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.