The ALPINE-ALMA [CII] Survey: Investigation of 10 Galaxies at $z\sim 4.5$ with [OII] and [CII] Line Emission $-$ ISM Properties and [OII]$-$SFR Relation

Abstract

We present $10$ main-sequence ALPINE galaxies (log($M/M_{\odot }$) = 9.2-11.1 and $\mathrm{SFR}=23-190{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$) at $z\sim 4.5$ with optical [OII] measurements from Keck/MOSFIRE spectroscopy and Subaru/MOIRCS narrow-band imaging. This is the largest such multi-wavelength sample at these redshifts, combining various measurements in the ultra-violet, optical, and far-infrared including [CII]${}_{158\hat{\mathrm{I}}¼\mathrm{m}}$ line emission and dust continuum from ALMA and H$\hat{I}\pm$ emission from Spitzer photometry. For the first time, this unique sample allows us to analyze the relation between [OII] and total star-formation rate (SFR) and the interstellar medium (ISM) properties via [OII]/[CII] and [OII]/\halpha luminosity ratios at $z\sim 4.5$. The [OII]$-$SFR relation at $z\sim 4.5$ cannot be described using standard local descriptions, but is consistent with a metal-dependent relation assuming metallicities around $50\%$ solar. To explain the measured dust-corrected luminosity ratios of $L[OII]/L[CII]\sim 0.98_{-0.22}^{+0.21}$ and $L[OII]/LHa\sim -0.22_{-0.15}^{+0.13}$ for our sample, ionization parameters $\log (U)<-2$ and electron densities $\log ({\mathrm{n}}_{\mathrm{e}}/[\mathrm{c}{\mathrm{m}}^{-3}])\sim 2.5-3$ are required. The former is consistent with galaxies at $z\sim 2-3$, 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($M/M_{\odot })$ > 9 galaxies at $z\sim 4.5$ to have broadly similar ISM properties as their descendants at $z\sim 2$ and suggest a strong evolution of ISM properties since the Epoch of Reionization at $z>6$.

14 pages, 8 figures, Accepted for publication to MNRAS