The nature of luminous Ly α emitters at z ∼ 2-3: maximal dust-poor starbursts and highly ionizing AGN

Abstract

Deep narrow-band surveys have revealed a large population of faint Ly α emitters (LAEs) in the distant Universe, but relatively little is known about the most luminous sources ({L}_{Lyα } ≳ 10^{42.7} erg s^-1; L_{Lyα }≳ L^*_{Lyα }). Here we present the spectroscopic follow-up of 21 luminous LAEs at z ∼ 2-3 found with panoramic narrow-band surveys over five independent extragalactic fields (≈4 × 10⁶ Mpc³ surveyed at z ∼ 2.2 and z ∼ 3.1). We use WHT/ISIS, Keck/DEIMOS, and VLT/X-SHOOTER to study these sources using high ionization UV lines. Luminous LAEs at z ∼ 2-3 have blue UV slopes (β =-2.0^{+0.3}_{-0.1}) and high Ly α escape fractions (50^{+20}_{-15} per cent) and span five orders of magnitude in UV luminosity (M_UV ≈ -19 to -24). Many (70 per cent) show at least one high ionization rest-frame UV line such as C IV, N V, C III], He II or O III], typically blue-shifted by ≈100-200 km s^-1 relative to Ly α. Their Ly α profiles reveal a wide variety of shapes, including significant blue-shifted components and widths from 200 to 4000 km s^-1. Overall, 60 ± 11 per cent appear to be active galactic nucleus (AGN) dominated, and at L_Lyα > 10^43.3 erg s^-1 and/or M_UV < -21.5 virtually all LAEs are AGNs with high ionization parameters (log U = 0.6 ± 0.5) and with metallicities of ≈0.5 - 1 Z_⊙. Those lacking signatures of AGNs (40 ± 11 per cent) have lower ionization parameters (log U=-3.0^{+1.6}_{-0.9} and log ξ_ion = 25.4 ± 0.2) and are apparently metal-poor sources likely powered by young, dust-poor `maximal' starbursts. Our results show that luminous LAEs at z ∼ 2-3 are a diverse population and that 2× L^*_{Lyα } and 2× M_UV^* mark a sharp transition in the nature of LAEs, from star formation dominated to AGN dominated.