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

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.

Recent Results in Dark Matter Direct Detection Experiments
July 10, 2012 | LASR conference room | 2:30 PM | Dan Hooper
Chris Kelso

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.

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

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

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.

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

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.

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

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.