Publication Date:
2016-02-06
Description:
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 L Hα 〉 $L_{\rm H\alpha }^{\ast }$ , 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 L Hα / $L_{\rm H\alpha }^{\ast }(z)$ . While L Hα ≤ $L_{\rm H\alpha }^{\ast }(z)$ Hα emitters are largely dominated by star-forming galaxies (〉80 per cent), the most luminous Hα emitters ( $L_{\rm H\alpha } 〉 10L_{\rm H\alpha }^{\ast }(z)$ ) 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 ( $L_{\rm H\alpha } 〉 L^{\ast }_{\rm H\alpha }(z)$ ) contribute the same fractional amount of 15 per cent to the total star formation rate density, at least up to z = 2.23.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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