The ALPINE-ALMA [C II] survey: Investigation of 10 galaxies at z 4.5 with [O II] and [C II] line emission - ISM properties and [O II]-SFR relation

We present 10 main-sequence ALPINE galaxies (log (M/M-circle dot) = 9.2-11.1 and SFR = 23 - 190 M-circle dot yr(-1)) at z similar to 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 mu m line emission and dust continuum from ALMA and H alpha 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 alpha luminosity ratios at z similar to 4.5. The [O II]-SFR relation at z similar to 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]) similar to 0.98(-0.22)(+0.21) and log(L-[OII]/L-H alpha) similar to -0.22(-0.15)(+0.13) for our sample, ionization parameters log (U) < -2 and electron densities log(ne/[cm(-3)])similar to 2.5 - 3 are required. The former is consistent with galaxies at z similar to 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-circle dot) > 9 galaxies at z similar to 4.5 to have broadly similar ISM properties as their descendants at z similar to 2 and suggest a strong evolution of ISM properties since the epoch of reionization at z > 6.