Ph.D. Thesis Defenses: 2009
Simulations of Binary Galaxy Cluster Mergers: Modeling Real Clusters and Exploring Parameter Spaces
January 23, 2009 | AAC 123 | 12:00 PM | PhD Advisor: Donald Q. Lamb | PhD Thesis Defense
John Zuhone

We present an investigation of controlled N -body/hydrodynamics high-resolution simulations of binary galaxy cluster mergers, performed using the FLASH code. In addition to analyzing the quantities directly from the simulation, we produce simulated X-ray observations of the cluster ICM and perform standard analyses of the surface brightness distribution and spectra of the X-ray photons emitted from the hot cluster gas. Several lines of evidence have suggested that the galaxy cluster Cl 0024+17, an apparently relaxed system, is actually a collision of two clusters, the interaction occurring along our line of sight. We present a high-resolution N -body/hydrodynamics simulation of such a collision. We analyze mock X-ray observations of our simulated clusters to generate radial profiles of the surface brightness and temperature to show that at later times the simulated surface brightness profiles are better fit by a superposition of two b-model profiles than a single profile, in agreement with the observations of Cl 0024+17. We determine from our fitted profiles that if the system is modeled as a single cluster, the hydrostatic mass estimate is a factor ~2-3 less than the actual mass, but if the system is modeled as two galaxy clusters in superposition, a hydrostatic mass estimation can be made which is accurate to within ~10%. Additionally, recent lensing observations of Cl 0024+17 suggest the presence of a ring-like dark matter structure, which has been interpreted as the result of such a collision. To determine the conditions under which such a feature would form, we vary the initial velocity anisotropy of the dark matter particles. Our simulations show a ring feature does not occur even when the initial particle velocity distribution is highly tangentially anisotropic. Only when the initial particle velocity distribution is circular do our simulations show such a feature, which is consistent with the halo velocity distributions seen in cosmological simulations. Lastly, we present a fiducial set of galaxy cluster merger simulations, where the initial mass ratio and the impact parameter have been varied. By projecting the simulated quantities along the axes of the computational domain, we produce maps of X-ray surface brightness, temperature, projected mass density, and simulated X-ray observations. From these observations we compute the observed X-ray luminosity and fitted spectral temperature, and fit b-model profiles to compute estimated hydrostatic masses. From this information we determine the effect of mergers viewed along different projections on these observed quantities. We also construct simulated maps of galaxies, and test the power of a commonly employed substructure statistic to probe for the existence of substructure along the different projections during the merger. Finally, we comment on other aspects of our simulations, such as comparisons to existing merging clusters; and the mixing of the intracluster medium due to merging, and resulting cluster entropy and cooling time profiles.

Structure Formation in Braneworld Cosmology
June 12, 2009 | AAC 123 | 1:30 PM | PhD Advisors: Scott Dodelson; Angela V. Olinto | PhD Thesis Defense
Fabian Schmidt

We perform cosmological N-body simulations of the Dvali-Gabadadze- Porrati braneworld model, by solving the full non-linear equations of motion for the scalar degree of freedom in this model, the brane bending mode. While coupling universally to matter, the brane-bending mode has self-interactions that become important as soon as the density field becomes non-linear. These self-interactions lead to a suppression of the field in high-density environments, and restore gravity to General Relativity. The code uses a multi- grid relaxation scheme to solve the non-linear field equation in the quasi- static approximation. We perform simulations of a flat self-accelerating DGP model without cosmological constant. However, the type of non-linear interactions of the brane-bending mode, which are the focus of this study, are generic to a wide class of braneworld cosmologies. The results of the DGP simulations are compared with standard gravity simulations assuming the same expansion history, and with DGP simulations using the linearized equation for the brane bending mode. This allows us to isolate the effects of the non-linear self-couplings of the field which are noticeable already on quasi-linear scales. We present results on the matter power spectrum and the halo mass function, and discuss the behavior of the brane bending mode within cosmological structure formation. We find that, independently of CMB constraints, the self-accelerating DGP model is strongly constrained by current weak lensing and cluster abundance measurements.

Measuring the Small Angular Scale Anisotropy with the QUaD Telescope
July 21, 2009 | LASR conference room | 5:00 PM | PhD Thesis Defense
Robert Friedman

I present measurements of the cosmic microwave background (CMB) radiation temperature anisotropy using the QUaD telescope, a several arcminute resolution bolometric polarimeter operating at 100 and 150 GHz, located at the South Pole. The results presented here are targeted to the multipole range 2000 < ℓ < 3000 using data from QUaD's second and third observing seasons. After masking the brightest point sources in the maps the results are consistent with the primary LCDM expectation alone. I further estimate the contribution of residual (un-masked) radio point sources using a model calibrated to our own bright source observations, and a full simulation of the source finding and masking procedure. Including this contribution slightly improves the h 2 . I then fit a standard SZ template to the bandpowers and see no strong evidence of an SZ contribution, which is as expected for s 8 [approximate] 0:8; the best- fit value for the template amplitude is A SZ = [Special characters omitted.] .

Cascades of VHE Gamma Rays from Blazars and the Extragalactic Gamma-ray Background
July 28, 2009 | AAC 123 | 4:00 PM | PhD Advisor: Angela V. Olinto | PhD Thesis Defense
Tonia Venters

As very-high-energy photons propagate through the extragalactic background light (EBL), they interact with the soft photons and initiate electromagnetic cascades of lower energy photons and electrons. The collective intensity of a cosmological population emitting at very-high energies (VHE) will be attenuated at the highest energies through interactions with the EBL and enhanced at lower energies by the resulting cascade. We calculate the cascade radiation created by VHE photons produced by blazars and investigate the effects of cascades on the collective intensity of blazars and the resulting effects on the extragalactic gamma-ray background. We find that cascade radiation greatly enhances the collective intensity from blazars at high energies before turning over due to attenuation. The prominence of the resulting features depends on the blazar gamma-ray luminosity function, spectral index distribution, and the model of the EBL. We additionally calculate the cascade radiation from the distinct spectral sub-populations of blazars, BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs), finding that the collective intensity of BL Lacs is considerably more enhanced by cascade radiation than that of the FSRQs. Finally, we discuss the implications that this analysis and upcoming Fermi observations could have for the nature of the EBL, the evolution of blazars, blazar spectra, and other sources of gamma-ray emission.