The Ly$α$ and Continuum Origins Survey. III. Investigating the Link between Galaxy Morphology, Merger Properties and LyC Escape

Characterizing the mechanisms and galaxy properties conducive to the emission and escape of ionizing (LyC) emission is necessary to accurately model the Epoch of Reionization, and identify the sources that powered it. The Ly$α$ and Continuum Origins Survey (LaCOS) is the first program to obtain uniform, multi-wavelength subkiloparsec imaging for a large sample (42) of galaxies observed in LyC and enable statistically robust studies between LyC and resolved galaxy properties. Here, we characterize the morphology and galaxy merger properties of LaCOS galaxies and investigate their connection with the escape fraction of LyC emission $f_{esc}^{LyC}$. We find strong anticorrelations between $f_{esc}^{LyC}$ and size ($r_{20}$, $r_{50}$, and $r_{80}$) measured in filters containing emission from star-forming regions, and with the asymmetry and clumpiness in F150LP, the bluest filter in our dataset, tracing UV continuum and Ly$α$. We find that $\geq48%$ of LaCOS galaxies, and $\geq41%$ of LaCOS LyC-emitters are visually classified as galaxy mergers. Galaxies robustly identified as mergers in LaCOS are at advanced stages of interaction, close to coalescence. The $f_{esc}^{LyC}$ properties of robust mergers and low-probability mergers cannot be differentiated statistically, and we only find significant difference between the two populations in terms of their of their sizes and LyC luminosity: robust mergers having larger values. We conclude that (i) $f_{esc}^{LyC}$ tends to be larger in galaxies with a small number of compact, centrally-located, UV-emitting star-forming regions, (ii) at advanced stages of interaction represent a sizable fraction of LyC-emitting samples at $z\sim0.3$, $z\sim0$, and (iii) mergers can facilitate the escape of LyC photons from galaxies.

💡 Research Summary

The Lyman‑α and Continuum Origins Survey (LaCOS) presents the first homogeneous, sub‑kiloparsec Hubble Space Telescope (HST) imaging of 42 low‑redshift (z ≈ 0.3) star‑forming galaxies that have been observed for Lyman‑continuum (LyC) leakage. Half of the sample (22 galaxies) are confirmed LyC‑emitters with escape fractions (fₑₛ𝚌^LyC) ranging from 0.01 to 0.49, while the remainder serve as a control set of non‑detections or low‑significance detections (fₑₛ𝚌^LyC < 0.02). The data set includes five filters: two rest‑frame UV bands (F150LP, F165LP) that trace recent star formation, and three optical bands (F438W, F547M, F850LP) that probe older stellar populations and overall morphology. With a point‑spread function of ~0.1″ (≈ 400 pc), the images enable detailed morphological analysis.

Non‑parametric morphometrics were measured in each filter: asymmetry (A), concentration (C), clumpiness (S), Gini coefficient (G), second‑order moment of the brightest 20 % of the light (M₂₀), and size quantiles (r₁₀, r₅₀, r₈₀). These metrics avoid assumptions about galaxy light profiles and are well suited for irregular, clumpy systems. Statistical tests reveal strong anti‑correlations between fₑₛ𝚌^LyC and the size measures (r₁₀, r₅₀, r₈₀) when measured in filters dominated by star‑forming regions. In other words, galaxies with compact, centrally concentrated UV clumps tend to let a larger fraction of ionizing photons escape. Additionally, the asymmetry and clumpiness measured in the bluest filter (F150LP) also show significant negative correlations with fₑₛ𝚌^LyC, indicating that smoother, more symmetric UV morphologies favor LyC leakage.

Visual inspection classifies ≥ 48 % of the full LaCOS sample as mergers, and ≥ 41 % of the confirmed LyC‑emitters fall into this category. The mergers are predominantly at advanced interaction stages, close to coalescence, displaying tidal tails and disturbed morphologies. When the merger sample is split into “robust” (high‑confidence) and “low‑probability” mergers, their fₑₛ𝚌^LyC distributions are statistically indistinguishable. However, robust mergers have larger r₅₀ values and higher intrinsic LyC luminosities than low‑probability mergers, suggesting that while the merger stage does not directly set the escape fraction, it does affect the overall LyC output and the spatial scale of star‑forming regions.

These findings align with theoretical expectations that gas‑rich interactions trigger intense, bursty star formation, generate strong stellar feedback, and can evacuate low‑density channels through which ionizing photons escape. The observed anti‑correlations between size, asymmetry, clumpiness and fₑₛ𝚌^LyC support models where a small number of compact, centrally located UV‑bright clumps dominate the ionizing output, and where merger‑driven dynamical processes help clear the interstellar medium.

In summary, the LaCOS study demonstrates that (i) high LyC escape fractions are associated with galaxies that host few, compact, centrally located UV‑emitting star‑forming regions; (ii) advanced‑stage mergers constitute a sizable fraction of LyC‑emitting galaxies at low redshift, mirroring trends seen at z ≈ 0.3 and z ≈ 0; and (iii) while mergers do not guarantee higher fₑₛ𝚌^LyC, they can facilitate LyC escape by reshaping the gas distribution and boosting LyC luminosity. The work provides a robust observational framework—combining non‑parametric morphometrics and merger classification—to predict LyC leakage in galaxies, which can be extrapolated to higher redshifts where direct LyC detection is impossible, thereby improving constraints on the sources responsible for cosmic reionization.