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

TL;DR

This study uses JWST/NIRCam and HST data to perform pixel-by-pixel SED fitting with Bagpipes on three LAE mergers at $3<z<4$ in GOODS-S. It finds two minor mergers and one major merger, with resolved and integrated stellar masses in close agreement and all components lying on the star-forming main sequence, indicating no strong starburst activity. Color and dust distributions show spatial gradients and disturbances linked to merger dynamics, suggesting that Lyα escape may be facilitated by gas redistribution and dust geometry rather than enhanced star formation. The results imply that mergers contribute to internal structural perturbations and Lyα visibility through non-starburst channels, offering a refined view of Lyα radiative transfer in the early universe.

Abstract

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.

Figures (10)

• Figure 1: From left to right: JWST/NIRCam broadband images in F090W, F115W, F150W, F200W, F277W, F356W, and F444W; pseudo-color composite (blue: F090W, F115W, F150W; green: F200W, F277W; red: F356W, F444W); MUSE Ly$\alpha$ narrow-band image and the emission-line spectrum with an asymmetric Gaussian fit. All panels show a $2" \times 2"$ field of view. Yellow stars mark the Ly$\alpha$ surface-brightness peaks in the narrow-band map and are overlaid on the F444W image.
• Figure 2: Illustration of the SED fitting results for the galaxy components in the LAE merger system MD-6666 at $z = 3.43$. The left panel shows the JWST/NIRCam F090W image, with MD-6666-N (upper component) and MD-6666-S (lower component) highlighted using distinct color maps. Black contours delineate the segmented regions used for photometry. The middle two panels display the Bagpipes SED fits for MD-6666-N and MD-6666-S, respectively, plotted on identical vertical scales to enable direct comparison. The right panels show the corresponding posterior redshift distributions from the SED fitting, with the median values and 16th -- 84th percentile credible intervals indicated.
• Figure 3: Illustration of the SED fitting results for the LAE merger system MD-82 at $z = 3.61$. The symbols and layout follow the same convention as in Figure \ref{['fig:6666']}.
• Figure 4: Illustration of the SED fitting results for the LAE merger system MD-1711 at $z=3.77$. The symbols and layout follow the same convention as in Figure \ref{['fig:6666']}.
• Figure 5: Resolved pixel-by-pixel SED fitting results for the galaxy MD-6666 at $z = 3.43$. The top-left panel shows the JWST/NIRCam F090W image; the remaining panels display maps of stellar mass, mass-weighted age, dust attenuation ($A_V$), star formation rate (SFR), specific SFR (sSFR), pseudo-color image, and UV slope ($\beta$). Stellar mass, age, $A_V$, SFR, and sSFR are derived directly from Bagpipes fits, while $\beta$ is computed from the best-fit spectra produced by Bagpipes. Each pixel corresponds to $0.03"$, and all panels span $1" \times 1"$. Note that the color scale is unique to each panel.