PhD Thesis Defenses, 2012
Hao Huan: "Cosmic Gamma-Ray Propagation as a Probe for Intergalactic Media and Interactions"
April 30, 2012 | 1:00 PM | LASR Conference room
Picture: Hao Huan: Cosmic Gamma-Ray Propagation as a Probe for Intergalactic Media and Interactions
Scientific Advisor: Scott P. Wakely

Ph.D. Committee: Paulo Privitera, Jonathan Rosner, Mark Oregia.

Thesis Abstract: Very-high-energy gamma rays from extragalactic sources travel a long way through the universe before being detected. Along this path the gamma-ray photons interact with the intergalactic photon field, including the cosmic microwave background and extragalactic background light, to initiate an electromagnetic cascade in the cosmic voids. The geometry of the cascade is affected by the presence of an extragalactic magnetic field, resulting in a characteristic morphology called a pair halo in observations. We employ both semi-analytic modeling and a full-scale Monte Carlo simulation to predict the distribution of cascade photons. In comparing predictions with observed data we are able to place a lower limit on the magnetic field strength at pG level, the weakest people have been able to probe so far.

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KICP Members: Paolo Privitera; Jonathan L. Rosner; Scott P. Wakely
KICP Students: Hao Huan
Scientific projects: Very Energetic Radiation Imaging Telescope Array System (VERITAS)

Immanuel Buder: "Measurement of the CMB Polarization at 95 GHz from QUIET"
June 8, 2012 | 2:30 PM | ACC 211
Picture: Immanuel Buder: Measurement of the CMB Polarization at 95 GHz from QUIET
Scientific Advisor: Stephan S. Meyer

PhD Committee: Paolo Privitera, Michael Turner, David Biron.

Thesis Abstract: Despite the great success of precision cosmology, cosmologists cannot fully explain the initial conditions of the Universe. Inflation, an exponential expansion in the first 10-30s, is a promising potential explanation. A generic prediction of inflation is odd-parity (B-mode) polarization in the cosmic microwave background (CMB). The Q/U Imaging ExperimenT (QUIET) aimed to limit or detect this polarization. We built a coherent pseudo-correlation microwave polarimeter. An array of mass-produced "modules" populated the focal plane of a 1.4-m telescope. We incorporated "deck" rotation, an absorbing ground screen, a new time-stream "double-demodulation" technique, and optimized optics into the design to reduce instrumental polarization. We observed with this instrument at the Atacama Plateau in Chile between August 2009 and December 2010. We collected 5336.9,hours of CMB observation and 1090,hours of astronomical calibration. This thesis describes the analysis and results of these data. We characterized the instrument using the astronomical calibration data as well as purpose-built artificial sources. We developed noise modeling, filtering, and data selection following a blind-analysis strategy. Central to this strategy was a suite of 32 null tests, each motivated by a possible instrumental problem or systematic effect. We also evaluated the systematic errors in the blind stage of the analysis before the result was known. We then calculated the CMB power spectra using a pseudo-$C_ell$ cross-correlation technique that suppressed contamination and made the result insensitive to noise bias. We measured the first three peaks of the E-mode spectrum at high significance and limited B-mode polarization. Systematic errors were well below ($r < 0.01$) our B-mode polarization limit. This systematic-error reduction was a strong demonstration of technology for application in more sensitive, next-generation CMB experiments.

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KICP Members: Stephan S. Meyer; Paolo Privitera; Michael S. Turner
KICP Students: Immanuel Buder
Scientific projects: Q/U Imaging ExperimenT (QUIET)

Eva Wuyts: "A Magnified View of High Redshift Star Formation"
July 3, 2012 | 3:00 PM | TAAC 67
Scientific Advisor: Michael D. Gladders

Thesis Abstract: This work takes advantage of the magnified view of the z=1-3 Universe provided by cluster-scale strong gravitational lensing to advance our understanding of the physical mechanisms driving the assembly of galaxies at this epoch of peak star formation. I will present measurements of the stellar mass and gas-phase metallicity for 10 lensed galaxies at 0.9<z<2.5, which extends the observed relation between stellar mass and metallicity for star-forming galaxies at z~2 to lower stellar masses than previously studied. I find less redshift evolution of the mass-metallicity relation in this mass range. There is a general agreement with the local fundamental relation between metallicity, stellar mass and SFR from Mannucci et al., though the scatter becomes large for the high specific star formation rates probed by our lensed galaxies. Using the Kennicutt-Schmidt law to infer gas fractions, I investigate the importance of gas inflows and outflows on the shape of the mass-metallicity relation using simple analytical models. I will also present a combined analysis of HST/WFC3 optical/near-IR imaging and Keck/OSIRIS near-IR IFU spectroscopy aided by laser-guide star adaptive optics for RCSGA0327, the brightest distant lensed galaxy currently known in the Universe. Due to the high lensing magnification of the system, these observations reach spatial scales of <100pc in the source-plane of the galaxy. The velocity field suggests we are witnessing a merger in progress, close in time to the first-pass encounter. Both the WFC3 images and the OSIRIS data show a clumpy morphology and resolve multiple individual star-forming regions. I measure the sizes, stellar masses, star formation densities and velocity dispersions of these clumps and compare them to scaling relations for local HII regions.

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KICP Members: Michael D. Gladders
KICP Students: Eva Wuyts

Christopher Kelso: "Recent Results in Dark Matter Direct Detection Experiments"
July 10, 2012 | 2:30 PM | LASR conference room
Picture: Christopher Kelso: Recent Results in Dark Matter Direct Detection Experiments
Scientific Advisor: Daniel Hooper

PhD Committee members: Juan Collar, Carlos Wagner, Wendy Zhang

"Chris' research has been focused on dark matter, and interpretations of data from a number of underground experiments designed to search for it. Particularly exciting has been his work on dark matter interpretations of the signals reported by the CoGeNT, CRESST and DAMA experiments. While we cannot say for certain yet whether these experiments are seeing dark matter particles, Chris' work has certainly helped to make these experiments and their anomalous results a "hot topic" among particle physicists and cosmologists."
- Dan Hooper, PhD advisor

Thesis Abstract: Three dark matter direct detection experiments (DAMA/LIBRA, CoGeNT, and CRESST-II) have each reported signals which resemble that predicted for a dark matter particle with a mass of roughly 10 GeV. I will review the detectors and their reported signals over the last few years. I will also compare the signals of these experiments and discuss whether they can be explained by a single species of dark matter particle, without conflicting with the constraints of other experiments. I will show that the spectrum of events reported by CoGeNT and CRESST-II are consistent with each other and with the constraints from CDMS-II, although some tension with xenon- based experiments remains. Similarly, the modulation signals reported by DAMA/LIBRA and CoGeNT appear to be compatible, although the corresponding amplitude of the observed modulations are a factor of at least a few higher than would be naively expected, based on the event spectra reported by CoGeNT and CRESST-II. I will also discuss some ways that this apparent discrepancy could potentially be resolved.

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KICP Members: Juan I. Collar; Daniel Hooper; Carlos E. M. Wagner
KICP Students: Christopher M. Kelso
Scientific projects: Coherent Germanium Neutrino Technology (CoGeNT)

Samuel Leitner: "The Star Formation History of Disk Galaxies & Implications for Simulations"
July 20, 2012 | 12:00 PM | ACC 211
Picture: Samuel Leitner: The Star Formation History of Disk Galaxies & Implications for Simulations
Scientific Advisor: Andrey V. Kravtsov

PhD Committee members: Fausto Cattaneo, Hsiao-Wen Chen, Nick Gnedin.

"In his thesis paper Sam has derived star formation histories of galaxies, which are still forming stars now, using a variety of observations on how star formation rate depends on stellar mass of galaxies as a function of cosmic epoch. Remarkably, the results show that galaxies such as our Milky Way have formed more than 80% of their stars over the last ten billion years of the evolution, and less than 20% during the first three billion years. This result is at variance with most simulations of galaxy formation and may help to explain why simulations generally fail to produce late-type disk galaxies with small bulges. In his paper, Sam has also uncovered a curious discrepancy between the star formation histories of dwarf galaxies derived using the method he used and those deduced from the color-magnitude diagram of stars for nearby dwarfs."
- Andrey Kravtsov, PhD advisor

Thesis Abstract: Disk galaxies are sensitive probes of the processes governing the growth of stellar mass in the universe. Hydrodynamic simulations still struggle to match the observed properties of these galaxies. I will present the mass-dependent star formation history of disk galaxies (deduced from surveys of the relation between star formation rate and stellar mass). These histories robustly trace present-day massive disks back to when they were close to 10% of their current mass. Based on these star formation histories, I will argue that limiting star formation at early epochs is crucial to forming more realistic disks in simulations. Simulation further show that gas accreted at high redshift cannot be allowed to settle at the centers of disks. Improved treatment of the star formation and the radiation feedback within molecular clouds provides a natural path to both restrained star formation and redistributed gas. I will briefly discuss subgrid treatment of these processes and some promising results from simulations.

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KICP Members: Hsiao-Wen Chen; Nickolay Y. Gnedin; Andrey V. Kravtsov
KICP Students: Samuel N. Leitner

Melanie Simet: "Galaxy cluster center detection methods with weak lensing"
August 3, 2012 | 2:00 PM | TAAC 67
Picture: Melanie Simet: Galaxy cluster center detection methods with weak lensing
Scientific Advisor: Scott Dodelson

PhD Committee members: Dan Hooper, Michael Gladders, Angela Olinto, Michael Turner.

"Melanie has studied weak gravitational lensing caused by galaxy clusters. She led the effort to measure this lensing signal in Stripe 82 of the Sloan Digital Sky Survey, with results that will help calibrate cluster masses and begin the community on the road to measuring tomography. For her thesis, Melanie studied the important effect that it is difficult to locate the true center of a galaxy cluster. This "mis-ceterning" problem leads to incorrect mass determinations. By using a combination of simulations and data from SDSS, Melanie quantified both the effect and various algorithms proposed to address it."
- Scott Dodelson, PhD advisor

Thesis Abstract: The precise location of galaxy cluster centers is a persistent problem in weak lensing mass estimates and in interpretations of clusters in a cosmological context. In this work, we test methods of centroid determination directly from weak lensing data and examine the effects of such self-calibration on the measured masses. Drawing on lensing data from the Sloan Digital Sky Survey Stripe 82, a 275 square degree region of coadded data in the Southern Galactic Cap, together with a catalog of MaxBCG clusters, we show that halo substructure as well as shape noise and stochasticity in galaxy positions limit the precision of such a self-calibration (in the context of Stripe 82, to $sim 500 h^{-1}$ kpc or larger) and bias the mass estimates around these points to a level that is likely unacceptable for the purposes of making cosmological measurements. We note two MaxBCG clusters that may be centered on a location other than the brightest cluster galaxy and discuss implications for future work.

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KICP Members: Scott Dodelson; Michael D. Gladders; Daniel Hooper; Angela V. Olinto; Michael S. Turner
KICP Students: Melanie Simet
Scientific projects: Sloan Digital Sky Survey (SDSS)

Christopher Greer, "Calibrating Optical Richness using Sunyaev-Zel'dovich Observations"
October 29, 2012 | 10:00 AM | LASR conference room
Picture: Christopher Greer, Calibrating Optical Richness using Sunyaev-Zeldovich Observations
Scientific Advisor: John E. Carlstrom

PhD Committee members: Al Harper, Wayne Hu, and Scott Dodelson.

"Chris Greer made critical contributions to the building and deploying of the Sunyaev-Zel'dovich Array (SZA), an interferometric array of 3.5 meter mm-wave telescopes located at the CARMA site in California. He used the SZA to image the SZ effect for a sample of galaxy clusters selected by their richness in the SDSS maxBCG cluster catalog. His thesis provides the first joint calibration of the SZ and optical mass-observable relation. This work is important for using cluster surveys for constraining cosmology, in particular the nature of dark energy, and it provides the ground work for the upcoming joint analysis of the large South Pole Telescope and Dark Energy Survey data sets."
- John Carlstrom, PhD advisor

Thesis Abstract: The advent of multiple large-area galaxy cluster surveys across multiple wavelengths means that galaxy cluster abundance measurements will play a key role in understanding the dark energy accelerating the universe. The main systematic limitation at the moment, however, is the understanding of the observable-mass relation. Recent theoretical work has shown that combining samples of clusters from surveys at different wavelengths can mitigate this systematic limitation. I present Combined Array for Research in Millimeter-wave Astronomy (CARMA) Sunyaev-Zel'dovich (SZ) observations of 28 galaxy clusters selected from the Sloan Digital Sky Survey (SDSS) maxBCG catalog. This cluster sample represents a complete, volume-limited sample of the richest galaxy clusters in the SDSS DR7 survey between redshifts 0.2 &ge z &ge 0.3, as measured by the RedMaPPer algorithm being developed for the Dark Energy Survey (DES, Rykoff et al., 2012).

I develop a formalism that uses the cluster abundance in tandem with the galaxy richness measurements from SDSS and the SZ signal measurements form CARMA to calibrate the SZ and optical mass-observable relations. We find that the scatter in richness at fixed mass is &sigma = 0.24+/-0.09 using SZ signal calculated by integrating a cluster pressure profile to a radius of 1 Mpc at the redshift of the cluster. I also calculate the SZ signal at R500 and find that the choice of scaling relation used to determined R500 has a non-trivial effect on the constraints of the observable-mass relationship. Finally, I investigate the source of disagreement between the positions of the SZ signal and SDSS Brightest Cluster Galaxies (BCG)s. Improvements to the richness calculator that account for blue BCGs in the cores of cool-core X-ray clusters, as well as multiple BCGs in merger situations will help reduce &sigma further. This work represents the first independent calibration of the RedMaPPer algorithm.

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KICP Members: John E. Carlstrom; Scott Dodelson; Wayne Hu
KICP Students: Christopher Greer