A Serendipitous Search for High‐Redshift Lyα Emission: Two Primeval Galaxy Candidates at z ≃ 3

Abstract

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, 2 σ) Lyα emission-line candidates at z ∼ 3 in a moderate dispersion (λ/Δλ ≃ 1200) spectrogram. Both lines have low velocity dispersions (σ v ∼ 60 km s -1 ) and deconvolved radii r ≈ 1 h -1 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 -2 per unit redshift at z ≃ 3, consistent with the recent estimate of Cowie & Hu. 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 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 about the first few times 10 7 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.