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

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 × 10^6 Mpc^3 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 (logU = −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