Highly Efficient Identification of Extreme Emission Line Galaxies in the Local Universe: >8000 New Green Pea Candidates at 0.12 < z < 0.36

TL;DR

This work addresses the incomplete census of Green Pea galaxies by developing a photometric identification pipeline that augments traditional spectroscopic selection. It combines SDSS DR18 photometry with WISE data and a novel SED-matching approach to distinguish Green Pea candidates from stars, QSOs, and other contaminants in the $0.12<z<0.36$ window, achieving about $84{-}89\%$ GP recovery in validation tests. The method yields ~9628 GP candidates (including ~8313 new) and provides 917 spectroscopically confirmed GPs plus 521 additional non-SDSS GPs, dramatically increasing the GP surface density to roughly $2$ deg$^{-2}$. External checks with LAMOST, KISS, H$pping$-Dot, and Gaia support the reliability of classifications while highlighting residual stellar contamination on the order of ~5%. Overall, the study delivers a powerful, scalable path to constructing large, well-characterized GP samples for studying local extreme emission line galaxies and their high-redshift parallels.

Abstract

The currently known compact extreme emission-line galaxies (the "Green Peas", GPs) in 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 an SED matching method, which separates candidate GPs from contaminants on the basis of SDSS and WISE 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 < z < 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 [OIII]5007 equivalent widths in excess of 500 A. 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.

Figures (10)

• Figure 1: Removal of non-Green Pea interlopers from a Cardamone09-like sample using [OIII]5007 equivalent width. Upper panel: EW([OIII]) vs. redshift plot allows us to refine the redshift window that yields GPs ($0.12<z<0.36$, red dashed lines). Lower Panel: EW([OIII]) vs. $(r-i)_{\mathrm{fiber}}$ color plot allows us to remove some non-GP interlopers (EW([OIII]) $<100$ Å, below black dashed line) based on red color within the 3 arcsec (fiber) aperture ($(r-i)_{\mathrm{fiber}}>0$, red dashed line). Red dots denote the objects removed by either the fiber color cut or lying outside of the $0.12<z<0.36$ redshift window. All EWs in this work are in rest frame.
• Figure 2: Removal of non-Green Pea contaminants using photometric information. Top: $r$ magnitude vs. redshift. Middle: fiber aperture (3") magnitude minus total magnitude ($r_{\mathrm{fiber}}-r_{\mathrm{model}}$) vs. redshift. Bottom: angular half-light radius in $r$ band ($R_{50}$) vs. redshift. Cuts in these three quantities (black dashed lines) allow us to remove additional non-GP contamination---mostly stars and low-$z$ galaxies or galaxy regions. The purpose of this cuts is to select a cleaner candidate pool for GPs when spectra are not available. Redshift is shown as the absolute value of redshift in order include the negative redshifts of many stars and nearby galaxies, which do not exceed $|z| = 0.004$. The redshift window ($0.12<z<0.36$) that yields GPs using the C09 color selection is shown with red dashed lines.
• Figure 3: Three types of color-color diagrams showing Green Peas and the two main classes of contaminants (stars and quasars). Different classes of objects have somewhat distinct loci, but also overlap enough that no color selection can cleanly select just the GPs. Cardamone09 color cuts are shown as dashed lines.
• Figure 4: Galactic maps showing the Green Pea candidate pool (left) and spectroscopically confirmed Green Peas (right). Removing the objects plotted in red from the candidate pool (low latitude objects and objects in isolated SEGUE stripes) helps reduce the stellar contamination and provides a more contiguous area.
• Figure 5: Color-color plots of Green Pea candidates identified using the SED matching method. Left: $g-r$ vs. r-i color-color plot. Middle: u-r vs. r-z color-color plot. Right: z-W1 vs. W1-W2 color-color plot. Distribution of GP candidates matches that of the spectroscopically confirmed GPs (second column of Fig. \ref{['fig: known ccplts']}). Dashed lines indicate Cardamone09 selection cuts.