The star formation history of the Hubble sequence: spatially resolved colour distributions of intermediate-redshift galaxies in the Hubble Deep Field

We analyse the spatially resolved colours of distant galaxies of known reds kift in the Hubble Deep Field, using a new technique based on matching reso lved four-band colour data to the predictions of evolutionary synthesis mod els, Given some simplifying assumptions, we demonstrate how our technique i s capable of probing the evolutionary history of high-redshift systems, not ing the specific advantage of observing galaxies at an epoch closer to the time of their formation, We quantify the relative age, dispersion in age, o n-going star formation rate and star formation history of distinct componen ts, We explicitly test for the presence of dust and quantify its effect on our conclusions. To demonstrate the potential of the method, we study the s pirals and ellipticals in the near-complete sample of 32 I-814 < 21.9 mag g alaxies with (z) over bar similar to 0.5 studied by Bouwens, Broadhurst & S ilk. The dispersion of the internal colours of a sample of 0.4 < z < 1 earl y-type field galaxies in the HDF indicates that similar to 40 per cent (4/1 1) show evidence of star formation which must have occurred within the past third of their ages at the epoch of observation. This result contrasts wit h that derived for HST-selected ellipticals in distant rich clusters, and i s largely independent of assumptions with regard to metallicity. For a samp le of well-defined spirals, we similarly exploit the dispersion in colour t o analyse the relative histories of bulge and disc stars, in order to resol ve the current controversy regarding the ages of galactic bulges. Dust and metallicity gradients are ruled out as major contributors to the colour dis persions that we observe in these systems. The median ages of bulge stars a re found to be significantly higher than those in galactic discs, and they exhibit markedly different star formation histories. This result is inconsi stent with a secular growth of bulges from disc instabilities, but is consi stent with gradual disc formation by accretion of gas on to bulges, as pred icted by hierarchical theories, We extend our technique in order to discuss the star formation history of the entire Bouwens et al. sample in the cont ext of earlier studies concerned with global star formation histories, Fina lly, we consider how to extend our method using near-infrared data and to d eeper samples.