The nature of luminous Lyα 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 Lyα emitters (LAEs) in the distant Universe, but relatively little is known about the most luminous sources (LLyα ≳ 1042.7 erg s-1; LLyα ≳ LLyα*). 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 = -2.0-0.1+0.3) and high Lyα escape fractions (50-15+20 per cent) and span five orders of magnitude in UV luminosity (MUV ≈ -19 to -24). Many (70 per cent) show at least one high ionization rest-frame UV line such as CIV, NV, CIII], HeII or OIII], typically blue-shifted by ≈100-200 km s-1 relative to Lyα. Their Lyα 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 LLyα > 1043.3 erg s-1 and/or MUV < -21.5 virtually all LAEs are AGNs with high ionization parameters (logU = 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 = -3.0-0.9+1.6 and log ξion = 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 × LLyα* and 2 × MUV* mark a sharp transition in the nature of LAEs, from star formation dominated to AGN dominated.