J1244-lyc1 Reveals How Galaxy Mergers Drive Intense Lyman Continuum Emission

Understanding how the earliest galaxies reionised the universe after the Big Bang remains a fundamental challenge in cosmology, and now a team led by Shengzhe Wang from the National Astronomical Observatories, Chinese Academy of Sciences, Xin Wang from the University of Chinese Academy of Sciences, and Matthew A. Malkan from the University of California, Los Angeles, has identified a particularly luminous source of ultraviolet radiation escaping from a distant galaxy. Their work, utilising data from the MAMMOTH-LyC survey, reveals J1244-LyC1, a galaxy at a redshift of exhibiting strong Lyman continuum (LyC) leakage, a key indicator of escaping ionising photons. This discovery is significant because J1244-LyC1 is not only the most massive LyC emitter found to date, but also displays clear evidence of a recent galactic merger, suggesting that such events may play a crucial role in facilitating the escape of radiation and contributing to the reionisation of the early universe, a process previously only theorised. The team’s observations confirm that the LyC emission originates from multiple locations within the disturbed galaxy, providing the first direct evidence of spatially resolved LyC escape in a merging system.

Keck MOSFIRE Data Reduction and Parameters

Data reduction employed the PypeIt pipeline, incorporating standard processing steps such as flat-fielding, dark subtraction, and cosmic ray detection, alongside precise wavelength calibration derived from OH sky lines and optimal spectral extraction techniques. Object identification utilized the pipeline’s default settings, but required adjustments for J1244-LyC1 due to initial tracing failures, achieved by lowering the signal-to-noise threshold and manually adjusting slit parameters for long-slit mode data. Flux calibration was performed using standard star observations processed identically to the science targets, ensuring accurate measurements of spectral features.

Hubble Maps Lyman Continuum Leakage in Merger

The MAMMOTH-LyC survey utilized deep imaging with the Hubble Space Telescope’s WFC3/UVIS instrument to investigate massive galaxy protocluster fields at a redshift of 2, leading to the discovery of J1244-LyC1, a strong Lyman continuum (LyC) emitter undergoing a galactic merger. The team confirmed a highly significant LyC detection with a signal-to-noise ratio exceeding 10, measuring an absolute escape fraction of 0.53, indicating substantial leakage of ultraviolet radiation. This emission is spatially resolved into multiple peaks correlating with the merging galaxies’ disturbed morphology, confirming genuine multi-site leakage.

To address challenges posed by long exposures and telescope stability, scientists developed a customized data reduction framework incorporating affine transformations, iterative drizzling, and robust cosmic-ray rejection, achieving relative astrometric precision of approximately 0.2 pixels. Complementing the Hubble data, Keck Observatory’s MOSFIRE instrument conducted observations in both Multi-Object Spectroscopy and Long-Slit Spectroscopy modes, processed with the PYPEIT pipeline for precise wavelength calibration and spectral extraction. Ground-based imaging from the Large Binocular Telescope and CFHT provided essential constraints for spectral energy distribution fitting.

High-resolution HST imaging revealed a 10σ detection of LyC emission, measuring mF225W = 27.81+0.11 −0.10 AB mag. Spectral analysis, combining HST grism data and Keck/MOSFIRE observations, detected key emission lines confirming a redshift of z = 2.39 and revealing double velocity components in the Hα emission, indicative of the ongoing merger.

Merging Galaxy Leaks All Ultraviolet Light

The MAMMOTH-LyC survey revealed J1244-LyC1, a luminous galaxy at a redshift of 2.39, exhibiting strong Lyman continuum (LyC) emission, marking the first confirmed high-redshift LyC-leaking galaxy undergoing a merger and the most massive LyC emitter detected to date. Scientists achieved a highly significant LyC detection, registering a signal strength of 10σ, corresponding to an absolute escape fraction of 1.0, confirming that nearly all ultraviolet light from the young stars is escaping into intergalactic space. The LyC emission is spatially resolved into multiple peaks coinciding with the disturbed morphology of the merging galaxy system, confirming that leakage originates from several distinct regions.

Emission lines of [O ii], Hγ, Hβ, and Hα were measured, solidifying the redshift determination and excluding foreground contamination. Detailed spectroscopic analysis using Keck/MOSFIRE detected a relative velocity difference of 116 ±3.6km s−1 between two components of the Hα emission, demonstrating the dynamic nature of the merger. Analysis of the HST imaging, achieving relative astrometric precision of approximately 0.2 pixel, showed that the LyC emission does not perfectly align with ultraviolet luminosity peaks.

The team measured a LyC magnitude of 27.81+0.11 −0.10 AB mag, establishing a baseline for future studies, supporting the interpretation of J1244-LyC1 as a merger-driven starburst where tidal interactions create pathways for LyC photons to escape, offering insight into the role of major mergers in cosmic reionization.

Merger Powers Bright Lyman Continuum Emission

This research presents the discovery of J1244-LyC1, a distant galaxy exhibiting strong Lyman continuum emission, representing a significant step forward in understanding the early universe. Identified within a survey of massive galaxy protoclusters, its emission was confirmed with high statistical confidence, establishing an absolute escape fraction of approximately 36 percent. The Lyman continuum emission originates from multiple locations within the galaxy, coinciding with evidence of a recent merger, suggesting that the merger’s disruption of the interstellar medium creates pathways for ultraviolet light to escape. This provides direct evidence that major mergers can facilitate Lyman continuum escape, potentially contributing to the reionization of the universe. Further investigation is needed to determine the prevalence of such galaxies,Ụ and future studies should not solely focus on protocluster environments. Observing background Lyman continuum leakers may offer additional insights into the intergalactic medium, and follow-up observations with advanced instruments are needed to fully characterize the dynamics and emission properties of J1244-LyC1.