STELLAR POPULATIONS AND GALAXY MORPHOLOGY AT HIGH REDSHIFT

Andrew Bunker ^1,2, Hyron Spinrad ¹, Daniel Stern ^1,3, Rodger Thompson ⁴, Leonidas Moustakas ^1,5, Marc Davis ¹ and Arjun Dey ^6,7

¹ Department of Astronomy, University of California at Berkeley,
601 Campbell Hall, Berkeley CA 94720, USA
² Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
email: bunker@ast.cam.ac.uk
³ Jet Propulsion Laboratory, California Institute of Technology, MS 169-327, Pasadena CA 91109, USA
⁴ Steward Observatory, University of Arizona, Tucson AZ 85721, USA
⁵ Astrophysics Department, 1 Keble Road, Oxford OX1 3RH, UK
⁶ Kitt Peak National Observatory, 950 N. Cherry Ave., Tucson AZ 85726, USA
⁷ Department of Physics & Astronomy, The John Hopkins University, Baltimore MD 21218, USA

Abstract. In this article we investigate the morphology and stellar populations of high-redshift galaxies through multi-waveband HST imaging and ground-based spatially-resolved spectroscopy. We study the redshift evolution of galaxy morphology in the Hubble Deep Field, using the deep IDT-NICMOS near-infrared HST imaging coupled with spectroscopic and photometric redshifts. Using the multi-waveband data to compare the appearance of galaxies at the same rest-frame wavelengths reveals that morphological k-corrections (the change in appearance when viewing high-z objects at shorter rest-frame wavelengths) are only important in a minority of cases, and that galaxies were intrinsically more peculiar at high redshift. One example of significant morphological k-corrections is spiral galaxies, which often show more pronounced barred structure in the near-infrared than in the optical. Therefore, the apparent decline in the fraction of barred spirals at faint magnitudes in the optical HDF may be due to band-shifting effects at the higher redshifts, rather than intrinsic evolution.

Using such features as the age-sensitive Balmer+4000 Å break, the spatially-resolved colours of distant galaxies in optical/near-infrared imaging can also be used to study their component stellar populations. We supplement this with deep Keck/LRIS spectroscopy of two extended sources: a chain galaxy at z = 2.8 (HDF4-555.1, the "Hot Dog" - the brightest U-drop Lyman-break galaxy in the HDF) and a pair of z = 4.04 gravitationally lensed arcs behind the cluster Abell 2390. The absence of measurable rotation across the z = 2.8 chain galaxy implies that it is unikely to be a disk viewed edge on. With the resolution enhancement from lensing, we detect stellar populations of different ages in the z = 4 arcs. The Ly- alpha emission powered by the H II regions is spatially offset from the star-forming knots in these arcs, possibly as a result of resonant scattering by neutral hydrogen.