Deep narrow-band surveys have revealed a large population of faint Ly \alpha emitters ( LAEs ) in the distant Universe , but relatively little is known about the most luminous sources ( L _ { Ly \alpha } ~ { } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } ~ { } 10 ^ { 42.7 } erg s ^ { -1 } ; L _ { Ly \alpha } ~ { } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } ~ { } L ^ { * } _ { % Ly \alpha } ) .
Here we present the spectroscopic follow-up of 21 luminous LAEs at z \sim 2 - 3 found with panoramic narrow-band surveys over five independent extragalactic fields ( \approx 4 \times 10 ^ { 6 } Mpc ^ { 3 } surveyed at z \sim 2.2 and z \sim 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 \sim 2–3 have blue UV slopes ( \beta = -2.0 ^ { +0.3 } _ { -0.1 } ) , high Ly \alpha 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 C iv , N v , C iii ] , He ii or O iii ] , typically blue-shifted by \approx 100 - 200 km s ^ { -1 } relative to Ly \alpha .
Their Ly \alpha profiles reveal a wide variety of shapes , including significant blue-shifted components and widths from 200 to 4000 km s ^ { -1 } .
Overall , 60 \pm 11 % appear to be AGN dominated , and at L _ { Ly \alpha } > 10 ^ { 43.3 } erg s ^ { -1 } and/or M _ { UV } < -21.5 virtually all LAEs are AGN with high ionisation parameters ( \log U = 0.6 \pm 0.5 ) and with metallicities of \approx 0.5 - 1 Z _ { \odot } .
Those lacking signatures of AGN ( 40 \pm 11 % ) have lower ionisation parameters ( \log U = -3.0 ^ { +1.6 } _ { -0.9 } and \log \xi _ { ion } = 25.4 \pm 0.2 ) and are apparently metal-poor sources likely powered by young , dust-poor ‘ ‘ maximal '' starbursts .
Our results show that luminous LAEs at z \sim 2–3 are a diverse population and that 2 \times L ^ { * } _ { Ly \alpha } and 2 \times M _ { UV } ^ { * } mark a sharp transition in the nature of LAEs , from star formation dominated to AGN dominated .