Ionized-gas Metallicity of the Strong [O III]λ5007 Emission-line Compact Galaxies in the LAMOST Survey

Abstract

This article reports a sample of 1830 strong [O III]λ5007 emission-line compact galaxies discovered with the LAMOST spectroscopic survey and the photometric catalog of the Sloan Digital Sky Survey. We newly identify 402 spectra of 346 strong [O III]λ5007 emission-line compact galaxies by finding compact isolated point sources. Combined with the samples in our previous work, this returns a sample of 1830 unique strong [O III]λ5007 emission-line compact galaxies with 2033 spectra of z ≤ 0.53. For the sources with 2σ[O III]λ4363 detections, we calculate the gas-phase metallicity with the direct-T _e method, and verify that the strong-line metallicity diagnostics calibrated with the direct-T _e method also applies to this sample. The strong [O III]λ5007 emission-line compact galaxies fall below several T _e -calibrated mass-metallicity relations. The N/O measurements of the strong [O III]λ5007 emission-line compact galaxies mainly locate at a plateau at low metallicity, indicating the product of primary nucleosynthesis. The Ne3O2 and O32 relation follows a tight linear relation with no redshift evolution. The Ne3O2 anticorrelates with the stellar mass, and at fixed stellar mass the Ne3O2 increases with the redshift. Eight sources with asymmetric [O III]λ5007 emission-line profiles have been identified, however with no [O III]λ4363 detection, which proves the rich metal content and complex ionized-gas kinematics within the galaxies. Higher-resolution spectroscopy will be necessary to identify the ionized-gas components in detail.