Ph.D. Thesis Defenses: 2011
Rayleigh-Taylor Unstable Flames: the Competition between Burning, Gravity and Self-Generated Flow
July 8, 2011 | Searle 240b | 4:00 PM | Robert Rosner
Elizabeth Hicks

Ph.D. Committee: Arieh Konigl, Fausto Cattaneo, Angela Olinto.

A comprehensive study of the cool gas content of massive dark matter halos
July 14, 2011 | AAC 123 | 11:00 AM | Hsiao-Wen Chen
Jean-Rene Gauthier

Ph.D. Committee: Michael Gladders, Andrey Kravtsov, Joshua Frieman.

"Jean-Rene's PhD thesis has provided new insights into the growth of massive galaxies, discovering the unexpected presence of extended reservoir of cold gas. In the fall, he will be starting a new position at Caltech as a Millikan fellow."
- Hsiao-Wen Chen, PhD advisor.

Thesis Abstract: Characterizing the structure and evolution of the cool gas in dark matter halos is a key element in current models of galaxy formation. Several physical mechanisms have been introduced to explain the origin of the cool, T ∼ 1e4 K gas in the dark matter halos of normal galaxies. Some of these mechanisms include cold accretion from the IGM, gas-rich galaxy mergers, starburst driven outflows, and thermal instabilities generated within the hot coronal gas. Unfortunately, direct empirical data are lacking and these mechanisms remain poorly constrained. My thesis focuses on the empirical determination of the cool gas content of massive dark matter and directly addresses the physical origin of the gas. Specifically, I will present a series of empirical constraints on the cool gas content of dark matter halos hosting Luminous Red Galaxies (LRGs) at z ≈ 0.5. In my talk, I will describe the outcome of a clustering analysis done on a population of MgII absorption doublets and its implications for the presence of cool gas in massive halos. I will then describe the main results of an on-going spectroscopic survey of LRGs and present measurements of the covering fraction of cool gas in these galaxies. I will also present the results of a stellar population synthesis analysis done on a sample of spectroscopic LRGs. This analysis allows me to directly assess whether or not there is a connection between the presence of the gas and the recent star formation activities of the galaxies. Finally, I will briefly introduce the preliminary results of a kinematical study of the MgII absorbers to explore the physical origin of the gas in more details.

The phenomenology of maverick dark matter
July 18, 2011 | LASR conference room | 3:00 PM | Edward ''Rocky'' W. Kolb
Zosia Krusberg

Ph.D. Committee: Jonathan Rosner, Henry Frisch, Ilya Gruzberg.

Thesis Abstract: Astrophysical observations from the galactic to the cosmological scales point to a substantial non-baryonic component to the universe's total matter density. Although very little is presently known about the physical properties of dark matter, its existence offers some of the most compelling evidence for physics beyond the standard model. In the weakly interacting massive particle (WIMP) scenario, the dark matter consists of particles that possess weak-scale interactions with the particles of the standard model, offering a compelling theoretical framework that allows us to understand the relic abundance of dark matter as a natural consequence of the thermal history of the early universe. From the perspective of particle physics phenomenology, the WIMP scenario is appealing for two additional reasons. First, many theories of physics beyond the standard model contain attractive WIMP candidates. Second, the enhanced interactions between WIMPs and standard model particles imply the possibility of detecting scatterings between WIMPs and detector nuclei in direct detection experiments, products of WIMP annihilations at locations throughout the galaxy, and WIMP production signals at high-energy particle colliders. In this work, we use an effective field theory approach to study model-independent dark matter phenomenology in direct detection and collider experiments. The maverick dark matter scenario is defined by an effective field theory in which the WIMP is the only new particle within the energy range accessible to the Large Hadron Collider (LHC). Although certain assumptions are necessary to keep the problem tractable, we describe our WIMP candidate model-independently by specifying only its spin and dominant interaction form with standard model particles. Constraints are placed on the masses and coupling constants of the maverick WIMPs using the Wilkinson Microwave Anisotropy Probe (WMAP) relic density measurement and direct detection exclusion data from both spin-independent (CDMSII and XENON100) and spin-dependent (COUPP) experiments. We further study the distinguishability of maverick WIMP production signals at the Tevatron and the LHC---at its early and nominal configurations---using standard simulation packages, place constraints on maverick WIMP properties using existing collider data, and determine projected mass reaches in future data from both colliders. We find ourselves in a unique era of theoretically motivated high-precision dark matter searches with the potential to give us important insights, not only into the nature of dark matter, but also into the physics that lies beyond the standard model.

Broadband Photometry of 105 Giant Arcs: Redshift Distribution Constraints and Implications for Giant Arc Statistics
July 18, 2011 | LASR conference room | 11:00 AM | Michael D. Gladders
Matthew Bayliss

Ph.D. Committee: Andrey Kravtsov, Richard Kron, Stephen Kent.

"Dr. Bayliss' Ph.D. research has focused on the identification and characterisation of several new large samples of gravitational lenses. In particular, his work with both imaging and spectroscopic data, from a range of telescopes, has established for the first time the redshift distribution of the lensed sources, which is a major milestone in using these lenses as cosmological probes and as environments in which to test our understanding of dark matter halo structure."
- Michael Gladders, PhD advisor

On direct and indirect searches for dark matter
July 18, 2011 | AAC 123 | 11:00 AM | Dan Hooper
Alexander Belikov

Ph.D. Committee: Carlos E. M. Wagner, Scott P. Wakely, Wendy W. Zhang.

"Sasha's work has explored many of the different facets of dark matter, from what forms it might take, to how it may have impacted the evolution of our universe. Ultimately, we hope that research such as this will help us to observe dark matter and finally find out what it is."
- Dan Hooper, PhD advisor.

Thesis Abstract: In addition to gamma rays, dark matter annihilation products can include energetic electrons which inverse Compton scatter with the cosmic microwave background to produce a diffuse extragalactic background of gamma rays and X-rays. In this dissertation we present an estimate of the extragalactic background flux of gamma rays including inverse Compton photons for a number of annihilation channels and review its dependence on such factors as the halo density profile, the mass-concentration ratio and others and find that the uncertainty of the estimate can reach about two orders of magnitude. For models in which the dark matter particles annihilate primarily to electrons or muons, the measurements of Fermi Gamma Ray Telescope and COMPTEL can provide significant constraints on the annihilation cross section.

We also study the effect of dark matter annihilation during the epoch of reionization. Although empirical evidence indicates that the universe's gas had become ionized by redshift $z approx 6$, the mechanism by which this transition occurred remains unclear. We explore the possibility that dark matter annihilations may have played a significant role in this process. Energetic electrons produced in these annihilations can scatter with the cosmic microwave background to generate relatively low energy gamma rays, which ionize and heat gas far more efficiently than higher energy prompt photons.

On the side of indirect detection, recent observations by the CoGeNT collaboration (as well as long standing observations by DAMA/LIBRA) suggest the presence of a $sim 5$-10 GeV dark matter particle with a somewhat large elastic scattering cross section with nucleons ($sigmasim 10^{-40}$ cm$^2$). Simple extensions of the MSSM, however, such as the extension of the MSSM by a chiral singlet superfield allow for the possibility that the dark matter is made up of a light singlino that interacts largely through the exchange of a fairly light ($sim$30-70 GeV) singlet-like scalar higgs. Such a scenario is consistent with all current collider constraints and can generate the signals reported by CoGeNT and DAMA/LIBRA and can naturally provide the thermal relic abundance consistent with the measured density of dark matter.

On the Imprints of Inflation in the Cosmic Microwave Background
July 20, 2011 | LASR conference room | 10:00 AM | Wayne Hu
Cora Dvorkin

Ph.D. Committee: John Carlstrom, Stephan Meyer, Jeff Harvey.

"Cora's Ph.D. research focused on testing inflation with the Cosmic Microwave Background beyond the slow roll approximation. Her work allows for the presence of features in the inflaton potential and enables tests of slow roll and single field inflation that do not depend on the specific form of the potential."
- Wayne Hu, PhD advisor.

Thesis Abstract: In this thesis, we introduce a general method for constraining the shape of the inflationary potential from Cosmic Microwave Background (CMB) temperature and polarization power spectra. This approach relates the CMB observables to the shape of the inflationary potential via a single source function that is responsible for the observable features in the initial curvature power spectrum. The source function is, to an excellent approximation, simply related to the slope and curvature of the inflaton potential, even in the presence of large or rapidly changing deviations from scale-free initial conditions. Oscillatory features in the CMB temperature power spectrum have been interpreted as possible evidence for new physics during inflation. It has been shown that a model with a sharp step in the inflationary potential can give rise to these oscillations, but such cases are typically studied on a case-by-case basis. This formalism generalizes previous studies by exploring the complete parameter space of inflationary models in a single analysis. Using a principal component basis that accommodates order unity features across more than 2 decades of the inflationary expansion, we test slow roll and single field inflation with the WMAP 7-year dataset. We present model-independent predictions for the matching glitches in the polarization spectrum that would test their inflationary origin. Even allowing for the presence of features in the temperature spectrum, single field inflation makes sharp falsifiable predictions for the acoustic peaks in the polarization whose violation would require extra degrees of freedom. This analysis greatly simplifies the testing of inflationary models in that it can be used to constrain parameters of specific models of inflation without requiring a separate likelihood analysis for each choice.

Radio Sources in Galaxy Clusters
July 20, 2011 | AAC 123 | 4:30 PM | Michael D. Gladders
Megan Gralla

Ph.D. Committee: John Carlstrom, Dan Hooper, and Don York.

"Dr. Gralla's thesis work has focused on the redshift evolution of the radio source population in galaxy clusters, over a broad swath of both redshift and cluster mass. Understanding the evolution of this active galaxy population is important for models of heating by AGN of the intra-cluster medium, as well as providing input to models addressing the impact of radio sources on Sunyaev-Zeldovich surveys for galaxy clusters."
- Michael Gladders, PhD advisor.

Thesis Abstract:
I present a statistical analysis of the radio source population in galaxy clusters by matching radio sources from two large-area surveys with optically-selected galaxy clusters spanning a wide range of redshift ($sim0.3$ to $sim$ 1) from the Red-Sequence Cluster Surveys (RCS). The first RCS (RCS1) is well-characterized within a cosmological context, with richness measurements calibrated over a wide redshift range. I have focused on the redshift evolution of the radio source population. The richest clusters in the sample have more radio sources than clusters with low or intermediate richness. When divided into bins according to cluster richness, there is no significant difference ($> 1.5sigma$) in the number of radio sources per unit of cluster mass for the galaxy clusters with $0.35 < z < 0.65$ as compared to the galaxy clusters with $0.65 < z < 0.95$. Thus the entire RCS1 sample can be characterized by the number of (L$_{1.4 GHz} > 4.1 imes 10^{24}$ W Hz$^{-1}$) radio sources per unit (10$^{14}$ M$_{odot}$) mass, which is measured to be 0.031$pm$0.004. I further characterize the population of galaxy cluster-related radio sources through visual inspection of the RCS1 images, finding that although the radio activity of brightest cluster galaxies (BCGs) also does not strongly evolve between the high and low redshift samples, the lower-redshift, richest clusters are more likely to host radio-loud BCGs than the higher-redshift, richest clusters or poorer clusters at the 2-$sigma$ level. The second RCS (RCS2) contains a much larger galaxy cluster sample selected from $>500$ square degrees of imaging data. I have refined and applied an automatic algorithm to identify the BCGs in clusters, and I find some 3$sigma$ evidence that the low redshift clusters have a higher fraction of radio-loud BCGs than the high redshift clusters, further supporting the initial evidence from RCS1. I also investigate the prevalence of extended sources in clusters and the radio activity in non-BCG red sequence galaxies, for which I find no evidence for evolution, in spite of the strong evolution that has been seen in the cluster X-ray AGN population.

Investigation of Molecular Cloud Structure around Infrared Bubbles: CARMA Observations of N14, N22, and N74
August 18, 2011 | LASR conference room | 3:00 PM | Doyal ''Al'' Harper
Reid Sherman

Ph.D. Committee: Roger Hildebrand, Stephan Meyer, Wayne Hu.

"An important unresolved problem in studies of star formation is the role of feedback. How does the formation of stars in a dense molecular cloud affect the evolution of the cloud and the formation of successive generations of stars?
Reid's research uses newly developed instrumental capabilities of the CARMA telescope array to explore the structure and motions in molecular clouds around "bubbles", pockets of hot ionized gas created by ultraviolet radiation from massive young stars. CARMA's high angular resolution and ability to observe many different parts of the millimeter-wavelength spectrum at the same time allow it to efficiently probe star-forming clouds at scales comparable to the size and density of the "molecular cores" required for the formation of additional high-mass stars."

- Doyal Harper, PhD advisor.

Thesis Abstract:
We present CARMA observations in 3.3 mm continuum and several molecular lines of the surroundings of N14, N22, and N74, three infrared bubbles from the GLIMPSE catalog. We have discovered 21 compact continuum sources and confirmed their associations with the bubbles using velocity information from HCO+ and HCN. We have also mapped fragments of quiescent clouds in the vicinity of the bubbles using N2H+. By combining our data with public survey data, we establish about half of our continuum sources as star-forming cores. We also use survey data with the velocity information from our molecular line observations to describe the morphology of the bubbles and the nature of the fragmentation. We conclude from the properties of the continuum sources that N74 lies at a much greater distance than previously assumed. We also present tentative evidence of molecular clouds being more fragmented on bubble rims compared to dark clouds, suggesting that triggered star formation may occur, though our findings do not conform to a classic collect-and-collapse model.