5. WHERE NEXT?

I would conclude that the much-heralded 'synthesis' of theory and data is premature. We have also only just scratched on the surface of the HDF data. Indeed, much of the recent progress is based merely on the integrated colours of HDF galaxies; we have hardly begun exploiting the 2-D resolved data which is the true benefit of using HST. I will conclude by illustrating the next step in this sense which is based on preliminary work done in Cambridge (Abraham et al. 1997, for a preliminary discussion see Abraham 1997).

Instead of using integrated 4-colour data to estimate redshifts, why not use those sources for which spectroscopic redshifts are available (Cowie et al. 1996) and analyze the resolved pixel-by-pixel 4-colour data in constraining the star formation history of each galaxy and its physical sub-components? As most of the faint galaxies are extraordinarily blue, the HDF colours are primarily sensitive to relative main sequence ages, modulo small uncertainties in dust, the IMF and metallicity. These effects would be worrisome if absolute ages were sought, but not if the goal is to examine the relative spread of burst ages (t / t) across a galaxy in relation to its constituent spatial components. The technique can be tested for intermediate redshift spirals and ellipticals where sensible results emerge for the bulge and disk components and so, in Fig. 6, we illustrate how this method can be applied to the enigmatic high z irregulars and so-called 'chain galaxies.' Our resolved photometric analysis determines that these systems are consistent with star formation progressing in distinct bursts occurring several hundred Myr apart, as might be expected if physically-independent young components are slowly assembling into larger structures.

Figure 6. Resolved stellar populations for 3 high redshift HDF galaxies from the forthcoming study of Abraham et al. (for a preliminary discussion see Abraham (1997)). The left panels show F814W images for 3 galaxies with Keck spectroscopy courtesy of Len Cowie. The right panels show the pixel-by-pixel HDF colours grouped in physically-distinct regions within each image as indicated by the colour scheme in the left hand panels. Model tracks show evolutionary changes expected for various stellar histories according to the cosmic clock in the vertical scale. The analysis suggest these three galaxies assembled from physically and temporally-distinct stellar components.