D. Sobral, S. A. Kohn, P. Best, I. Smail, C. M. Harrison, J. P. Stott, J. Calhau, J. Matthee
Abstract
We use new near-infrared spectroscopic observations to investigate the nature and evolution of the most luminous Hα emitters at z ̃ 0.8-2.23, which evolve strongly in number density over this period, and compare them to more typical Hα emitters. We study 59 luminous Hα emitters with LHα 〉 L_{Hα }^{ast }, 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 ± 8 per cent are active galactic nuclei [AGNs; 80 ± 30 per cent of these AGNs are broad-line AGNs, BL-AGNs], and we find little to no evolution in the AGN fraction with redshift, within the errors. However, the AGN fraction increases strongly with Hα luminosity and correlates best with LHα/L_{Hα }^{ast }(z). While LHα ≤ L_{Hα }^{ast }(z) Hα emitters are largely dominated by star-forming galaxies (〉80 per cent), the most luminous Hα emitters (L_{Hα }〉10L_{Hα }^{ast }(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 Hα luminosity, we find a factor of ̃1300 evolution in the star formation rate density from z = 0 to 2.23. This is much stronger than the evolution from typical Hα star-forming galaxies and in line with the evolution seen for constant luminosity cuts used to select `ultraluminous' infrared galaxies and/or sub-millimetre galaxies. By taking into account the evolution in the typical Hα luminosity, we show that the most strongly star-forming Hα-selected galaxies at any epoch (L_{Hα }〉L^{ast }_{Hα }(z)) contribute the same fractional amount of ≈15 per cent to the total star formation rate density, at least up to z = 2.23.
Keywords
galaxies: evolution, galaxies: high-redshift, cosmology: observations
Monthly Notices of the Royal Astronomical Society
Volume 457, Issue 2, Page 1739
2016 February
