Talks & Events
Ph.D. Thesis Defenses: 2005
Constraints on Cluster Structure & Cosmology from X-ray and Sunyaev-Zel’dovich Effect Properties of Galaxy Clusters
We present a study of 36 massive galaxy clusters at redshifts 0.14 <= z <= 0.89, whose properties are used to constrain cluster structure and cosmological models. The clusters are studied at both X-ray wavelengths using data from the Chandra X-ray Observatory , and at centimeter wavelengths using Sunyaev- Zel`dovich Effect (SZE) data from the BIMA and OVRO interferometric arrays. Likelihood analysis is performed using a Markov chain Monte Carlo (MCMC) method, and we motivate and adopt an isothermal b-model with central 100 kpc excluded from the X-ray data to describe the intracluster medium in all 36 clusters. This model is shown to provide consistently good fits to clusters with a wide range of morphological properties. Best-fit cluster gas masses, total masses, and integrated SZE fluxes are determined from this model with in the MCMC framework. X-ray and SZE results are then used in an effort to constrain cluster structure and cosmology. First, the integrated SZE flux is compared with X-ray derived gas mass, spectroscopic temperature, and total mass to determine how firmly connections between observables and derived cluster properties can be established in SZE surveys. These properties are found to display tight scaling relations over the full range of redshift. Cluster gas mass fractions from both X-ray and SZE data are then compared, and the implications for cluster structure are noted. The gas mass fractions are used to obtain constraints on O M and O L both by comparing the mean f gas values to O B /O M for assumed O B , and by assuming that f gas should be constant with redshift and determining what cosmology best facilitates this. Cosmological constraints are found to be consistent with CMB results, but a better understanding of the cluster baryon budget is needed to achieve the same level of precision.
Sunyaev-Zeldovich Scaling Relations in Cosmological Cluster Simulations
We study the effects of radiative cooling and galaxy formation on the Sunyaev- Zel'dovich (SZ) observable-mass relations using high-resolution cosmological simulations. The simulations of eleven individual clusters spanning a decade in mass are performed with the shock-capturing eulerian adaptive mesh refinement N-body+gasdynamics ART code. To assess the impact of galaxy formation, we compare two sets of simulations performed in the adiabatic regime (without dissipation) and with radiative cooling and several physical processes critical to various aspects of galaxy formation: star formation, metal enrichment and stellar feedback. We show that the SZ signal integrated to sufficiently large fraction of the cluster volume correlates strongly with the enclosed cluster mass, regardless of the details of the cluster physics or dynamical state of the cluster. The slope and redshift evolution of the SZ flux-mass relation are also insensitive to the details of the cluster gas physics, and they are well characterized by the simple self-similar cluster model. While the tightness, slope and redshift evolution are relatively unaffected, we show that the radiative cooling and galaxy formation significantly modify the normalization of the SZ scaling relations. In our simulations, we find that the gas cooling and associated star formation suppress the normalization by [approximate]30- 40%. The effect is due to the decrease in the hot gas fraction, which is offset slightly by the increase in the gas temperature. The baryon dissipation also slightly modifies the cluster mass and affects the normalization by non- negligible amount. Finally, we show that the simulations that include gas cooling and star formation are in good agreement with the recent observational results on the SZ scaling relations obtained using 36 OVRO/BIMA SZ+Chandra X- ray cluster observations, while the simulations neglecting galaxy formation are inconsistent with the observed correlation. The comparison highlights the importance of galaxy formation in theoretical modelling of clusters and shows that the current generation of simulations produce clusters with gross properties quite similar to their observed counterparts.
Modified-Gravity as an Alternative to Dark Energy
We consider general curvature-invariant modifications of the Einstein-Hilbert action that become important only in regions of extremely low space-time curvature. We investigate the late-time evolution of the universe in such models, examining the possibilities for cosmic acceleration and other ultimate destinies. The models generically possess de Sitter space as an unstable solution and exhibit an interesting set of attractor solutions which, in some cases, provide alternatives to dark energy models. We show that modifications of the form f ( R ) are ruled out by solar system tests of gravitation. In addition, we also review the Palatini method of variation for such theories and contrast it with the metric variation approach.
Lyman-alpha radiative transfer in cosmological simulations and application to a z [approximate] 8 Ly-alpha emitter
I develop a Lyman-a radiative transfer (RT) Monte Carlo code for cosmological simulations. High resolution, along with appropriately treated cooling, can result in simulated environments with very high optical depths. Thus, solving the Lyman-a RT problem in cosmological simulations can take an unrealistically long time. For this reason, I develop methods to speed up the Lyman-a RT. With these accelerating methods, along with the parallelization of the code, I make the problem of Lyman-a RT in the complex environments of cosmological simulations tractable.
I test the RT code against simple Lyman-a emitter models, and then I apply it to the brightest Lyman-a emitter of a gas dynamics+N-body Adaptive Refinement Tree (ART) simulation at z ~= 8. I find that recombination rather than cooling radiation Lyman-a photons is the dominant contribution to the intrinsic Lyman- a luminosity of the emitter, which is ~= 2.3 x 10 44 ergs/s. The size of the emitter is pretty small, making it unresolved for currently available instruments. Its spectrum before adding the Lyman-a Gunn-Peterson absorption (GP) resembles that of static media, despite some net inward radial peculiar motion. This is because for such high optical depths as those in ART simulations, velocities of order some hundreds km/s are not important.
I add the GP in two ways. First I assume no damping wing, corresponding to the situation where the emitter lies within the HII region of a very bright quasar, and second I allow for the damping wing. Including the damping wing leads to a maximum line brightness suppression by roughly a factor of ~ 62. The line fluxes, even though quite faint for current ground-based telescopes, should be within reach for JWST.