Physical conditions of the interstellar medium in star-forming galaxies at z~1.5

TL;DR

This study uses near-infrared spectroscopy to analyze the physical conditions of the interstellar medium in star-forming galaxies at redshift ~1.5, revealing high ionization parameters and strong emission lines indicative of young stellar populations.

Contribution

It provides new insights into the ionization state and physical conditions of high-redshift star-forming galaxies using detailed emission line diagnostics.

Findings

[OII] is a reliable star formation rate indicator at z~1.5

High [OIII]/[OII] ratios indicate elevated ionization parameters

Electron densities are similar to local galaxies

Abstract

We present results from Subaru/FMOS near-infrared (NIR) spectroscopy of 118 star-forming galaxies at $z\sim1.5$ in the Subaru Deep Field. These galaxies are selected as [OII]$\lambda$3727 emitters at $z\approx$ 1.47 and 1.62 from narrow-band imaging. We detect H$\alpha$ emission line in 115 galaxies, [OIII]$\lambda$5007 emission line in 45 galaxies, and H$\beta$, [NII]$\lambda$6584, and [SII]$\lambda\lambda$6716,6731 in 13, 16, and 6 galaxies, respectively. Including the [OII] emission line, we use the six strong nebular emission lines in the individual and composite rest-frame optical spectra to investigate physical conditions of the interstellar medium in star-forming galaxies at $z\sim$1.5. We find a tight correlation between H$\alpha$ and [OII], which suggests that [OII] can be a good star formation rate (SFR) indicator for galaxies at $z\sim1.5$. The line ratios of H$\alpha$/[OII] are consistent with those of local galaxies. We also find that [OII] emitters have strong [OIII] emission lines. The [OIII]/[OII] ratios are larger than normal star-forming galaxies in the local Universe, suggesting a higher ionization parameter. Less massive galaxies have larger [OIII]/[OII] ratios. With evidence that the electron density is consistent with local galaxies, the high ionization of galaxies at high redshifts may be attributed to a harder radiation field by a young stellar population and/or an increase in the number of ionizing photons from each massive star.