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Star‑forming galaxies that brightened the young Universe during its reionization era are too faint to study in detail, even with the James Webb Space Telescope (JWST). To overcome this, astronomers have turned to *Extreme Emission Line Galaxies* (EELGs) at redshifts between about 1 and 1.7. Their compact sizes, low masses, and powerful nebular emission make them excellent laboratories for the conditions that drove reionization.
Using near‑infrared imaging from the CANDELS Survey of the Extended Groth Strip, the team applied a J‑band colour‑excess criterion that isolates galaxies whose flux rises steeply in the J filter because of strong rest‑frame optical lines. This selection picks up both Hβ + [O III] emitters at z≈1.7 and Hα emitters at z≈1.0. The method was calibrated with spectroscopic follow‑up from a range of ground‑based and space‑based instruments, including MMT/MMIRS, Keck/MOSFIRE, HST/WFC3, JWST/NIRSpec and GTC/OSIRIS+. The colour selection is shown in Figure 1, which plots the J‑band excess against adjacent filter colours and displays the two distinct EELG populations.
Out of the 229 photometrically selected candidates, 27 had secure spectroscopic redshifts from the literature and the follow‑up campaign: 19 Hβ + [O III] objects and 8 Hα objects. The purity calculation, based on 1,000 Monte‑Carlo realizations, gives 84.3 ± 1.5 % for the Hβ + [O III] window and 76.8 ± 2.0 % for the Hα window. The spectroscopic confirmation rates of 94.1 % and 62.5 % match these purities, indicating the colour method efficiently isolates the intended redshift ranges.
The spectroscopic subsample also reveals the physical properties of these galaxies. Table 4 lists their emission‑line fluxes, stellar masses, and star‑formation rates. The median stellar mass is about 10^8.5 M☉, while star‑formation rates derived from Hα range from ≈0.3 to 15 M☉ yr^–1. These values are comparable to those measured for z>5 galaxies observed by JWST, supporting the idea that EELGs at z∼1–1.7 are valid low‑redshift analogues of the earliest star‑forming galaxies.
Potential contamination by active galactic nuclei (AGN) is low. In the spectroscopic sample, only one galaxy falls into the AGN region of the Mass‑Excitation diagram, and at most one source satisfies the Donley et al. mid‑infrared colour criteria for AGN. Mid‑IR photometry also shows that no source has a pronounced AGN spectral energy distribution. Thus the majority of contamination comes from lower‑redshift line interlopers rather than from AGN.
The selection purity for Hα emitters is lower because roughly 77 % of candidates were rejected in an initial photometric‑redshift cut, and only eight Hα sources have spectroscopic confirmation. This small sample increases statistical uncertainty for the Hα population.
These findings confirm that EELGs at intermediate redshifts are robust, low‑mass analogues of the galaxies that likely reionized the Universe. They provide a practical laboratory for studying intense star formation, ionizing photon production, and feedback in conditions similar to the early cosmos, with full spectroscopic capability available from today’s telescopes.