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The most luminous H α emitters at z ∼ 0.8–2.23 from HiZELS : evolution of AGN and star-forming galaxies.

Sobral, D. and Kohn, S.A. and Best, P.N. and Smail, I. and Harrison, C.M. and Stott, J. and Calhau, J. and 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). pp. 1739-1752.


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 LHα∗ , 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) LHα∗(z) . While LHα ≤ L ∗ Hα (z) LHα∗(z) Hα emitters are largely dominated by star-forming galaxies (>80 per cent), the most luminous Hα emitters (L Hα >10L ∗ Hα (z) LHα>10LHα∗(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) LHα>LHα∗(z) ) contribute the same fractional amount of ≈15 per cent to the total star formation rate density, at least up to z = 2.23.

Item Type:Article
Keywords:Galaxies: evolution, Galaxies: high-redshift, Cosmology: observations.
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Publisher statement:This article has been accepted for publication in Monthly notices of the Royal Astronomical Society ©: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:04 January 2016
Date deposited:22 February 2016
Date of first online publication:04 February 2016
Date first made open access:22 February 2016

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