A Comprehensive Study of Chemical Evolution in Galaxy Mergers

University Of Hawaii, Honolulu

Investigators

Abstract

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.