Keck Spectroscopy and NICMOS Photometry of a Redshift Z = 5.60 Galaxy

Abstract

We present Keck Low Resolution Imaging Spectrometer spectroscopy along with Near-Infrared Camera and Multiobject Spectrometer (NICMOS) F110W (~J) and F160W (~H) images of the galaxy HDF 4-473.0 in the Hubble Deep Field (HDF), with a detection of an emission line consistent with Lyα at a redshift of z=5.60. Attention to this object as a high-redshift galaxy was first drawn by Lanzetta, Yahil, & Fernandez-Soto and appeared in their initial list of galaxies with redshifts estimated from the Wide Field Planetary Camera 2 (WFPC2) HDF photometry. It was selected by us for spectroscopic observation, along with others in the HDF, on the basis of the NICMOS F110W and F160W and WFPC2 photometry. For H₀=65 km s^-1 Mpc^-1 and q₀=0.125, the use of simple evolutionary models along with the F814W (~I), F110W, and F160W magnitudes allow us to estimate the star formation rate (~13 M_solar yr^-1). The colors suggest a reddening of E(B-V)~0.06. The measured flux in the Lyα line is approximately 1.0×10^-17 ergs cm^-2 s^-1, and the rest-frame equivalent width, correcting for the absorption caused by intervening H I, is ~90 Å. The galaxy is compact and regular, but resolved, with an observed FWHM of ~0.44". Simple evolutionary models can accurately reproduce the colors, and these models predict the Lyα flux to within a factor of 2. Using this object as a template shifted to higher redshifts, we calculate the magnitudes through the F814W and two NICMOS passbands for galaxies at redshifts 6<z<10.

Optical 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. The near-infrared observations were obtained with the Near-Infrared Camera and Multiobject Spectrometer on the NASA/ESA Hubble Space Telescope, which is operated by AURA Inc., under contract with NASA.