Highly Efficient Identification of Extreme Emission-line Galaxies in the Local Universe: &gt;8000 New Green Pea Candidates at 0.12 &lt;  <i>z</i>  &lt; 0.36

Abstract The currently known compact extreme emission-line galaxies (the “Green Peas” (GPs)) in the Sloan Digital Sky Survey (SDSS) are rare and were mostly found among serendipitous spectroscopic targets, thus leaving open the possibility that a substantial population of GPs is missed. A significantly larger number of identified GPs in the Local Universe might provide a better characterization of their high-redshift analogs and Lyman continuum escape. In this paper, we confront the challenges of robustly identifying GPs without spectroscopic information, a needed approach considering the incompleteness of spectroscopic surveys for compact sources. The principal difficulty stems from a significant contamination of photometric candidates by stars and quasars of similar color. To solve this, we introduce a spectral energy distribution matching method, which separates candidate GPs from contaminants on the basis of SDSS and Wide-field Infrared Survey Explorer photometry of spectroscopically confirmed stars, quasars, and galaxies. The method has an effectiveness of 85% and a contamination rate of ∼10%. With it, we identify ∼9600 GP candidates expected to lie in the 0.12 &lt; z &lt; 0.36 range—a tenfold increase over what would be selected using SDSS DR18 spectra. Some of the new GPs are as bright as r ∼ 19, and 1200 are predicted to have [O III ]5007 equivalent widths in excess of 500 Å. The new population contains many “Extended Peas,” which are absent among known GPs and possibly represent merging systems. We provide catalogs containing 8313 newly identified GP candidates, as well as 917 GPs confirmed using SDSS spectroscopy and 521 GPs with spectroscopic redshifts from LAMOST and other sources.