PhD Thesis Defenses, 2006
 
Eric T. Linton, "A search for primordial black holes and a study of spectral variation in Markarian 421 using the Whipple 10-M telescope"
July 2, 2006
Online Materials

Scientific Advisor: Simon P. Swordy

The development of ground based atmospheric Cerenkov telescopes has opened up astronomy to the world of TeV photons and allowed for the study of a wide range of high energy physical processes. In this work two different examples are explored. The first is a search through 5.5 years of archival data from the Whipple 10-m telescope for TeV gamma-ray bursts on 1-s, 3-s, and 5- s timescales. The motivation for this search is that such a signal might be expected from the evaporation of primordial black holes (PBHs) in the local region of our galaxy. Based on a null result from this direct search for PBH evaporations, an upper limit of 1.08 10 6 pc -3 yr -1 (99% CL) on the evaporation rate is set, assuming the Standard Model of particle physics. This is more than a factor of two better than the previous limit at this energy range and includes longer time scales than have previously been explored.

The second topic covered in this work is a study of TeV observations of the blazar Markarian 421 made by the Whipple 10-m telescope during its 2003-2004 season. This work is the first to study this full data set. One result has been to confirm that the relationship between spectral shape and flux for month- scale flares is stable over the period of years. However, for data averaged over sub-month scales, this relationship fails to fit the data, indicating that a different mechanism is responsible for flares on the shorter time scales. Another interesting fact to come from this study is that, given the most recent constraints on the extra galactic background light, absorption by this photon field is insufficient to completely account for the cutoff observed in the spectrum of Markarian 421 at a few TeV. This finding provides further support for an intrinsic cause for this cutoff.

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KICP Members: Simon P. Swordy
KICP Students: Eric T. Linton
 
Zhaoming Ma, "Matter Power Spectrum 101"
July 19, 2006 | 2:00 PM | AAC 123
Online Materials

Scientific Advisor: Wayne Hu

We modify the public PM code developed by Anatoly Klypin and Jon Holtzman to simulate cosmologies with arbitrary initial power spectrum and equation of state of dark energy. With this tool in hand, we perform the following studies on the matter power spectrum.

With an artificial sharp peak at k ~ 0.2 h Mpc -1 in the initial power spectrum, we find that the position of the peak is not shifted by nonlinear evolution. An upper limit of the shift at the level of 0.02% is achieved by fitting the power spectrum local to the peak using a power law plus a Gaussian. This implies that, for any practical purpose, the baryon acoustic oscillation peaks in the matter power spectrum are not shifted by nonlinear evolution which would otherwise bias the cosmological distance estimation. We also find that the existence of a peak in the linear power spectrum would boost the nonlinear power at all scales evenly. This is contrary to what HKLM scaling relation predicts, but roughly consistent with that of halo model.

We construct two dark energy models with the same linear power spectra today but different linear growth histories. We demonstrate that their nonlinear power spectra differ at the level of the maximum deviation of the corresponding linear power spectra in the past. Similarly, two constructed dark energy models with the same growth histories result in consistent nonlinear power spectra. This is hinting, not a proof, that linear power spectrum together with linear growth history uniquely determine the nonlinear power spectrum. Based on these results, we propose that linear growth history be included in the next generation fitting formulas of the nonlinear power spectrum.

For simple dark energy models parametrized by w 0 and w a , the existing nonlinear power spectrum fitting formulas, which are calibrated for ACDM model, work reasonably well. The corrections needed are at percent level for the power spectrum and 10% level for the derivative of the power spectrum. We find that, for Peacock & Dodds (1996) fitting formula, the corrections to the derivative of the power spectrum are independent of w a but changing with redshift. As a short term solution, a fitting form could be developed for w 0 , w a models based on this fact.

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KICP Members: Wayne Hu
KICP Students: Zhaoming Ma
 
Eduardo Rozo, "Cosmological Constraints from Photometrically-Selected Cluster Catalogs"
July 24, 2006 | 3:00 PM | LASR conference room
Online Materials

Scientific Advisors: Andrey V. Kravtsov, Scott Dodelson

We develop a new maximum likelihood method for estimating cosmological parameters using observed cluster abundances. Our model allows us to include and properly marginalize over systematic uncertainties associated with the cluster selection function. We apply our method to the maxBCG cluster catalog, and explicitly demonstrate that when the cluster selection function is accurately known, the maxBCG catalog can provide percent level constraints on particular combinations of cosmological and Halo Occupation Distribution (HOD) parameters. Even when the selection function is only known with poor accuracy, we show that our method can provide interesting constraints on s 8 if one assumes cosmological priors on O m h 2 and h as determined from the Cosmic Microwave Background (CMB) and supernovae data. Our best estimate for s 8 is s 8 = [Special characters omitted.] (1-s), though it is also subject to an additional 5% prior on the slope of the HOD.

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KICP Members: Scott Dodelson; Andrey V. Kravtsov
KICP Students: Eduardo Rozo
 
Eun-Joo Ahn, "Early stages of ultra high energy cosmic ray air showers as a diagnostic of exotic primaries"
July 31, 2006 | 2:00 PM | AAC 123
Online Materials

Scientific Advisor: Angela V. Olinto

The nature of ultra high energy cosmic rays (UHECRs) remains an enigma. UHECR detection rate is increasing with new generation detectors which will speed up the process of understanding these energetic particles. After a review on the field of UHECRs, we focus on air shower characterisation of primaries. We study both common primaries as well as more exotic possibilities. One such case is the TeV black hole (BH) creation which can happen in models of large extra dimensions. High energy neutrinos interacting with air molecules may form these objects in the Earth's atmosphere, and a good way of discriminating them from other backgrounds is through air shower studies. Full scale Monte Carlo simulations of air shower cascades are the best way to predict air shower characteristics. However, these are very computer-time consuming. The first interactions of an air shower is instructive as it gives information on how the shower will develop without the full scale simulation. The first interaction study is a far less time consuming method that is advantageous for testing new models with many parameters. Hadronic models are used to interact particles in simulation softwares, where low energy experimental data are extrapolated up to high energies using various models, such as minijets and pomerons. Here, we study the first interactions of well studied cosmic ray primaries - photon, proton, iron nucleus - are performed with SIBYLL 2.1, a hadronisation Monte Carlo which uses the minijet model. "Templates" from common primaries can then be used to compare with new primaries. The TeV BHs are used as an example of a new primary. Air shower simulations have been carried out, which showed that these neutrino induced BH air shower resembles a hadronic air shower. Using the first interaction templates, we show that BHs indeed resemble protons closely.

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KICP Members: Angela V. Olinto
KICP Students: Eun-Joo Ahn
 
Jacqueline Chen, "The Galaxy Cross-Correlation Function as a Probe of the Spatial Distribution of Galactic Satellites"
August 23, 2006 | 1:30 PM | AAC 123
Online Materials

Scientific Advisor: Andrey V. Kravtsov

The spatial distribution of satellite galaxies around host galaxies can illuminate the relationship between satellites and dark matter subhalos and aid in developing and testing galaxy formation modes. The projected cross- correlation of bright and faint galaxies offers a promising avenue to putting constraints on the radial distribution of satellite galaxies. Previous efforts to constrain the distribution attempted to eliminate interlopers from the measured projected number density of satellites and found that the distribution is generally consistent with the expected dark matter halo profile of the parent hosts. The measured projected cross-correlation can be used to analyze contributions from satellites and interlopers together, using a halo occupation distribution (HOD) based analytic model for galaxy clustering. Tests on mock catalogs constructed from simulations show promise in this approach. Analysis of Sloan Digital Sky Survey (SDSS) data shows results generally consistent with interloper subtraction methods, although the radial distribution is poorly constrained with the current dataset and larger samples are required.

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KICP Members: Andrey V. Kravtsov
KICP Students: Jacqueline Chen
Scientific projects: Sloan Digital Sky Survey (SDSS)
 
Nicolas G. Busca, "The ultra high energy cosmic ray flux from the southern Pierre Auger Observatory data"
October 26, 2006
Online Materials

Scientific Advisor: Angela V. Olinto

The Pierre Auger Observatory is currently the largest ground based cosmic ray observatory. Its goal is to characterize the properties of ultra high energy cosmic rays (of energies above ~ 10 18 eV) in order to understand their origin, composition and acceleration mechanisms.

In this thesis, the cosmic ray flux is calculated using data from the first two and a half years of the production phase of the Auger observatory. The Auger observatory is a hybrid observatory combining a ground array of particle detectors and a set of fluorescence telescopes that overlook this ground array. The constant intensity cut method is used to measure the attenuation curve from ground array data. The energy calibration curve is measured using events that the trigger both the ground array and the fluorescence detector. A simple proportionality between energy and signal at ground is found. The energy resolution is shown to be between 18% at low energies and 10% at higher energies, with an overall systematic uncertainty of 25%.

The resulting flux exhibits an ankle at energies around 3 10 18 eV, and shows signs of a GZK cutoff at around 40 10 18 eV. Both these energy estimates are uncertain by 25%. The integrated flux of events above 10 19 eV is (25 5(stat) 6(syst))/km 2 /sr/century.

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KICP Members: Angela V. Olinto
KICP Students: Nicolas G. Busca
Scientific projects: Pierre Auger Observatory (AUGER)