The nature of luminous Lyman-alpha emitters at z~2-3: maximal dust-poor starbursts and highly ionising AGN

Deep narrow-band surveys have revealed a large population of faint Lyman-alpha (Lya) emitters (LAEs) in the distant Universe, but relatively little is known about the most luminous sources ($L_{Ly\alpha}>10^{42.7}$ erg/s; $L_{Ly\alpha}>L^*_{Ly\alpha}$). 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 (~4x10$^6$ 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 ionisation UV lines. Luminous LAEs at z~2-3 have blue UV slopes ($\beta=-2.0^{+0.3}_{-0.1}$), high Lya escape fractions ($50^{+20}_{-15}$%) and span five orders of magnitude in UV luminosity ($M_{UV}\approx-19$ to -24). Many (70%) show at least one high ionisation rest-frame UV line such as CIV, NV, CIII], HeII or OIII], typically blue-shifted by ~100-200 km/s relative to Lya. Their Lya profiles reveal a wide variety of shapes, including significant blue-shifted components and widths from 200 to 4000 km/s. Overall, 60+-11% appear to be AGN dominated, and at $L_{Ly\alpha}>10^{43.3}$ erg/s and/or $M_{UV}<-21.5$ virtually all LAEs are AGN with high ionisation parameters (log U=0.6+-0.5) and with metallicities of ~0.5-1 Zsun. Those lacking signatures of AGN (40+-11%) have lower ionisation parameters ($\log U=-3.0^{+1.6}_{-0.9}$ and $\log\xi_{\rm ion}=25.4\pm0.2$) 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 2xL$^*_{Ly\alpha}$ and 2xM$_{UV}^*$ mark a sharp transition in the nature of LAEs, from star formation dominated to AGN dominated.