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

Abstract

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] (λ 3726, λ 3729) ratio for cluster and field galaxies at z ∼ 2. We measure a median electron density of n_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_e = 113 ± 63 cm⁰³ and 1 &sigma 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_e = 30 ± 1 cm^-3 for z ∼ 0 to n_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.