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 , [ O iii ] \lambda 4363 , is detected at \geq 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 } ) \leq 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 M _ { \sun } yr ^ { -1 } 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 lower than the local M _ { \star } – Z – ( SFR ) relation by \approx 0.2 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 [ O iii ] \lambda 4363 detections .
Finally , the observed higher ionization parameter and high electron density suggest that they are lower redshift analogs to typical z \gtrsim 1 galaxies .