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A Lyman-Leaking Galaxy Merger Confirmed with JWST

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Epoch of Reionization
A scientific simulation still of the epoch of reionization when galaxies leaked high-energy photons into the universe, changing it from neutral to ionized. [M. Alvarez (http://www.cita.utoronto.ca/~malvarez), R. Kaehler, and T. Abel/ESO; CC BY 4.0]

Exactly how the universe became reionized after cooling to form stars and galaxies remains an open question. A recent study investigates how a merging set of galaxies may have spurred an onslaught of ionizing photons into the intergalactic medium.

Lyman Continuum Leakers and Merging Galaxies

After its chaotic and blistering beginning, the universe cooled and began to coalesce, stars and galaxies flickering on to set the universe alight. From a few hundred million to about a billion years after the Big Bang, galaxies pumped high-energy ionizing Lyman continuum (LyC) photons into the intergalactic medium during what is known as the epoch of reionization. The exact mechanisms that produced and allowed LyC emission to escape galaxies during this era remain elusive.

Galaxy Merger

Hubble Space Telescope image of the interacting galaxy group Arp 194. The blue stream connecting the galaxies is where tidally stripped gas formed millions of new stars. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]

One promising driver of LyC leakage is galaxy mergers — strong gravitational interactions and interstellar medium mixing can induce bursts of star formation, creating the hot, young stars that emit LyC photons. In particular, galaxy mergers can create tidal tails, streams of wispy gas, that can sustain brief but intense bouts of star formation that quickly use up surrounding gas. With no interstellar gas left to absorb radiation, the fresh stars in these streams can easily pump their high-energy photons into the intergalactic medium, contributing to cosmic reionization.

While observational studies have increasingly pointed to galaxy mergers as considerable drivers of LyC escape, only a small number have explicitly considered the role of mergers in the epoch of reionization due to spectral and spatial resolution limitations. Leveraging the high-resolution capabilities of JWST, Shengzhe Wang (University of Chinese Academy of Sciences; National Astronomical Observatories, Chinese Academy of Sciences) and collaborators performed detailed analyses of two LyC-leaker candidates to better understand the role of galaxy mergers in heating up the universe.

Confirming (and Refuting) Merger-Driven Leaks with JWST

Using candidate LyC leakers first identified in the LymAn Continuum Escape Survey (LACES) carried out with the Hubble Space Telescope, Wang and team obtained high-resolution JWST spectroscopy for two galaxies at redshift z = 3.1 (about 2 billion years after the Big Bang), LACES94460 and LACES104037. After carefully identifying and measuring emission lines in each candidate, the authors confirmed LACES104037 as a merger-driven LyC leaker. However, the apparent LyC emission from LACES94460 was determined to be contamination from a low-redshift object.

LACES 104037

Hubble and JWST observations of LACES104037. The top panels show the Hubble images (first two panels) and the JWST spectral observations (last three panels). The bottom panel shows a composite three-color image marking the location of two primary interacting galaxies and the LyC emission along with the associated spectra. Click to enlarge. [Wang et al 2026]

Based on the JWST spectroscopy, LACES104037 is an early-stage merger system with two primary interacting galaxies (LACES104037-bulk and LACES104037s) with a tidal-tail structure connecting them (LACES104037-LyC) where the LyC emission originates. Using model predictions based on the JWST spectroscopy and photometry from previous studies, the team found that LACES104037-LyC is a star-forming clump in a tidal tail with a stellar age of about 5 million years and an extreme LyC photon escape fraction of approximately 0.99. This points to merger-induced star formation being an extremely efficient way to dump ionizing photons into the intergalactic medium.

LACES104037-LyC represents the first confirmed LyC leaker of its kind: an external clump of star formation in a tidal tail. This discovery aids in our understanding of how mergers can drive high-energy photon emission, and highlights the critical role galaxy mergers may play in the epoch of reionization. Future high-resolution observations will hopefully reveal more knots of LyC-leaking stars within merging galaxies and provide a deeper understanding of this important evolutionary phase of the universe.

Citation

“Confirmation and Refutation of Lyman Continuum Leakers at z ~ 3 with JWST NIRSpec IFU,” Shengzhe Wang et al 2026 ApJL 1006 L17. doi:10.3847/2041-8213/ae8523