Abstract
Using deep narrow-band imaging and optical spectroscopy from Keck and MMT, we
identify a sample of 20 emission-line galaxies at z=0.065-0.90 where the weak
auroral emission line, O III4363, is detected at >3\sigma. These detections
allow us to determine the gas-phase metal abundances using the "direct" method.
After correcting for dust attenuation using Balmer decrements, we find that 10
of these low-mass galaxies are extremely metal-poor with 12+log(O/H) <= 7.65
dex or one-tenth solar. Considering measurement uncertainties, we argue that 8
and 4 of them are extremely metal-poor at the 85% and 95% confidence levels,
respectively. Our three most metal-deficient galaxies have
12+log(O/H)=7.05^+0.43_-0.12, 7.09^+0.53_-0.25, and
7.12^+0.33_-0.24 dex (95% confidence), similar to some of the lowest
metallicity galaxies identified in the local universe. We also 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=0.4-0.8. We also examine how the gas-phase metallicities compare
with the stellar mass and the SFR of the galaxies, and find that while roughly
half of our galaxies lie along the M-Z-(SFR) relation, the other half have
metallicities that are 0.1-1 dex lower than predicted. Our analysis suggests
that this discrepancy is real at the 95% confidence. This indicates that the
local M-Z-(SFR) relation may not hold for all low-mass galaxies (<10^9 Msun) at
z=0.4-0.8. The high ionization parameter and high electron density seen in our
galaxies suggest that they are lower redshift analogs to typical z > 1
galaxies.
Description
[1307.7712] "Direct" Gas-phase Metallicities, Stellar Properties, and Local Environments of Emission-line Galaxies at Redshift below 0.90
Links and resources
Tags