A Z = 5.34 Galaxy Pair in the Hubble Deep Field

Abstract

We present spectrograms of the faint V-drop (V_606 = 28.1, I_814 = 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 sigma) for these galaxies; our integrated spectrograms show a large and abrupt discontinuity near 7710 +/- 5 Å. This break is almost certainly due to the Lyalpha forest because its amplitude (1-f^short_nu/f^long_nu>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 (V_606 - I_814) color, consistent with this large break. A more accurate measure of the continuum depression blueward of Lyalpha 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^-2_50 M_⊙ yr^-1 (q_0 = 0.5) assuming the absence of dust extinction. The two individual galaxies are quite small (size scales <~1 h^-1_50 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.