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Tracing Galaxy Assembly: A Study of Merging & Emission-Line Galaxies with HST

The Hubble Space Telescope (HST) has revolutionized our view of the universe since the early 1990s, and deep imaging has provided a detailed view of galaxies that existed when the universe was only a fraction of its current age, allowing investigation of the galaxy assembly process over cosmic time. In the nearby universe, galaxies are neatly classified in the familiar Hubble types, yet as we look into the higher redshift universe— particularly at z > 1–2—we see a much more peculiar universe filled with irregularly-shaped galaxies that do not resemble many of the large, organized ones seen nearby (Driver et al. 1998). Additionally, we observe significant growth of galaxies’ supermassive black holes (SMBHs). One well-supported theory is that the process by which these transitions take place is through hierarchical galaxy merging and the subsequent merging and feeding of the central SMBHs. This process of galaxy merging triggers several specific physical processes, including not only SMBH growth, but also enhanced star formation (e.g., Larson & Tinsley 1978; Kennicutt et al. 1987; Hopkins et al. 2005). An efficient way to probe galaxies that are actively forming stars is through investigation of their spectral emission lines. These emission-line galaxies (ELGs) yield insight into the mechanism by which galaxies are believed to have built up over cosmic time. This dissertation summary, aimed at study of galaxy assembly and star-formation processes, describes results obtained on a specific class of merging/interacting galaxies—“tadpole” galaxies—as well as a sample of starforming ELGs. Tadpole galaxies from the Hubble Ultra Deep Field (HUDF; Beckwith et al. 2006) appear dynamically unrelaxed—presumably in an early merging stage—where tidal interactions likely created the distorted knot-plus-tail morphology. In Straughn et al. (2006) we systematically select tadpole galaxies from the HUDF and study their properties as a function of their photometric redshifts. In a companion HUDF variability study, Cohen et al. (2006) reveal a total of 45 variable objects believed to be Active Galactic Nuclei (AGN). This faint AGN sample has no overlap with the tadpole galaxy sample, as predicted by recent theoretical work (di Matteo et al2005; Springel et al. 2005; Hopkins et al. 2005). The tadpole morphology—combined with the lack of overlap with the variable objects—supports the idea that these galaxies are in the process of an early-stage merger event, i.e., at a stage that likely precedes the “turn-on” of any AGN component and the onset of any point-source variability. The redshift distribution of tadpole galaxies (Fig. 1) generally follows that of the field galaxy population, indicating that—if most of the tadpole galaxies are indeed dynamically young—the process of galaxy assembly generally kept up with the reservoir of available field galaxies as a function of cosmic epoch. These observational results highlight the importance of merger-driven processes throughout cosmic history, and are consistent with a variety of theoretical and numerical predictions. The HST grism PEARS (Probing Evolution And Reionization Spectroscopically; Malhotra et al., in preparation) survey provides a large data set of low-resolution spectra from thousands of galaxies in the GOODS North and South fields. ELGs are an important subset of objects in these data and we have investigated several different methods aimed at systematically selecting these galaxies (Straughn et al. 2008). We present a new methodology and results of a search for these ELGs in the PEARS HUDF using a 2D detection method that utilizes the observation that many emission lines originate from clumpy knots within galaxies. This 2D line-finding method proves to be