This project seeks to clarify our understanding of how galaxies are altered, and how the histories of their stellar populations are affected, by galaxy merging. Taking a primarily observational approach, the project is motivated by extensive theoretical work of that predicts that galaxy mergers should drive large amounts of diffuse interstellar gas to the central regions of the merging systems, resulting in a burst of star formation, fueling of an active nucleus (supermassive black hole), or both. In a pilot study of merging systems, the Principal Investigator has found that the heavy element abundances (metallicities) of the interstellar gas vary with radius in the galaxy much less steeply than in ordinary isolated spiral galaxies. This result is consistent with theoretical expectations that flow of metal-poor gas into the metal-rich central regions should smooth out the radial gradients. To test this picture more completely, the team will undertake a spectroscopic study, using the LRIS spectrograph on the 10-meter Keck telescope, of galaxies covering a wide range in merger stage, from first passage to final coalescence. These galaxies will be compared against a control sample of isolated galaxies with ordinary metallicity gradients. In parallel with the observational work, the proposing team will work on the theory-based development of new metallicity diagnostics for ionized hydrogen (HII) regions. These new diagnostics will take into account the ionization of the gas produced by starlight, and use population synthesis models to additionally include the age of the stellar population; the goal is to reduce the scatter in radial metallicity gradients currently measured from HII regions. Finally, the observations will be compared quantitatively with the predictions of hydrodynamical simulations of galaxy mergers. In addition to support for graduate student training, this project will also fund the team's activities in the HI STAR outreach program. In particular, the team will develop a Galaxy Collisions Discovery Tutorial using their spectroscopic data and incorporates physics at the K-12 level.
Galaxy interactions and mergers are fundamental to the formation and evolution of galaxies. In this proposal, we exploit a new technique to systematically probe the major gas flows in galaxy mergers: metallicity gradients. Metallicity gradients measure the amount of oxygen to the amount of hydrogen in galaxies as a function of radius. Galaxies with constant oxygen to hydrogen across the galaxy are said to have "flat" metallicity gradients, while galaxies with more oxygen in their centers compared with their outskirts are said to have "steep" gradients. In this project, we measure the oxygen abundance gradient in ~50 galaxies undergoing mergers to test whether the mergers cause large inflows of pristine gas from the outer regions of the galaxies. We obtained spectra of star-forming regions in merging galaxies using the Keck telescope on Mauna Kea. We use the latest sophisticated merger models to compare with our observations. We show that the mergers trigger widespread gas inflows after the galaxies pass close to each other for the first time. The oxygen abundance gradients in mergers become shallower and shallower until the latest merger stages. These results are a smoking gun for massive gas infall. In our merger models, the gravitational effects of a merger cause pristine gas to stream down the spiral arms and into the galaxy center. Our results show that these merger models are correct, and that gas is distributed throughout the galaxy during the merger, and is not simply concentrated in the center. During this project, we discovered that at late merger stages, massive shock fronts are generated, which drive large amounts of material out of the galaxies. These shocks can either be caused by gas collisions during the merger, or they can be caused by numerous supernovae and stellar winds due to the new star formation triggered by the merger. This project was the training ground for 3 PhD students, and 2 masters students at the University of Hawaii.
