Boötes-HiZELS: An optical to near-infrared survey of emission-line galaxies at z = 0.4-4.7

We present a sample of~1000 emission-line galaxies at z=0.4-4.7 from the~0.7deg² High-z Emission-Line Survey in the Boötes field identified with a suite of six narrow-band filters at ≈0.4-2.1 μm. These galaxies have been selected on their Lyα (73), [O_II] (285), Hβ/[O_III] (387) or Hα (362) emission line, and have been classified with optical to near-infrared colours. A subsample of 98 sources have reliable redshifts from multiple narrow-band (e.g. [O_II]-Hα) detections and/or spectroscopy. In this survey paper, we present the observations, selection and catalogues of emitters. We measure number densities of Lyα, [O_II], Hβ/[O_III] and Hα and confirm strong luminosity evolution in star-forming galaxies from z ~ 0.4 to ~5, in agreement with previous results. To demonstrate the usefulness of dual-line emitters, we use the sample of dual [O_II]-Hα emitters to measure the observed [O_II]/Hα ratio at z = 1.47. The observed [O_II]/Hα ratio increases significantly from 0.40±0.01 at z = 0.1 to 0.52±0.05 at z = 1.47, which we attribute to either decreasing dust attenuation with redshift, or due to a bias in the (typically) fibre measurements in the local Universe that only measure the central kpc regions. At the bright end, we find that both the Hα and Lyα number densities at z ≈ 2.2 deviate significantly from a Schechter form, following a power law. We show that this is driven entirely by an increasing X-ray/active galactic nucleus fraction with line luminosity, which reaches ≈100 per cent at line luminosities L ≳ 3 × 10⁴⁴ erg s^-1.