Tracing galaxy assembly: Tadpole galaxies in the hubble ultra deep field

In the Hubble Ultra Deep Field ( HUDF) an abundance of galaxies is seen with a knot at one end plus an extended tail, resembling a tadpole. These '' tadpole galaxies '' appear dynamically unrelaxed-presumably in an early merging stage-where tidal interactions likely created the distorted knot-plus-tail morphology. Here 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 presented in this issue, Cohen et al. revealed a total of 45 variable objects believed to be active galactic nuclei ( AGNs). Here we show that this faint AGN sample has no overlap with the tadpole galaxy sample, as predicted by recent theoretical work. 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. We show that the redshift distribution of tadpole galaxies follows that of the general 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 new observational results highlight the importance of merger-driven processes throughout cosmic history and are consistent with a variety of theoretical and numerical predictions.