In this article, we have explored spatially-resolved stellar populations at high-redshift, and addressed the impact on studies of galaxy morphology. The deep, high-resolution IDT-NICMOS near-infrared imaging of a portion of the northern Hubble Deep Field has been combined with the WFPC 2 data and photometric redshift estimates to study the redshift evolution of morphology, comparing galaxy appearance at the same rest-wavelengths. Some Hubble tuning-fork galaxies only reveal their true morphology in the near-infrared images. This is particularly so for galaxies with a large dispersion in stellar ages and spatially-distinct stellar populations, such as spiral galaxies which sometimes exhibit galactic bars in the NICMOS images which are invisible at shorter wavelengths. However, galaxies which do undergo a morphological metamorphosis from the WFPC 2 to NIC 3 images are in the minority; most galaxies retain the same appearance in all wavebands, or are too compact for the structural parameters to be determined. Once the morphological k-corrections have been accounted for, it appears that the fraction of galaxies falling outside the Hubble sequence does increase at faint magnitudes/high-z. Many of these "true peculiars" show evidence of being dynamically disturbed (possibly through mergers) with recent star formation activity. From the HST imaging and resolved spectroscopy with Keck/LRIS, we have shown that a z = 2.8 chain galaxy in the HDF has a predominantly young stellar population and no significant rotation, and is thus unlikely to be an edge-on disk galaxy. Using gravitational amplification to increase our resolution, we have also resolved the stellar populations on sub-kpc scales in a system of z = 4.04 lensed arcs.
The analysis of galaxy morphologies and colours in multi-waveband imaging, coupled with resolved spectroscopy, provides a valuable probe into the stellar populations and evolution of galaxies. A natural progression is to use the integral field unit spectrographs currently being developed. Spectroscopy of spatially-resolved stellar populations in the high-redshift Universe will be a major scientific goal for NGST and the next generation of large ground-based telescopes with adaptive optics.
AJB acknowledges a NICMOS postdoctoral fellowship while at Berkeley (grant NAG 5-3043), and a U.K. PPARC observational rolling grant at the Institute of Astronomy in Cambridge (ref. no. PPA/G/O/1997/00793). The observations were obtained in part with the NASA/ESA Hubble Space Telescope operated by the Space Telescope Science Institute manged by the Association of Universities for Research in Astronomy Inc. under NASA contract NAS 5-26555. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We are grateful to Mark Dickinson, Chuck Steidel, Lisa Storrie-Lombardi & Ray Weymann for useful discussions, and Brenda Frye and Tom Broadhurst for providing details of their z = 4 arcs in advance of publication. We have made use of the spectral evolutionary models of Gustavo Bruzual and Stéphane Charlot. We thank Hans Hippelein and Klaus Meisenheimber at the Max-Planck-Institut für Astronomie for organizing an enjoyable and informative meeting at Ringberg.