"Direct" Gas-phase Metallicities, Stellar Properties, and Local Environments of Emission-line Galaxies at Redshift below 0.90

TL;DR

This study uses deep narrow-band imaging and spectroscopy to analyze 20 emission-line galaxies at redshift below 0.9, revealing extremely metal-poor galaxies, their star formation properties, and environmental factors, providing insights into low-redshift analogs of high-redshift galaxies.

Contribution

First direct metallicity measurements of low-mass, low-metallicity emission-line galaxies at z<0.9, revealing their star formation and environmental characteristics.

Findings

Identification of extremely metal-poor galaxies with 12+log(O/H) <= 7.65.

Galaxies exhibit high specific star formation rates (~100 Myr^{-1}).

Gas-phase metallicities are systematically lower than local relations by ~0.2 dex.

Abstract

Using deep narrow-band (NB) imaging and optical spectroscopy from the Keck telescope and MMT, we identify a sample of 20 emission-line galaxies (ELGs) at z=0.065-0.90 where the weak auroral emission line, [OIII]4363, is detected at >3\sigma. These detections allow us to determine the gas-phase metallicity using the "direct'' method. With electron temperature measurements and dust attenuation corrections from Balmer decrements, we find that 4 of these low-mass galaxies are extremely metal-poor with 12+log(O/H) <= 7.65 or one-tenth solar. Our most metal-deficient galaxy has 12+log(O/H) = 7.24^{+0.45}_{-0.30} (95% confidence), similar to some of the lowest metallicity galaxies identified in the local universe. We find that our galaxies are all undergoing significant star formation with average specific star formation rate (SFR) of (100 Myr)^{-1}, and that they have high central SFR surface densities (average of 0.5 Msun/yr/kpc^2. In addition, more than two-thirds of our galaxies have between one and four nearby companions within a projected radius of 100 kpc, which we find is an excess among star-forming galaxies at z=0.4-0.85. We also find that the gas-phase metallicities for a given stellar mass and SFR lie systematically below the local M-Z-(SFR) relation by \approx0.2 dex (2\sigma\ significance). These results are partly due to selection effects, since galaxies with strong star formation and low metallicity are more likely to yield [OIII]4363 detections. Finally, the observed higher ionization parameter and electron density suggest that they are lower redshift analogs to typical z>1 galaxies.