Wednesday, July 31, 2013

1307.7712 (Chun Ly et al.)

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

Chun Ly, Matthew A. Malkan, Tohru Nagao, Nobunari Kashikawa, Kazuhiro Shimasaku, Masao Hayashi
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 III]4363, 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.
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