SPECTROSCOPIC LIMITS ON HIGH-REDSHIFT LY-ALPHA EMISSION

We have conducted a deep long-slit spectroscopic search for high-redshift (2.7 < z < 4.7) Lyα emission. Four pairs of deep, high-resolution (R ≈ 2000), long-slit CCD frames were taken at the Multiple Mirror Telescope. The "blank sky" in each pair of frames was searched for faint emission features unresolved spatially and spectrally. No emission features were found down to a limiting line surface brightness (1 σ) of 1-4 × 10-18 ergs s-1 cm-2 arcsec-2. The sensitivity of the search was calibrated using simulations with synthetic features added to the data frames. The data set upper limits on the mean space density 〈n〉 and line flux of randomly distributed Lyα-emitting clouds; for example, at z ≈ 4.5 we have a 95% confidence limit of 〈n〉 < 1 Mpc-3 h1003 at a total line flux level of ∼3 × 10-17 ergs s-1 cm-2. These limits approach expected emission levels for fairly conservative published primeval galaxy models. In addition, Lyα emission was searched for, but not detected, from a known Lyα-limit absorption cloud toward QSO 0731+653. Assuming that the gas in this cloud is spatially resolved (size ≳ 4h-1 kpc) we set an upper limit on the diffuse ionizing flux density at z = 2.912 of Jv ≲ 2 × 10-19 ergs s-1 cm-2 sr-1 Hz-1.