2022MNRAS.511.1303V

We present 10 main-sequence ALPINE galaxies (log (M/M_☉) = 9.2-11.1 and SFR=23-190 M_☉ yr^–1) at z ∼ 4.5 with optical [O II] measurements from Keck/MOSFIRE spectroscopy and Subaru/MOIRCS narrow-band imaging. This is the largest such multiwavelength sample at these redshifts, combining various measurements in the ultraviolet, optical, and far-infrared including [C II]_158 µm_ line emission and dust continuum from ALMA and H α emission from Spitzer photometry. For the first time, this unique sample allows us to analyse the relation between [O II] and total star-formation rate (SFR) and the interstellar medium (ISM) properties via [O II]/[C II] and [O II]/H α luminosity ratios at z ∼ 4.5. The [O II]-SFR relation at z ∼ 4.5 cannot be described using standard local descriptions, but is consistent with a metal-dependent relation assuming metallicities around 50 per cent solar. To explain the measured dust-corrected luminosity ratios of log(L_[OII]/L_[CII]) ∼0.98^+0.21_–0.22 and log(L_[OII]/L_Hα) ∼-0.22^+0.13_–0.15 for our sample, ionization parameters log (U) < -2 and electron densities log( n_e/[cm^–3]) ∼2.5-3 are required. The former is consistent with galaxies at z ∼ 2-3, however lower than at z > 6. The latter may be slightly higher than expected given the galaxies' specific SFR. The analysis of this pilot sample suggests that typical log (M/M_☉) > 9 galaxies at z ∼ 4.5 to have broadly similar ISM properties as their descendants at z ∼ 2 and suggest a strong evolution of ISM properties since the epoch of reionization at z > 6.