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.

Testing flatness of the universe with probes of cosmic distances and growth
July 15, 2009 | LASR conference room | 11:00 AM | PhD Advisor: Wayne Hu | PhD Thesis Defense
Michael Mortonson

When using distance measurements to probe spatial curvature, the geometric degeneracy between curvature and dark energy in the distance-redshift relation typically requires either making strong assumptions about the dark energy evolution or sacrificing precision in a more model-independent approach. Measurements of the redshift evolution of the linear growth of perturbations can break the geometric degeneracy, providing curvature constraints that are both precise and model-independent. Future supernova, CMB, and cluster data have the potential to measure the curvature with an accuracy of s(O K ) 0.002, without specifying a particular dark energy phenomenology. In combination with distance measurements, the evolution of the growth function at low redshifts provides the strongest curvature constraint if the high-redshift universe is well approximated as being purely matter dominated. However, in the presence of early dark energy or massive neutrinos, the precision in curvature is reduced due to additional degeneracies, and precise normalization of the growth function relative to recombination is important for obtaining accurate constraints. Curvature limits from distances and growth compare favorably to other approaches to curvature estimation proposed in the literature, providing either greater accuracy or greater freedom from dark energy modeling assumptions, and are complementary due to the use of independent data sets. Model-independent estimates of curvature are critical for both testing inflation and obtaining unbiased constraints on dark energy parameters.

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.

Instabilities in free-surface Hartmann flow at low magnetic Prandtl numbers
September 17, 2009 | PhD Advisor: Robert Rosner | PhD Thesis Defense
Dimitris Giannakis

Free-surface Hartmann flow is the parallel flow of a viscous, electrically conducting, capillary fluid on a planar surface, subject to gravity and a flow- normal magnetic field. This type of flow arises in a variety of industrial and astrophysical contexts, including liquid-metal walls in fusion devices, heavy- ion accelerator targets, and surface layers of white dwarfs and neutron stars. Typically, the Reynolds number, Re >10 4 , is high, and the background magnetic field is strong ( Ha >100, where the Hartmann number, Ha , measures the square root of the ratio of electromagnetic to viscous forces). On the other hand, the magnetic Prandtl number, Pm (the ratio of viscous to magnetic diffusivity), of laboratory fluids is small (e.g., Pm <10 -4 for liquid metals), as is the case in a number of astrophysical models.

When the background magnetic field is zero, free-surface Hartmann flow exhibits the so-called soft and hard instability modes; the former being a surface wave destabilized by viscous stresses acting on the free surface, whereas the latter is a shear mode destabilized by positive Reynolds stress associated with an internal critical layer. We study in detail the influence of the external magnetic field on these two instabilities, working in the regime Pm <10^-4. We also consider flows in the inductionless limit, Pr [arrow right]0, where magnetic field perturbations diffuse infinitely fast, and the sole MHD effect is a Lorentz force arising from currents induced by the perturbed fluid motion within the background magnetic field.

We have developed a spectral Galerkin method to solve the coupled Orr- Sommerfeld and induction equations, which, in conjunction with suitable stress conditions at the free surface and continuity conditions for the magnetic field, govern the linear stability of free-surface Hartmann flow. Our scheme's discrete bases for the velocity and magnetic fields consist of linear combinations of Legendre polynomials, chosen according to the order of the Sobolev spaces of the continuous problem. The orthogonality properties of the bases solve the matrix-coefficient growth problem of the discrete stability operators, and eigenvalue-eigenfunction pairs can be computed stably at spectral orders at least as large as p =3000 with p -independent roundoff error.

We find that, because it is a critical-layer instability (moderately modified by the presence of the free surface), the hard mode exhibits similar behavior to the even unstable mode in the corresponding closed-channel flow, in terms of both the weak influence of Pm on its neutral-stability curve and the monotonic increase of its critical Reynolds number, Re c , with the Hartmann number. In contrast, the soft mode's stability properties exhibit the novel behavior of differing markedly between problems with small, but nonzero, Pm and their counterparts in the inductionless limit. Notably, the critical Reynolds number of the soft mode grows exponentially with Ha in inductionless problems, but when Pm is nonzero that growth is suppressed to either a sublinearly increasing, or a decreasing function of Ha (respectively when the lower wall is an electrical insulator or a perfect conductor). In the insulating-wall case, we also observe pairs of counter-propagating Alfvén waves, the upstream- propagating wave undergoing an instability at high Alfvén numbers.

We attribute the observed Pm -sensitivity of the soft instability to the strong-field behavior of the participating inductionless mode, which, even though stabilized by the magnetic field, approaches neutral stability as Ha grows. This near-equilibrium is consistent with a balance between Lorentz and gravitational forces, and renders the mode susceptible to effects associated with the dynamical response of the magnetic field to the flow (which vanishes in the inductionless limit), even when the magnetic diffusivity is large. The boundary conditions play a major role in the magnetic field response to the flow, since they determine (i) the properties of the steady-state induced current, which couples magnetic perturbations to the velocity field, and (ii) the presence or not of magnetic modes in the spectrum (these modes are not part of the spectrum of conducting-wall problems), which interact with the hydrodynamic ones, including the soft mode. In general, our analysis indicates that the inductionless approximation must be used with caution when dealing with free-surface MHD.