The Density of Lyα Emitters at Very High Redshift

We describe narrowband and spectroscopic searches for emission-line star-forming galaxies in the redshift range 3-6 with the 10 m Keck II Telescope. These searches yield a substantial population of objects with only a single strong (EW ≫100 Å) emission line, lying in the 4000-8500 Å range. Spectra of the objects found in narrowband-selected samples at λ~5390 and ~6741 Å show that these very high equivalent width emission lines are generally redshifted Lyα 1216 Å at z~3.4 and 4.5. The density of these emitters above the 5 σ detection limit of 1.5×10−17 ergs cm-2 s-1 is roughly 15,000 deg-2 per unit z at both z~3.4 and 4.5. A complementary deeper (1 σ~10−18 ergs cm-2 s-1) slit spectroscopic search covering a wide redshift range but a more limited spatial area (200 arcsec2) shows that such objects can be found over the redshift range z=3-6, with the currently highest redshift detected being at z=5.64. The Lyα flux distribution can be used to estimate a minimum star formation rate in the absence of reddening of roughly 0.01 M☉ Mpc-3 yr-1 (H0=65 km s-1 Mpc-1, q0=0.5). Corrections for reddening are likely to be no larger than a factor of 2, since observed equivalent widths are close to the maximum values obtainable from ionization by a massive star population. Within the still significant uncertainties, the star formation rate from the Lyα-selected sample is comparable to that of the color-break-selected samples at z~3 but may represent an increasing fraction of the total rates at higher redshifts. This higher z population can be readily studied with large ground-based telescopes.