[O II] emitters in the GOODS field at z∼ 1.85: a homogeneous measure of evolving star formation

Abstract

We present the results of a deep, near-infrared, narrow-band imaging survey at a central wavelength of 1.062 μm (FWHM = 0.01 μm) in the GOODS-South field using the ESO VLT instrument, HAWK-I. The data are used to carry out the highest redshift search for [O II] λ3727 Å emission line galaxies to date. The images reach an emission line flux limit (5σ) of 1.5 × 10^-17 erg cm^-2 s^-1, additionally making the survey the deepest of its kind at high redshift. In this paper we identify a sample of [O II] λ3727 Å emission line objects at redshift z∼ 1.85 in a comoving volume of ∼4100 Mpc^-3. Objects are selected using an observed equivalent width (EW_obs) threshold of EW_obs> 50Å. The sample is used to derive the space density and constrain the luminosity function of [O II] emitters at z= 1.85. We find that the space density (ρ) of objects with observed [O II] luminosities in the range ? is log(ρ) =-2.45 ± 0.14 Mpc^-3, a factor of 2 greater than the observed space density of [O II] emitters reported at z∼ 1.4. After accounting for completeness and assuming an internal extinction correction of A_Hα= 1 mag (equivalent to ?= 1.87), we report a star formation rate density of ? M_⊙ yr^-1 Mpc^-3. We independently derive the dust extinction of the sample using 24-μm fluxes and find a mean extinction of ? magnitudes (A_Hα= 0.52). This is significantly lower than the A_Hα= 1 (?= 1.86) mag value widely used in the literature. Finally, we incorporate this improved extinction correction into the star formation rate density measurement and report ? M_⊙ yr^-1 Mpc^-3.