Mergers Drive Structural Complexity but Not Starbursts in Lyman-$α$ Emitters at $3 < z < 4$: A JWST Spatially Resolved View

Recent observations with the James Webb Space Telescope (JWST) reveal that the merger fraction among Ly$α$ emitters (LAEs) at redshifts $z > 3$ is significantly higher than previously estimated. In this study, we focus on three high signal-to-noise merging LAE systems at $3 < z < 4$, selected from the VLT/MUSE-Deep survey in the GOODS-S field. We combine new \textit{JWST}/NIRCam broadband and medium-band imaging with archival \textit{HST}/ACS data to perform spatially resolved spectral energy distribution (SED) fitting using the \textsc{Bagpipes} software package. Our analysis reveals that two of the systems are minor mergers, while the third is a major merger. The close agreement between spatially resolved and integrated stellar mass estimates indicates that recent star formation does not significantly outshine the light from older stellar populations in these systems. Moreover, both the individual components and the systems as a whole lie on the star-forming main sequence, further supporting the conclusion that these mergers have not yet triggered substantial starburst activity. Furthermore, we detect prominent color gradients and disturbed dust distributions in these merging systems, indicating that the mergers have already induced significant internal structural perturbations. These morphological and dust-related changes may facilitate the escape of Ly$α$ photons – potentially through mechanisms such as gas redistribution or a reduced covering fraction of neutral hydrogen – thereby playing a key role in shaping the observed properties of LAEs.

💡 Research Summary

This paper presents a spatially resolved analysis of three Lyman‑α emitting galaxies (LAEs) at redshifts 3 < z < 4 that show clear signs of merging. The authors combine new JWST/NIRCam broadband and medium‑band imaging with archival HST/ACS data, achieving a uniform pixel scale of 0.03″ and matching all point‑spread functions to the F444W band (FWHM = 0.163″). Effective pixels are defined by a signal‑to‑noise ratio greater than five in the F150W and F200W bands, yielding 215–287 usable pixels per galaxy (≈220 pc physical resolution at z ≈ 3).

The sample is drawn from the VLT/MUSE‑Deep spectroscopic survey. After applying stringent criteria—secure Ly α detection, dual‑nucleus morphology with projected separations of 0.15″–0.30″, and S/N > 15 in all JWST and HST filters—only three systems (MD‑6666, MD‑82, MD‑1711) remain. These objects have Ly α equivalent widths of 30–60 Å and luminosities around 10⁴² erg s⁻¹.

Spectral energy distribution fitting is performed on a pixel‑by‑pixel basis using the BAGPIPES code. The model includes flexible star‑formation histories, variable metallicity, and V‑band dust attenuation (A_V). From the fits the authors derive stellar mass, star‑formation rate (SFR), mass‑weighted age, specific SFR (sSFR), and UV continuum slope (β) for each pixel and for the integrated galaxy.

Key results: two systems are classified as minor mergers (mass ratios ≈ 1:3–1:5) and one as a major merger (mass ratio ≈ 1:1). The sum of the pixel‑wise stellar masses matches the integrated mass to within a few percent, indicating that recent star formation does not dominate the light. All components and the whole systems lie on the star‑forming main sequence, with sSFR values consistent with typical main‑sequence galaxies at these redshifts; no evidence for a starburst phase is found.

Despite the lack of a starburst, the authors detect pronounced color gradients and irregular dust attenuation maps. The dust is redistributed asymmetrically, and the UV–optical colors become markedly redder toward the outskirts of the nuclei. These structural perturbations suggest that the mergers have already altered the interstellar medium, potentially creating low‑density channels or reducing the covering fraction of neutral hydrogen. Such changes could facilitate the escape of Ly α photons, offering a plausible explanation for the high Ly α visibility of these merging systems.

The study demonstrates that, even at early cosmic times, galaxy mergers can generate significant internal complexity without necessarily triggering intense star formation. It underscores the importance of high‑resolution, multi‑wavelength imaging for disentangling the interplay between morphology, dust, and Ly α radiative transfer. The authors advocate for follow‑up JWST/NIRSpec IFU spectroscopy to map gas kinematics and metallicity, which would further clarify how merger‑driven structural changes impact Ly α escape and the evolution of high‑redshift galaxies.