A Serendipitous Search for High - Redshift Ly α Emission : Two Primeval Galaxy Candidates at z ≃ 3

In the course of our ongoing search for serendipitous high-redshift Lyα emission in deep archival Keck spectra, we discovered two very high equivalent width (W obs λ ∼> 450 Å, 2σ) Lyα emission line candidates at z ∼ 3 in a moderate dispersion (λ/∆λ ≃ 1200) spectrogram. Both lines have low velocity dispersions (σv ∼ 60 km s−1) and deconvolved radii r ≈ 1 h 50 kpc. We argue that the lines are Lyα, and are powered by stellar ionization. The surface density of robust, high equivalent width Lyα candidates is estimated to be ∼ 3±2 arcmin per unit redshift at z ≃ 3, consistent with the estimate of Cowie & Hu (1998). The Lyα emission line source characteristics are consistent with the galaxies undergoing their first burst of star formation, i.e., with being primeval. Source sizes and velocity dispersions are comparable to the theoretical primeval galaxy model of Lin & Murray (1992) based on the inside-out, self-similar collapse of an isothermal sphere. In this model, star formation among field galaxies is a protracted process. Galaxies are thought to be able to display high equivalent widths for only the first ∼ few ×107 yr. This time is short in relation to the difference in look back times between z = 3 and z = 4, and implies that a substantial fraction of strong line-emitting galaxies at z = 3 were formed at redshifts z ≤ 4. We discuss the significance of high-equivalent width Lyα-emitting galaxies in terms of the emerging picture of the environment, and the specific characteristics of primeval galaxy formation at high redshift. Subject headings: Cosmology: observations — galaxies: compact — galaxies: evolution — galaxies:formation — galaxies: starburst — line: profiles Based on observations at the W.M. Keck Observatory, which is operated as a scientific partnership among the University of California, the California Institute of Technology, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Current address: Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 169-327, Pasadena, CA 91109; [email protected] Current address: Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, England; email: [email protected]