Session K4: DAP Thesis Prize

New Frontiers for Diagnosing the Turbulent Nature of the Multiphase Magnetized Interstellar Medium

Magnetic fields and turbulence are vital components in galactic processes, including cosmic ray transport, ISM structure formation and star formation. However turbulence is difficult to measure observationally and the role of simulations is vital for both testing theories of ISM turbulence and gauging observational diagnostics via synthetic observations. In this talk I will discuss the origins of turbulence in galaxies, and its connection to the star formation process, both from observations and the turbulence simulations. I will also highlight how turbulence can be measured using novel techniques applied to spectral line observations of molecular clouds and diffuse gas in galaxies in order to constrain and test simulations as well as obtain important properties of turbulence such as the injection scale, spectral index and Mach number.   

Understanding non-Gaussianity signatures in general relativity

Possible departure from Gaussian statistics in cosmological perturbations can shed much light on the physics of their generation in the primordial Universe. Many of the forthcoming surveys of the large-scale structure with unprecedented survey volume aim at detecting these signatures. However, ignoring the ``gauge artifacts'' in general relativity that arise from the freedom to choose arbitrary space-time coordinates to describe the perturbed Universe can lead to incorrect interpretation on the observational consequences of these non-Gaussian signatures. I present two important examples of non-Gaussianity signatures. I show that in the ``separate universes'' formalism it can be clarified that they are strictly forbidden in canonical inflation scenarios involving only one scalar degree of freedom. One is a quadrupolar direction-dependence in the power spectrum of matter density, which is naively expected from a non-Gaussian correlation between a primordial gravitational wave of super-horizon wavelength and two density perturbations of shorter wavelengths. The other is a galaxy biasing that grows toward large scales, which is naively expected from nonlinearity in general relativity that couples a long-wavelength gravitational potential with two short-wavelength density fluctuations. Conversely, general models of single-field inflation can be falsified if it turns out that either of those signatures is actually observed.   

Unveiling the Progenitors of Short-duration Gamma-ray Bursts

Gamma-ray bursts (GRBs) are relativistic explosions which originate at cosmological distances, and are among the most luminous transients in the universe. Following the prompt gamma-ray emission, a fading synchrotron ``afterglow'' is detectable at lower energies. While long-duration GRBs (duration$>$2 sec) are linked to the deaths of massive stars, the progenitors of short-duration GRBs (duration$<$2 sec) have remained elusive. Predictions formulated over the past two decades have suggested that they are the mergers of two compact objects, involving either two neutron stars or a neutron star and a black hole. Such merging systems are also important to understand because they are premier candidates for gravitational wave detections with current facilities and are likely sites of heavy element nucleosynthesis. The launch of NASA's Swift satellite in 2004, with its rapid multi-wavelength monitoring and localization capabilities, led to the first discoveries of short GRB afterglows and therefore robust associations to host galaxies. At a detection rate of roughly 10 events per year, the growing number of well-localized short GRBs has enabled comprehensive population studies of their afterglows and environments for the first time. In this talk, I describe my multi-wavelength observational campaign to address testable predictions for the progenitors of short GRBs. My work comprises several lines of independent evidence to demonstrate that short GRBs originate from the mergers of two compact objects, and also provides the first constraints on the explosion properties for a large sample of events. With the direct detection of gravitational waves from compact object mergers on the horizon, these studies provide necessary inputs to inform the next decade of joint electromagnetic-gravitational wave search strategies.