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The Star Formation and Merger Evolution of Interacting Galaxies

Carrie R. Bridge

Doctor of Philosophy 2007
Graduate Department of Astronomy and Astrophysics, University of Toronto

Hierarchical models and observations show that galaxy mergers and interactions play a key role in galaxy assembly and star formation, but to what extent is still unclear. This thesis attempts to quantify their contribution to galaxy evolution by probing the number of interactions and mergers, along with their star forming properties as a function of redshift. The presence of long tidal tails and bridges are robust signatures of recent merger activity. This completely dynamical phenomenon was used to develop a new classification scheme to identify interacting galaxies and probe the interaction fraction and merger rate. We applied this new technique to large area, multi-band imaging obtained via the Canada France Hawaii Telescope Legacy Survey (CFHTLS-Deep), yielding the first statistically secure, lower limit of the galaxy interaction fraction between 0.1 <= z <= 1.0. Optically, the fraction of galaxies undergoing an interaction evolves moderately with redshift as (1 + z)2.24 +- 0.24.

The Spitzer 24μm coverage of both the Extragalactic First Look Survey (XFLS) and CFHTLS Deep Survey were used to carry out one of the first and largest merger studies of IR bright galaxies. Within the ACS component of the XFLS, interactions were identified over the full merger sequence using traditional techniques, finding a merger rate increase for 24μm galaxies of (1 + z)~2. This result implies that merging is an increasingly important process in the evolution of luminous IR galaxies (LIRGs), contributing 40-60% of the IR luminosity density and at least 30-40% of the star formation rate density at z ~ 1.

Galaxy interactions at all stages are found to have elevated star formation rates greater than a factor of two-four (on average) and a higher incidence of AGN activity compared to non-interacting field galaxies. This result supports a causal connection between galaxy merging, induced star formation, and AGN activity. Ultimately, major mergers provide a moderate contribution to the evolution of the cosmic star formation rate density and IR luminosity density to z ~ 1, with an increasing trend suggesting that merging plays a larger role at higher redshifts (z > 1). It is also clear that merging plays a significant role in triggering the processes that power the IR emission of LIRG galaxies at z > 0.5.

Reproduced with permission. library@astro.utoronto.ca
November 20 2007