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Session 70 - Cosmic Backgrounds and the Distant Universe.
Oral session, Thursday, June 11
The faint (I \sim 26) close pair of galaxies near the edge of chip 3 in the HDF (HDF 3--951) was selected for spectroscopic study because of its peculiar color, with V - I \approx 2.5 and fairly flat AB--colors into the near--IR. The resulting photometric redshift is z = 5.7. Deep Keck/LRIS spectroscopic observations yield a low signal--to--noise spectrum with a large discontinuity at 7720 Å\, and a flat continuum at longer wavelengths. The break has an amplitude > 7, and is thus unlikely to be any of the familiar discontinuities seen in optical spectra of low redshift galaxies. The most likely interpretation is the sudden onset of the Ly\alpha forest for a young, star--forming galaxy at z = 5.35. The amplitude of the break is larger than seen in other distant galaxies and quasars, with D_A \approx 0.9. For H_0 = 50 H_\circ km\,s^-1\,Mpc^-1, q_0 = 0.5, we derive a projected physical separation of \sim 3 kpc for the galaxy pair, and \sim 0.5 L_1500 luminosities (relative to the z \sim 3 Lyman--break population; Dickinson 1998). We speculate that the Ly\alpha clouds which stochastically depress the \lambda_rest < 1216 Å\, continuum may have an additional continuous H\scriptsize I absorption due to the long ``missing'' Gunn--Peterson effect.
Program listing for Thursday