Star formation at z=1.47 from HiZELS : An Hα+[Oii] double-blind study

Sobral, David and Best, Philip N. and Matsuda, Yuichi and Smail, Ian and Geach, James E. and Cirasuolo, Michele (2012) Star formation at z=1.47 from HiZELS : An Hα+[Oii] double-blind study. Monthly Notices of the Royal Astronomical Society, 420 (3). pp. 1926-1945. ISSN 0035-8711

Abstract

This paper presents the results from the first wide and deep dual narrow-band survey to select Hα and [Oii] 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 F Hα≈ 7 × 10 -17ergs -1cm -2 (corresponding to a limiting star formation rate (SFR) in Hα of≈7M ⊙yr -1) and detects ≈200 Hα emitters over 0.7deg 2, while the much deeper [Oii] survey reaches an effective flux of ≈7 × 10 -18ergs -1cm -2 (SFR in [Oii] of ∼1M ⊙yr -1), detecting ≈1400 z= 1.47 [Oii] emitters in a matched comoving volume of ∼2.5 × 10 5Mpc 3. The combined survey results in the identification of 190 simultaneous Hα and [Oii] emitters at z= 1.47. Hα and [Oii] 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 ρ SFR= 0.16 ± 0.05M ⊙yr -1Mpc -3 and the [Oii] analysis ρ SFR= 0.17 ± 0.04M ⊙yr -1Mpc -3. The measurements are combined with other studies, providing a self-consistent measurement of the star formation history of the Universe over the last ∼11Gyr. By using a large comparison sample at z∼ 0.1, derived from the Sloan Digital Sky Survey (SDSS), [Oii]/Hα line ratios are calibrated as probes of dust extinction. Hα emitters at z∼ 1.47 show on average A Hα≈ 1mag, 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 ∼0.5mag 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 ∼9Gyr.