The Hubble Deep Field North (HDF-N) is one of the most important, publicly available resources ever generated in the history of astronomy (see Ferguson, Dickinson & Williams 2000 and references therein). What initially appeared to be a small, undistinguished 2.5' × 2.5' patch of the celestial sphere was transformed during a 10-day long Hubble Space Telescope (HST) deep field integration (Williams et al. 1996).
The key advance of the HDF images was not only the depth which they reached (I < 29m) but perhaps more crucially, the superb angular resolution that accompanied them. On inspection it immediately became clear (see Fig. 1), that the most distant galaxies in the field were also the most morphologically disturbed. For example, familiar "grand design" spirals observed locally, all but disappear beyond z ~ 0.3. In terms of morphology, these distant, disturbed systems are most akin to nearby Ultra Luminous Infrared Galaxies (ULIG) and interacting starburst systems (Abraham & van den Bergh 2001).
Figure 1. Close-up of a small region of the HDF-N (Williams et al. 1996). More distant galaxies also tend to be more morphologically disturbed (NASA/STScI). One of the radio sources (VLA J123644+621133) detected by the European VLBI Network is circled (see section 4).