3D-HST Emission Line Galaxies at z ~ 2: Discrepancies in the Optical/UV Star Formation Rates

TL;DR

This study uses Hubble Space Telescope spectroscopy to compare star formation rates derived from H-beta emission and UV flux in galaxies at z ~ 2, revealing a significant discrepancy influenced by metallicity.

Contribution

It identifies a systematic offset between optical and UV star formation indicators at high redshift and links it to metallicity effects, informing future SFR measurements.

Findings

H-beta-based SFR is ~1.8 times higher than UV-based SFR at z ~ 2.

Metallicity influences the enhancement of H-beta emission in these galaxies.

The relation between stellar and nebular extinction at z ~ 2 matches local universe observations.

Abstract

We use Hubble Space Telescope near-IR grism spectroscopy to examine the H-beta line strengths of 260 star-forming galaxies in the redshift range 1.90 < z < 2.35. We show that at these epochs, the H-beta star formation rate (SFR) is a factor of ~1.8 higher than what would be expected from the systems' rest-frame UV flux density, suggesting a shift in the standard conversion between these quantities and star formation rate. We demonstrate that at least part of this shift can be attributed to metallicity, as H-beta is more greatly enhanced in systems with lower oxygen abundance. This offset must be considered when measuring the star formation rate history of the universe. We also show that the relation between stellar and nebular extinction in our z ~ 2 sample is consistent with that observed in the local universe.