Tracing galaxy assembly: A study of merging and emission-line galaxies

One astrophysical problem that remains largely unsolved is that of galaxy formation and evolution. These outstanding questions can be probed through investigation of galaxy mergers and physical processes induced by merging events. The Hubble Space Telescope (HST) has revolutionized our view of the universe since the early 1990's, and deep imaging over the past decade has allowed a much more detailed view of galaxies that existed when the universe was only a fraction of its current age. Through a study of early--stage mergers ("tadpole" galaxies), observational support is found for recent numerical simulations which predict that Active Galactic Nuclei (AGN) activity will only be detectable well after the galaxies are in a morphologically--recognizable merger stage. Additionally, the percentage of galaxies that appear in the tadpole phase is roughly constant at ~6% for the redshift range probed here, implying that galaxy assembly generally kept up with the supply of available field galaxies over cosmic time. Merging is known to induce episodes of active star formation, which can be probed through spectral emission lines. The HST Probing Evolution And Reionization Spectroscopically (PEARS) grism survey data is used to investigate emission--line galaxies (ELGs) in the widely--studied CDF-S field. Three ELG detection methods are investigated in detail; a 2- dimensional method is efficient at detecting individual star--forming galaxy knots out to redshifts ~1.5. Many of the emission lines detected have very high equivalent widths (EW), and potential evolution of [O II ] EW with redshift is detected--suggesting substantial evolution of galaxies' star-forming properties since redshift ~1.5. The total sample includes 230 star--forming knots in 203 individual ELGs. One hundred eighteen of these galaxies previously had no spectroscopic redshift; the line identifications now provide secure grism-- spectroscopic redshifts for these objects. By adjusting the fitting procedure to include the H-beta line (which is blended with [O III ]), estimates of excitation are obtained and thus AGN candidates are selected. Additionally, the radial distribution of giant star--forming knots in PEARS galaxies up to redshift ~0.5 tend to peak near the half-light radii--similar to that of giant HII regions in local galaxies.