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A.N. Straughn, S.H. Cohen, R.E. Ryan, N.P. Hathi, R.A. Windhorst, R.A. Jansen (Arizona State University)
The abundance of dynamically unrelaxed galaxies in several deep fields (including the Hubble Ultra Deep Field, GOODS, and the COSMOS Field) suggests that they play a major role in the overall scheme of galaxy evolution. In particular, galaxies with knot-plus-tail morphologies--tadpole galaxies--constitute a large, well-defined subset of irregular objects that is uniquely measurable as is discussed here. The selection of this specific morphology also ties closely to recent theoretical predictions and numerical simulations that predict a stage of merger-driven galaxy evolution that closely resembles these tadpole galaxies in a distinct phase. In particular, this morphology appears to represent an \emph{early} stage in the merging of two galaxies of nearly-equal mass. The numerical simulations mentioned above predict a tadpole-like stage \~0.7 Gyr after the merger begins, with AGN activity being displayed only \emph{well after} the tadpole-like phase. A partner variability study led by Cohen et al. (2005) has concluded that for the HUDF, \~1% of objects (45 in all) to i_AB'=28 are variable due to AGN activity. This sample of likely-AGN objects has no overlap with the tadpole sample, suggesting that we may be seeing two phases in galaxy evolution, in agreement with these theoretical predictions.
This research was supported in part by the NASA Space Grant Graduate Fellowship at Arizona State University, the Harriet G. Jenkins Predoctoral Fellowship, and HST Grant GO.9793.07.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.