Clumpy Galaxies in GOODS and GEMS: Massive Analogs of Local Dwarf Irregulars

Clumpy galaxies in the GEMS and GOODS fields are examined for clues to their evolution into modern spirals. The magnitudes of the clumps and the surface brightnesses of the interclump regions are measured and fitted to models of stellar age and mass. There is an evolutionary trend from clump clusters with no evident interclump emission to clump clusters with faint red disks, to spiral galaxies of the flocculent or grand design types. Along this sequence, the interclump surface density increases and the mass surface density contrast between the clumps and the interclump regions decreases, suggesting a gradual dispersal of clumps to form disks. Also along this sequence, the bulge-to-clump mass ratios and age ratios increase, suggesting a gradual formation of bulges. All of these morphological types occur in the same redshift range, indicating that the clump cluster morphology is not the result of bandshifting. Comparisons to local galaxies with the same rest wavelength and spatial resolution show that clump clusters resemble local dwarf Irregulars. This resemblance is consistent with a model in which the clumpy morphology comes from gravitational instabilities in gas with a high turbulent speed compared to the rotation speed and a high mass fraction compared to the stars.Comment: scheduled for ApJ 701, August 20, 2009, 40 pages, 20 figure

The State of the Blazar: Investigations of Variability Patterns in the Very High Energy Gamma-ray Emission from Northern Blazars

University of Minnesota Ph.D. dissertation. December 2017. Major: Astrophysics. Advisor: Lucy Fortson. 1 computer file (PDF); ix, 160 pages.Blazars are active galactic nuclei with jets aimed in our direction. They are one of the most energetic astrophysical phenomena and in the very-high-energy (VHE; > 100 GeV) gamma-ray regime comprise the dominant extragalactic source class. Variability in the non-thermal emission of blazars is one of the key observables for discerning the details of their emission regions. In this thesis, I explore VHE gamma-ray emission properties of blazars in a variety of states. After a detailed description of the VERITAS instrument and data analysis, I present an in-depth study of an unusually stable extreme blazar (formerly, an unidentified source), HESS J1943+213, for which no concrete evidence of variability has been detected to date. The study includes the steps towards identifying HESS J1943+213 as a blazar using VLBA observations and estimation of the source redshift with gamma-ray spectra. Models of leptonic emission from the source and hadronic emission from cosmic-ray interactions along the line of sight are presented for explaining the gamma-ray observations. I then proceed to characterize the emission of two blazars in rapidly flaring states, Mrk 421 and MS 1221.8+2452. The dataset for Mrk 421, the first blazar detected in VHE gamma rays, comes from one of the brightest VHE gamma-ray flares ever observed and offers a unique opportunity for exploring VHE gamma-ray and multi-wavelength emissions of the source on timescales of few minutes. The flare of MS 1221.8+2452, on the other hand, allows for the construction of the first VHE gamma-ray spectrum of the source, which is used along with contemporaneous multi-wavelength observations to construct and model the SED of the source during the flare. Moreover, the source exhibits a spectral hysteresis pattern in X-rays during the flaring period, which is tied to the acceleration and cooling timescales of particles in the emission zone. Finally, I investigate, PG 1553+113, a blazar exhibiting long-term periodic flux modulations in lower-energy gamma rays – due to periodic accretion flow instabilities, jet precession, or the presence of a binary supermassive black hole system – and search for a similar periodicity signal in the VHE gamma rays