Are High-EW LAEs good analogs for the earliest galaxies? A study on the ionizing conditions and gas properties of LAEs at $2\lesssim z\lesssim7

Abstract

Lyman-$\alpha$ emitters (LAEs) are frequently proposed as analogs of the low-mass, metal-poor galaxies thought to dominate cosmic reionization. In particular, galaxies with large Ly$\alpha$ equivalent widths (EWs) are expected to have not experienced significant chemical enrichment. We test this hypothesis using a sample of 88 LAEs at $2 \lesssim z\lesssim 7$, selected from MUSE observations that are undetected in deep HST imaging and subsequently followed up with JWST/NIRSpec spectroscopy. Despite the unprecedented sensitivity of JWST, only 50 of the LAEs exhibit secure counterparts in available deep NIRCam imaging reaching typical 3$\sigma$ depths of $m_\text{AB}\sim30.3$ mag, highlighting the intrinsically faint nature of the population. Gas-phase metallicities are inferred by fitting optical emission lines with a grid of BPASS+cloudy photoionization models, while stellar masses are derived from NIRCam photometry through SED fitting. The final sample with secure stellar masses and gas-phase metallicities encompasses only 16 LAEs, occupying regions of standard emission-line diagnostic diagrams consistent with low-metallicity star-forming systems. Although the strong-line metallicity diagnostic R3 appears to decrease with increasing EW, we find no compelling evidence for a strong monotonic relation between EW and gas-phase metallicity when metallicities are inferred through full emission-line fitting. However, galaxies with stronger Ly$\alpha$ emission tend to lie below the average mass-metallicity relation at fixed stellar masses, suggesting that they are preferentially less chemically evolved than typical galaxies of the same mass. We conclude that high-EW LAEs provide useful, though imperfect, analogs of the galaxies thought to drive cosmic reionization, and combining EW with stellar mass may provide a more effective route for identifying the most chemically primitive galaxies.