Star formation at z=1.47 from HiZELS: an Hα+[O ii] double-blind study★

This paper presents the results from the first wide and deep dual narrow-band survey to select Hα and [O ii] line emitters at z= 1.47 ± 0.02, exploiting synergies between the United Kingdom Infrared Telescope and the Subaru telescope by using matched narrow-band filters in the H and z′ bands. The Hα survey at z= 1.47 reaches a 3σ flux limit of FHα≈ 7 × 10−17 erg s−1 cm−2 (corresponding to a limiting star formation rate (SFR) in Hα of ≈7 M⊙ yr−1) and detects ≈200 Hα emitters over 0.7 deg2, while the much deeper [O ii] survey reaches an effective flux of ≈7 × 10−18 erg s−1 cm−2 (SFR in [O ii] of ∼1 M⊙ yr−1), detecting ≈1400 z= 1.47 [O ii] emitters in a matched comoving volume of ∼2.5 × 105 Mpc3. The combined survey results in the identification of 190 simultaneous Hα and [O ii] emitters at z= 1.47. Hα and [O ii] luminosity functions are derived and both are shown to evolve significantly from z∼ 0 in a consistent way. The SFR density of the Universe at z= 1.47 is evaluated, with the Hα analysis yielding ρSFR= 0.16 ± 0.05 M⊙ yr−1 Mpc−3 and the [O ii] analysis ρSFR= 0.17 ± 0.04 M⊙ yr−1 Mpc−3. The measurements are combined with other studies, providing a self-consistent measurement of the star formation history of the Universe over the last ∼11 Gyr. By using a large comparison sample at z∼ 0.1, derived from the Sloan Digital Sky Survey (SDSS), [O ii]/Hα line ratios are calibrated as probes of dust extinction. Hα emitters at z∼ 1.47 show on average AHα≈ 1 mag, the same as found by SDSS in the local Universe. It is shown that although dust extinction correlates with SFR, the relation evolves by about ∼0.5 mag from z∼ 1.5 to ∼0, with local relations overpredicting the dust extinction corrections at high z by that amount. Stellar mass is found to be a much more fundamental extinction predictor, with the same relation between mass and dust extinction being valid at both z∼ 0 and ∼1.5, at least for low and moderate stellar masses. The evolution in the extinction–SFR relation is therefore interpreted as being due to the evolution in median specific SFRs over cosmic time. Dust extinction corrections as a function of optical colours are also derived and shown to be broadly valid at both z∼ 0 and ∼1.5, offering simpler mechanisms for estimating extinction in moderately star-forming systems over the last ∼9 Gyr.

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