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Durham University

Department of Physics

Staff profile

Publication details for Professor Ian Smail

Sobral, D., Kohn, S.A., Best, P.N., Smail, I., Harrison, C.M., Stott, J., Calhau, J. & Matthee, J. (2016). The most luminous H α emitters at z ∼ 0.8–2.23 from HiZELS: evolution of AGN and star-forming galaxies. Monthly Notices of the Royal Astronomical Society 457(2): 1739-1752.

Author(s) from Durham


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
, 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 (z)
. While LHα ≤ L ∗ Hα (z)
Hα emitters are largely dominated by star-forming galaxies (>80 per cent), the most luminous Hα emitters (L Hα >10L ∗ Hα (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 ∗ 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.