Lyman-α at Cosmic Noon I: Lyα Spectral Type Selection of z ∼ 2 – 3 Lyman Break Galaxies with Broadband Imaging

Abstract High-redshift Lyman break galaxies (LBGs) are efficiently selected in deep images using as few as three broadband filters, and have been shown to have multiple intrinsic and small- to large-scale environmental properties related to Lyman- $\alpha$ . In this paper we demonstrate a statistical relationship between net Lyman- $\alpha$ equivalent width (net Ly $\alpha$ EW) and the optical broadband photometric properties of LBGs at $z\sim2$ . We show that LBGs with the strongest net Ly $\alpha$ EW in absorption (aLBGs) and strongest net Ly $\alpha$ EW in emission (eLBGs) separate into overlapping but discrete distributions in $(U_n-\mathcal{R})$ colour and $\mathcal{R}$ -band magnitude space, and use this segregation behaviour to determine photometric selection criteria by which sub-samples with a desired Ly $\alpha$ spectral type can be selected using data from as few as three broadband optical filters. We propose application of our result to current and future large-area and all-sky photometric surveys that will select hundreds of millions of LBGs across many hundreds to thousands of Mpc, and for which spectroscopic follow-up to obtain Ly $\alpha$ spectral information is prohibitive. To this end, we use spectrophotometry of composite spectra derived from a sample of 798 LBGs divided into quartiles on the basis of net Ly $\alpha$ EW to calculate selection criteria for the isolation of Ly $\alpha$ -absorbing and Ly $\alpha$ -emitting populations of $z\sim3$ LBGs using ugri broadband photometric data from the Vera Rubin Observatory Legacy Survey of Space and Time (LSST).