A z = 5.34 galaxy pair in the Hubble Deep Field

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 z_ph = 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 - f^short_v/f^long_v > 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 D_A = 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.