discovery in 1965, Cosmic Microwave Background (CMB) has provided a plethora of insights into the origin, composition, and the development of structure in the Universe.  The new generation of the CMB experiments (e.g., ACT, Planck, SPT) continue to push the sensitivities and angular resolution of  experiments. One of the new frontiers is the study of the small-scale secondary anisotropy signals sourced by ionized gas associated with intervening large-scale structures, including galaxy clusters, intergalactic medium, and expanding bubbles around first stars and galaxies during the epoch of reionization. However, interpretation of these measurements requires equally significant advances in theoretical modeling.  Over the last several years, our group has focused on developing models of the secondary CMB anisotropy signals, including the Sunyaev-Zel’dovich effect, generated by scattering of CMB photons off of hot electrons in galaxy clusters, and gravitational lensing of CMB photons by the large-scale structure. Our work has enabled the use of small-scale CMB measurements as probes of both cosmology and astrophysics and helped define new scientific goals and designs of future CMB experiments.

Clusters of Galaxies as Cosmological Probes


My research is broadly focused on understanding the origin, composition, and structure formation of the Universe, specializing in theoretical and computational modeling of galaxy clusters and their application to cosmology.  Knowledge of how these so-called large-scale structures develop has the potential to enlighten us about the fundamental physics of the cosmos, including the nature of mysterious dark energy and dark matter.  This requires modeling all important astrophysical processes in simulations of the formation of large-scale structure. My ultimate goal is to advance the use of complex systems, such as galaxy clusters, as laboratories for addressing the fundamental questions in cosmology.

energy feedback. The numerical simulations of cluster formation start from the well-defined cosmological initial conditions. In our work, we use the technique called Adaptive Mesh Refinement to greatly increase the resolution in the high density regions within clusters. The high resolution is required to follow formation and evolution of cluster galaxies and their impact on the hot intracluster medium (ICM). The panel illustrates the complexity of physical processes involved in a typical simulation. It shows distribution of dark matter, stars, and the properties of gas at the present epoch in one of the simulated clusters forming in the concordance Cold Dark Matter model with vacuum energy. In the hierachical structure formation scenario, clusters form and grow through continuous mergers and accretion of small clumps and diffuse matter from voids and filaments. The figure reveals a rich and complex structure of the gas density and temperature distributions, such as strong and highly aspherical accretion shocks surrounding the cluster and turbulent gas motions within the cluster. The cluster gas is also enriched with heavy elements ("metals"), as the metal-enriched gas is stripped off from galaxies when they orbit within the cluster.

SZA image of A1914

Numerical Simulations

Dark Matter

Stars

Research

Probing Dark Matter with Gravitational Lensing


Gravitational lensing is the deflection of a light ray by a curved space-time around a massive body - a phenomenon predicted by the Einstein’s general relativity.  Being the largest gravitationally bound objects in the Universe largely made up of elusive dark matter, clusters of galaxies serve as efficient “lenses”, making them excellent probes of the underlying mass distribution of dark matter within clusters as well as baby galaxies in the early universe using clusters as magnifying lenses.  Multi-wavelength observations of merging galaxy clusters are especially unique and interesting, because it is possible to measure the distribution of dark matter (by lensing), gas (by X-ray/SZ), and stars (by optical) individually.  Notably, the apparent offset between dark matter and gaseous baryonic components observed in the famous Bullet cluster (shown on the right) has provided one of the strongest evidence for the existence of dark matter, because the mass distribution mapped out by gravitational lensing cannot be explained by the baryonic component alone.  A major challenge is to use this type of observations to derive quantitative constrains on the properties (e.g., annihilation and collisional cross-section) of dark matter.  Since the merger dynamics of the multi-component system is quite complicated, such constraints are highly model dependent and rely critically on detailed numerical simulations.  Hydrodynamical cluster simulations developed by our group provide necessary theoretical support for realizing the power of cluster lensing observations.

Physics of Astrophysical Plasma


Being the diffuse medium in the largest virialized structures in the Universe, intracluster medium (ICM) is a unique laboratory for physics ranging from the largest scales in our Universe down to the scale of particle interactions.  On the scale of galaxy clusters, gravity is the dominating force which leads to nearly universal thermodynamical profiles of the ICM and enables galaxy clusters to be a useful probe for cosmology. At the same time, ICM exhibits rich phenomena at small scales such as turbulence, magnetic-fields, cosmic rays, as well as a variety of astronomical phenomena (including radio halos, radio relics, cold fronts, cool cores, and energy injection from black holes). To understand them fully we may need to treat the ICM in its full complexity, as a magnetized, weakly collisional plasma. Do we understand microphysics of the ICM well enough for precision cosmology?  How can we tackle this multi-scale problem using classical/quantum computers?  What clues can we gain from astronomical observations?  By bringing together astrophysicists and plasma physicists, we are exploring ways to use the ICM as a laboratory for plasma physics and cosmology.

Computational & Data Sciences


Computational astrophysics has recently emerged as a new branch of astronomy alongside observational and theoretical approaches.  We need computer simulation because it is impossible to perform laboratory experiments of astronomical phenomena due to the extreme time and scales involved.  Today, we create universes in supercomputers, reproduce astronomical phenomena, and study their behaviors.  In these virtual universes, we examine astrophysical processes responsible for the origin, evolution and fate of planets, galaxies, and the large-scale structure of the universe and compare their results to astronomical observations.  Collaborating with computer scientists, statisticians, and data scientists, we develop algorithms and software for the next-generation of computational astrophysics, perform the largest and ever more sophisticated numerical experiments ever attempted, and apply novel statistical techniques to analyze large astronomical datasets.  By using supercomputer simulations as experimental tools, our mission is to tackle longstanding questions about what our universe is made of and how the structure forms and grows in our universe.  In the process, we are also training the next-generation of computational and data scientists who will help shape the emerging data-driven society.

Modeling the equilibration process of protons and electrons in cosmological simulations

(Rudd & Nagai 2009)