The most luminous H$\alpha$ emitters at z~0.8-2.23 from HiZELS:
  evolution of AGN and star-forming galaxies
  We use new near-infrared spectroscopic observations to investigate the nature
and evolution of the most luminous H\alpha (Ha) emitters at z~0.8-2.23, which
evolve strongly in number density over this period, and compare them to more
typical Ha emitters. We study 59 luminous Ha emitters with
$L_{H\alpha}>L_{H\alpha}^*$, roughly equally split per redshift slice at z~0.8,
1.47 and 2.23 from the HiZELS and CF-HiZELS surveys. We find that, overall,
30$\pm$8% are AGN (80$\pm$30% of these AGN are broad-line AGN, BL-AGN), and we
find little to no evolution in the AGN fraction with redshift, within the
errors. However, the AGN fraction increases strongly with Ha luminosity and
correlates best with $L_{H\alpha}/L_{H\alpha}^*(z)$. While $L_{H\alpha}<L_{\rm
H\alpha}^*(z)$ Ha emitters are largely dominated by star-forming galaxies
(>80%), the most luminous Ha emitters ($L_{H\alpha}>10L_{H\alpha}^*(z)$) at any
cosmic time are essentially all BL-AGN. Using our AGN-decontaminated sample of
luminous star-forming galaxies, and integrating down to a fixed Ha luminosity,
we find a factor of ~1300x evolution in the star formation rate density from
z=0 to z=2.23. This is much stronger than the evolution from typical Ha
star-forming galaxies and in line with the evolution seen for constant
luminosity cuts used to select "Ultra-Luminous" Infrared Galaxies and/or
sub-millimetre galaxies. By taking into account the evolution in the typical Ha
luminosity, we show that the most strongly star-forming Ha-selected galaxies at
any epoch ($L_{H\alpha}>L^*_{H\alpha}(z)$) contribute the same fractional
amount of ~15% to the total star-formation rate density, at least up to z=2.23.
