Radiation hydrodynamic simulation of the Haro 11 galaxy: the escape of LyC and Ly$α$ in a dwarf galaxy merger

The Haro 11 galaxy merger is the closest known Lyman Continuum (LyC) leaker and a strong Lyman-$α$ (Ly$α$) emitter, making it an important analogue of the high-$z$ galaxies that reionised the early Universe. To investigate how Haro 11’s properties arise, we perform a radiation hydrodynamics simulation of the merger, and create mock observations of LyC, Ly$α$, and H$α$, from which we compute their luminosities ($L$) and escape fractions ($f_{\rm esc}$). We track these quantities along multiple sightlines as the two progenitor galaxies merge, from the first interaction until the system resembles present-day Haro 11. We find that $L$ and $f_{\rm esc}$ vary by 1-2 orders of magnitude for LyC due to sightline variations. At the two pericentre passages, the total $f_{\rm esc}^{\rm LyC}$ increases by roughly an order of magnitude. Conversely, $f_{\rm esc}^{\rm Lyα}$ shows a moderate increase at the pericentre passages, which affects the inference of LyC properties from Ly$α$. We attribute this to a displacement of the LyC-emitting stars relative to the \Lya-emitting gas, combined with an increased density from gas compression. Furthermore, $f_{\rm esc}^{\rm LyC}$ is boosted during star formation bursts, likely due to stellar feedback. As direct comparison with Haro 11, the simulation qualitatively matches its morphology and luminosities. We find that among the dense stellar knots, knot C is the main contributor to both intrinsic and escaping LyC emission. Additionally, the Ly$α$ spectra displays distinct features found in observations, implying similar gas conditions are present.

💡 Research Summary

This paper presents a high‑resolution radiation‑hydrodynamics (RHD) simulation of the nearby dwarf galaxy merger Haro 11, the closest known Lyman‑continuum (LyC) leaker and a strong Lyman‑α (Lyα) emitter, to investigate how its observed properties arise. Using the RAMSES‑RT code, the authors model two disc galaxies that merge in isolation, allowing a clean dissection of merger‑driven effects without cosmological complications. Star formation follows a Schmidt law with a density threshold of 100 cm⁻³ and an efficiency per free‑fall time of 10 %, producing star particles of 10³ M⊙ that represent single‑age stellar populations with a Chabrier IMF. Stellar feedback includes winds, Type Ia/II supernovae (with a resolution‑dependent treatment of thermal vs. momentum injection), and on‑the‑fly radiative transfer. The RT module employs a moment‑based M1 closure, four photon groups (IR, H I‑ionising, He I‑ionising, He II‑ionising), and a reduced speed of light (c̃ = 0.005 c) to keep the Courant condition tractable. Metal cooling is handled with CLOUDY tables above 10⁴ K and fine‑structure cooling below that temperature.

Post‑processing is performed with the Monte‑Carlo code RASCAS, which propagates LyC, Lyα, and Hα photons through the simulated 3‑D mesh. Emissivities for recombination and collisional lines are derived directly from the RAMSES‑RT output, while Lyα resonant scattering, deuterium interactions, and dust extinction (SMC law scaled with metallicity) are treated in detail. The authors generate mock HST‑like images and Lyα spectra for many sightlines, enabling a direct comparison with observations.

The time evolution of intrinsic and escaping luminosities, as well as escape fractions (fesc), is tracked from the first close encounter through the two pericentre passages to the final stage that resembles present‑day Haro 11. The key findings are:

1. Strong sightline dependence: Both LyC and Lyα luminosities and fesc vary by 1–2 dex across different viewing angles, reflecting the clumpy, anisotropic ISM created by the merger.

2. Pericentre boosts: At each pericentre passage, gas compression displaces the LyC‑producing stars relative to the neutral gas, opening low‑column‑density channels. Consequently, the total LyC escape fraction rises by roughly an order of magnitude, while Lyα fesc shows only a moderate increase.

3. Star‑formation burst effect: Periods of elevated star formation trigger intense stellar feedback (winds, supernovae) that carve out cavities and low‑density tunnels, further enhancing LyC escape. This confirms the theoretical expectation that LyC leakage is tightly coupled to recent SFR spikes.

4. Knot C dominance: Among the three bright stellar knots observed in Haro 11 (A, B, C), knot C is identified in the simulation as the primary source of both intrinsic LyC photons and escaping LyC radiation. Its high SFR surface density and relatively low neutral‑hydrogen column make it the most efficient leaker.

5. Lyα spectral reproduction: The simulated Lyα line profiles exhibit multiple peaks, asymmetries, and velocity offsets that match observed spectra of Haro 11, indicating that the simulated gas kinematics, density structure, and dust content are realistic.

6. Implications for indirect diagnostics: Because LyC and Lyα escape respond differently to merger‑induced gas dynamics (LyC being more sensitive to the relative positioning of stars and gas, Lyα to overall column density and scattering), using Lyα properties alone to infer LyC escape can be misleading, especially during pericentre phases.

Overall, the simulation reproduces Haro 11’s morphology, luminosities, and spectral features, providing a physically motivated framework that links merger dynamics, star‑formation feedback, and radiative transfer to the observed escape of ionising and resonant photons. The authors discuss limitations—single merger configuration, reduced speed of light approximation, and lack of a surrounding CGM—but argue that the controlled setup offers valuable insight into how dwarf galaxy mergers at high redshift could have contributed to cosmic reionisation. Future work is suggested to explore a broader parameter space (mass ratios, orbital geometries) and to couple such high‑resolution merger simulations with cosmological environments to assess the cumulative impact on the ionising background.