MAMMOTH-LyC: Investigating the Role of Galaxy Mergers in a Strong Lyman Continuum Leaker at $z=2.39$

TL;DR

The study presents the discovery and comprehensive characterization of J1244-LyC1, a strong Lyman continuum leaker at $z=2.39$ in the MAMMOTH-LyC survey. Using ultra-deep HST LyC imaging, high-resolution multi-band photometry, and Keck/MOSFIRE plus HST spectroscopy, the authors confirm a major-merger system with dual H$\alpha$ components and a three-clump LyC leakage morphology. SED fitting with CIGALE yields a young, dusty, massive galaxy ($M_* \sim 10^{10.15} M_\odot$, $E(B-V)_{gas} \approx 0.21$) with an inferred absolute escape fraction $f_{\mathrm{esc}} \approx 0.36$, consistent with the multi-site LyC leakage. The spatially resolved LyC emission and merger-driven ISM disturbances imply that mergers can play a crucial role in enabling LyC escape in massive galaxies, providing important constraints on reionization-era processes and the environmental factors that govern LyC leakage. The work also emphasizes the significant model-dependent uncertainties in translating LyC measurements into $f_{esc}$ due to IGM transmission along different sightlines.

Abstract

The MAMMOTH-LyC survey is a cycle 30 Hubble Space Telescope (HST) medium program obtaining 18-orbit-deep WFC3/UVIS F225W imaging in two massive galaxy protocluster fields at $z\sim2.2$. We introduce this survey by reporting the discovery of J1244-LyC1, a strong Lyman continuum (LyC) leaker at $z = 2.39$, exhibiting clear merger signatures. J1244-LyC1 has a highly significant ($10σ$) LyC detection, corresponding to an absolute escape fraction of $f_{\mathrm{esc}} \! =\!36\%\pm4\%$ ($1σ$). The LyC emission is spatially resolved into multiple peaks that coincide with the system's disturbed morphology, confirming genuine multi-site LyC leakage. With a stellar mass of $10^{10.2}{M_\odot}$, J1244-LyC1 is both the first confirmed high-redshift LyC-leaking merger and the most massive LyC emitter known to date. We interpret J1244-LyC1 as a merger-driven starburst system in which tidal interactions have disrupted the interstellar medium, creating multiple low-column-density pathways that facilitate LyC escape. This discovery provides the first direct evidence of spatially resolved LyC escape in a merging system, offering new insight into the potential role of major mergers in driving the cosmic reionization.

Figures (6)

• Figure 1: HST WFC3 coverage of the BOSS1244 protocluster field and high-resolution multi-band imaging of J1244-LyC1. Left: The MAMMOTH-Grism and MAMMOTH-LyC programs target overdense fields of extreme emission-line galaxies (EELGs) at $z \approx 2.2$, corresponding to the BOSS1244 protocluster. Black circles mark spectroscopically confirmed H$\alpha$ emitters (HAEs; Shi_2021), while the magenta diamonds indicate EELGs with ${\rm EW}$([Oiii])$> 225\,\text{\AA}$ following Tang_2019_OIII, identified using the MAMMOTH-Grism deep HST grism spectroscopy. The red star denotes the location of J1244-LyC1. Right: HST imaging of J1244-LyC1 in WFC3/UVIS F225W (rest-frame LyC; PSF-smoothed), F475W (rest-frame UV), and WFC3/IR F125W and F160W (rest-frame optical). The FWHM of the PSF is shown by the black circle. The 0.6 aperture is indicated by the black dashed circle. Each cutout is 1.8$\times$ 1.8 in size.
• Figure 2: Keck/MOSFIRE $K$ band spectroscopy (R$\sim3600$) confirming the double-velocity components of J1244-LyC1 through the H$\alpha$ emission line. Left: Slit positions from two MOSFIRE observations overlaid on the pseudo-color image of J1244-LyC1. The yellow and green rectangles correspond to the slit orientations for the first and second observations, respectively, and match the colors used for the spectra on the right. Right: H$\alpha$ emission-line profiles and Gaussian fitting results. For the first observation, a single-Gaussian model is adopted, and the derived line centroid is used to constrain the double-Gaussian fit in the second observation. Because the blue component is closer to the systemic redshift of J1244-LyC1 ($z=2.387$), we adopt its centroid as the velocity zero-point to illustrate the relative velocity offset between the two components. The expected locations of the [Nii] $\lambda\lambda6548,6583$ lines are marked; no significant [Nii] emission is detected in either observation.
• Figure 3: The best-fit SED model (blue) of J1244-LyC1 at $z=2.39$ using CIGALE, fit to the existing broad-band photometry covering a wide wavelength range of $600$--$7000\,\AA$ in the rest-frame. The SED fitting results are shown in Table \ref{['tab:SED_result']}, and the multi-band photometry is shown in Table \ref{['tab:measure']}. The SED of J1244-LyC1 is dominated by a young stellar population with an age $330 \pm 120$ Myr and a recent SFR of $65 \pm 33\,M_\odot\,\mathrm{yr^{-1}}$.
• Figure 4: GALFIT decomposition of J1244-LyC1 in HST F160W imaging. All images are normalized to the peak value of the observed image. Single-component ( top) and Double-component ( bottom) GALFIT models for the F160W image. The Double-component fit provides a significantly better description of the main body of J1244-LyC1, whereas the single-component model leaves behind prominent structural residuals. The red arrows in the residual map highlight the tidal-tail structure, a clear signature of galaxy mergers.
• Figure 5: UV morphology and [Oii] emission distribution of J1244-LyC1. We present the HST/WFC3 F475W image (rest-frame UV continuum) together with the [Oii] emission map extracted from the HST WFC3/G141 slitless spectroscopic data. The blue dashed contours trace the spatial distribution of the LyC emission. The green crosses mark the two luminosity centers identified in the F475W image. Three black circles (diameter 0.15) indicate the locations of the three LyC-emitting clumps defined in our analysis. At all three clump positions, the flux distribution in F475W differs significantly from that in the [Oii] map, highlighting spatial variations in recent ($\sim$3–10 Myr) versus longer-timescale ($\sim$10–100 Myr) star-formation activity.