The COSMOS [OII] Survey: Evolution of Electron Density with Star Formation Rate

TL;DR

This study investigates how electron densities in star-forming galaxies evolve with redshift, revealing that observed differences are primarily due to variations in star formation rates rather than intrinsic evolution.

Contribution

The paper provides the largest high-redshift sample with [OII] emission measurements and demonstrates that electron density evolution is driven by star formation rate, not intrinsic galaxy evolution.

Findings

High-redshift galaxies have five times higher electron densities than local ones.

No significant difference in electron densities when controlling for star formation rate.

Electron density correlates strongly with star formation rate across redshifts.

Abstract

Star-forming galaxies at $z > 1$ exhibit significantly different properties to local galaxies of equivalent stellar mass. Not only are high-redshift star-forming galaxies characterized by higher star formation rates and gas fractions than their local counterparts, they also appear to host star-forming regions with significantly different physical conditions, including greater electron densities. To understand what physical mechanisms are responsible for the observed evolution of star-forming conditions we have assembled the largest sample of star-forming galaxies at $z\sim 1.5$ with emission-line measurements of the $\mathrm{[OII]} \lambda \lambda 3726,3729$ doublet. By comparing our $z\sim 1.5$ sample to local galaxy samples with equivalent distributions of stellar mass, star formation rate and specific star formation rate we investigate the proposed evolution in electron density and its dependence on global properties. We measure an average electron density of $114_{-27}^{+28} \, \mathrm {cm}^{-3} $ for our $z\sim 1.5$ sample, a factor of five greater than the typical electron density of local star-forming galaxies. However, we find no offset between the typical electron densities of local and high-redshift galaxies with equivalent star-formation rates. Our work indicates that the average electron density of a sample is highly sensitive to the star formation rates, implying that the previously observed evolution is mainly the result of selection effects.