Andrew Bunker

Abstract:

We present spectrograms of the faint V-drop (V606 = 28.1, I814 = 25.6) galaxy pair HDF 3-951.1 and HDF 3-951.2 obtained at the Keck II Telescope. In a recent study, Fernández-Soto, Lanzetta, & Yahil derive a photometric redshift of zph = 5.28+0.34-0.41 (2 σ) for these galaxies; our integrated spectrograms show a large and abrupt discontinuity near 7710 ± 5 Å. This break is almost certainly due to the Lyα forest because its amplitude (1 - fshortv/flongv > 0.87, 95% confidence limit) exceeds any discontinuities observed in stellar or galactic rest-frame optical spectra. The resulting absorption break redshift is z = 5.34 ± 0.01. Optical/near-IR photometry from the HDF yields an exceptionally red (V606 - I814) color, consistent with this large break. A more accurate measure of the continuum depression blueward of Lyα utilizing the imaging photometry yields DA = 0.88. The system as a whole is slightly brighter than L*1500 relative to the z ∼ 3 Lyman break population, and the total star formation rate inferred from the UV continuum is ≈22 h-250 M⊙ yr-1 (q0 = 0.5) assuming the absence of dust extinction. The two individual galaxies are quite small (size scales ≲ 1 h-150 kpc). Thus these galaxies superficially resemble the "building blocks" of Pascarelle and coworkers; if they comprise a gravitationally bound system, the pair will likely merge in a timescale ∼100 Myr.

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. ForH0 = 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⊙ 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. © 1998. The American Astronomical Socicly. All rights reserved.