2.1. Multi-wavelength follow-up observations of the HDF

The HDF represented a huge investment in HST observing time, and this was matched by an equally ambitious multi-wavelength follow-up campaign. The latter included every part of the e-m spectrum accessible via both ground and space based instrumentation. In many cases, extremely long integrations produced the very deepest view of the Universe in a given wave-band. In addition, extensive efforts were made to obtain spectroscopic and photometric redshifts of sources in the field (including the adjacent Hubble Flanking Fields - HFF). A pictorial summary of a limited subset of these multi-wavelength follow-up observations is shown in Fig. 2.

Figure 2. Multi-wavelength images of the HDF-N in the radio (Garrett et al. 2000a - includes the HFF), sub-mm (Hughes et al. 1998), Mid-IR (Rowan-Robinson et al. 1997 - includes part of the HFF) and x-rays (Brandt et al. 2001 - includes the HFF).

It comes as no surprise that the source counts in the HST images (both in the optical and Near-IR) are considerably greater than that observed at other wavelengths. What is surprising, however, is that despite this fact, a considerable fraction (10-20%) of the faint radio, Mid-IR and x-ray detections, appear to be heavily obscured in the optical i.e. they have no obvious optical counterparts (R > 25^m). An even larger fraction of the faint SCUBA source population fall into this category, suggesting that the sub-mm observations reveal a completely different (unobscured) view of the high redshift, dusty, star-forming Universe (Hughes et al. 1998). We will return to the nature of these faint sub-mm sources in section 3.3.