Preliminary results from a spectroscopic Ly alpha survey at redshift 5.7 with IMACS

We describe preliminary results from an ultra-sensitive, spectroscopic emission-line survey and illustrate the challenges inherent in identifying high-redshift Ly alpha emitters. Our multi-slit windows technique complements other types of emission-line surveys. Narrowband imagine surveys cover large areas of sky but only detect much brighter objects. Longslit spectra taken along cluster caustics yield intrinsically fainter lensed Ly alpha emitters but probe small volumes of space. We have observed the COSMOS deep field and a field at 15 h +00. To a line flux of a few similar to x 10(-18) ergs s(-1) cm(-2), we found 150 emission-line sources (with no detectable continuum) among 4 masks. These candidates are being re-observed with broad spectral coverage to determine the line identity, To date, the interloper to Ly alpha ratio is about 8:1. The sky positions of the Lya candidates generally do not coincide with those of foreground objects in ultra-deep r band or i' images consistent with the presence of a strong Lyman break. (c) 2005 Published by Elsevier B.V.