ZFIRE: Measuring Electron Density with [O ii] as a Function of Environment at z=1.62

The global star formation rates (SFR) of galaxies at fixed stellar masses increase with redshift and are known to vary with environment up to z ∼ 2. We explore here whether the changes in the SFRs also apply to the electron densities of the interstellar medium by measuring the [O ii] ($\lambda 3726$, $\lambda 3729$) ratio for cluster and field galaxies at z ∼ 2. We measure a median electron density of ${n}_{{\rm{e}}}$ = 366 ± 84 cm${}^{-3}$ for six galaxies (with 1σ scatter = 163 cm${}^{-3}$) in the Ultra-Deep Survey (UDS) protocluster at z = 1.62. We find that the median electron density of galaxies in the UDS protocluster environment is three times higher compared to the median electron density of field galaxies (${n}_{{\rm{e}}}$ = 113 ± 63 cm${}^{-3}$ and 1σ scatter = 79 cm${}^{-3}$) at comparable redshifts, stellar mass, and SFR. However, we note that a sample of six protocluster galaxies is insufficient to reliably measure the electron density in the average protocluster environment at z ∼ 2. We conclude that the electron density increases with redshift in both cluster and field environments up to z ∼ 2 (${n}_{{\rm{e}}}$ = 30 ± 1 cm${}^{-3}$ for z ∼ 0 to ${n}_{{\rm{e}}}$ = 254 ± 76 cm${}^{-3}$ for z ∼ 1.5). We find tentative evidence (∼2.6σ) for a possible dependence of electron density on environment, but the results require confirmation with larger sample sizes.K.T. acknowledges support by the National Science Foundation under grant No. 1410728. T.Y. acknowledges support from an ASTRO 3D fellowship. G.G.K. acknowledges the support of the Australian Research Council through the Discovery Project DP170103470. T.N. acknowledges the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) top grant TOP1.16.057