[O II] emitters at z ∼ 4.6 in the GOODS field: a homogeneous measure of evolving star formation

Abstract

We present the results of a high-redshift, z = 4.6, survey of [O II] λ3727 emission line galaxies in the GOODS-S field. The survey uses deep near-infrared data in the NB2090 (λ_c = 2.095 μm, Δ λ = 0.02 μm) and Ks (λ_c = 2.146 μm, Δ λ = 0.324 μm) filters taken with the European Southern Observatory instrument, HAWK-I. The images reach an emission line flux limit (5σ) of 3.16 × 10^-18 erg s^-1 cm ^-2. At z = 4.6, the survey probes a comoving volume of ∼6680 Mpc³.

Three [O II] emission line candidates at z ∼ 4.6 are selected using the Lyman-break criteria. Photometric redshift analysis supports the conclusion that these are genuine [O II] emitters, ruling out a z < 3 solution entirely for one of the candidates. In the analysis presented in this paper, two scenarios are considered: first, all three candidates are genuine [O II] emitters and secondly, only the most likely candidate is a genuine [O II] emitter.

We use the line fluxes of these objects to place confidence limits on the star formation rate density (SFRD) in bright (log(L[O ii ])>42.0) [O II] emission line galaxies. Assuming an observed [O II]/Hα line ratio of 0.45 and A(Hα) = 1.0 mag, we report an SFRD of ρdot_*(log(L[O ii ])>42.0)=0.058 M yr -1 Mpc -3 in our objects. Using small number statistics, we then place a 50 per cent confidence interval on the global star formation rate of ρdot;_*(log(L[O ii ])>42.0)=0.058±0.013 M yr -1 Mpc -3. By combining our results with those from low-z surveys, we compile the first homogeneous set of measurements of the SFRD in bright [O II] emitters from z = 0 to 4.6. From this, we conclude that there was an increase in the SFRD in the brightest [O II] emitters of at least a factor of 2 between z = 4.6 and 1.85.