Advances in the Study of Ionizing Radiation Escape in Galaxies:

Perspectives from Simulations and Observations

Bojin Chen

Santa Monica College, U.S.A.

Keywords: Ionization Radiation Escape, Galaxy Simulations, Escape Fraction.

Abstract: The study of ionizing radiation escape in galaxies is crucial for understanding galaxy evolution and cosmic

reionization. Ionizing radiation, produced by stars, affects the surrounding interstellar medium by ionizing

hydrogen atoms. The escape fraction (fesc) is a key parameter that measures the proportion of ionizing

photons that successfully escape, and it directly influences the ionization state of the universe. This study

summarizes advancements in simulation and observational techniques, enhancing the understanding of the

mechanisms behind ionizing radiation escape. High-resolution simulations (e.g., IllustrisTNG-50 and Thesan-

1), have revealed complex structures in hydrogen distribution within galaxies, highlighting the impact of

hydrogen ionization state, gas density, and dust content on the escape fraction. Advanced observational tools

like JWST and ALMA have provided direct measurements of escape fractions from high-redshift galaxies,

validating theoretical models. This research explores various physical factors that influence radiation escape,

analyses the limitations of current research, and discusses future research directions in this field, emphasizing

the importance of further refined simulations and more accurate observational data.

1 INTRODUCTION

In the past few decades, astronomers have

continuously deepened their study of the escape

mechanism of ionizing radiation in galaxies through

simulations and observations. Ionizing radiation

refers to high-energy photons that can ionize

hydrogen atoms, typically produced by stars within

galaxies, which directly affect the surrounding

interstellar medium (Cullen, et al., 2023; Haardt &

Madau, 2012). Understanding the escape fraction

(fesc) of ionizing radiation is key to studying galaxy

evolution because it is directly related to the ionizing

influence galaxies have on the cosmic environment

(Hopkins, et al., 2023).

Initially, research mainly focused on star

formation within galaxies and the distribution of the

interstellar medium. However, as simulation

technologies have advanced, scientists have gradually

discovered that the escape of ionizing radiation is not

only influenced by stellar activity and material

distribution within galaxies but also regulated by

many other complex factors. Contemporarily, with

the application of high-resolution simulations such as

IllustrisTNG-50, Thesan-1, FIRE-2, and the advent of

observational instruments like the James Webb Space

Telescope (JWST) and ALMA, scientists can more

deeply explore these complex processes (Osterbrock

& Ferland, 2006).

In recent years, the development of simulation

tools and observational facilities has provided new

perspectives for understanding the escape mechanism

of ionizing radiation in galaxies. For example, the

IllustrisTNG-50 simulation, through high-resolution

3D models, reveals the distribution structure of

hydrogen within galaxies, showing a "sponge-like"

structure where ionized hydrogen regions are

connected through narrow channels, which become

crucial pathways for the escape of ionizing radiation

(Springel, et al., 2005; Zhang, et al., 2021).

Similarly, the Thesan-1 simulation also provides

an in-depth analysis of the evolution of the hydrogen

ionization front during the cosmic reionization period,

pointing out that the "ionization halo" outside

galaxies is crucial for the escape of ionizing radiation.

Furthermore, the appearance of advanced

observational instruments such as JWST/NIRSpec

and ALMA has enabled us to directly measure the

escape fraction of high-redshift galaxies and verify

the accuracy of theoretical models. Naidu et al.

directly detected the leakage of the Lyman continuum

spectrum from the galaxy LEO-1 at z = 8.5 using

Chen, B.

Advances in the Study of Ionizing Radiation Escape in Galaxies: Perspectives from Simulations and Observations.

DOI: 10.5220/0013823300004708

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 2nd International Conference on Innovations in Applied Mathematics, Physics, and Astronomy (IAMPA 2025), pages 267-271

ISBN: 978-989-758-774-0

267

JWST/NIRSpec for the first time, revealing that the

escape fraction of this galaxy is as high as 25±5%

(Naidu, et al., 2018).

The motivation of this study is to explore in depth

the multiple factors that influence the escape of

ionizing radiation, particularly the ionization state of

hydrogen, gas density, dust content, and others. This

paper will combine existing simulation tools and

observational data to analyze how these factors

interact and jointly influence the escape fraction of

ionizing radiation in galaxies. Furthermore, the paper

will summarize the limitations of current research and

outline the future research directions in this field.

2 DESCRIPTIONS OF IONIZING

RADIATION IN GALAXIES

The process of ionizing radiation formation in

galaxies can be traced back to the birth and evolution

of stars within galaxies. When a star forms and burns

hydrogen, it releases large amounts of high-energy

photons, which can excite and eject electrons from

hydrogen atoms, thereby producing ionized hydrogen

(HⅡ). These ionizing photons can freely propagate

depending on their energy until they encounter

regions of neutral hydrogen (HⅠ), where they are

absorbed (Inoue, et al., 2020). Therefore, the

ionization state of hydrogen in galaxies directly

influences the escape of ionizing radiation (Pillepich,

et al., 2021).

3 SIMULATION TOOLS AND

OBSERVATION FACILITIES

3.1 Thesan Simulation System

The Thesan simulation system is one of the key tools

currently used to study the escape of ionizing

radiation (Kannan, et al., 2023). This system not only

simulates the transition of hydrogen from neutral to

ionized but also accurately depicts the dynamic

changes in the ionization front within galaxies. By

coupling dark matter, baryonic matter, and radiation

transfer, the Thesan simulation reveals the evolution

of the ionization front around galaxies, especially its

impact during the reionization period.

3.2 IllustrisTNG and FIRE-2

Simulations

The IllustrisTNG-50 simulation provides higher

resolution, revealing the "sponge-like" structure of

ionized hydrogen regions. Additionally, the FIRE-2

simulation, with its ultra-high spatial resolution,

precisely models the impact of supernova explosions

on the hydrogen distribution within galaxies,

providing important evidence for understanding the

variations in escape fractions across different galaxies.

3.3 Observational Facilities

For observational tools, JWST/NIRSpec and ALMA

are currently the most advanced instruments. JWST

is capable of capturing spectral features from distant

galaxies and directly measuring the escape fraction of

these galaxies. For example, through JWST/NIRSpec

observations, scientists have for the first time

detected the leakage of the Lyman continuum

spectrum from high-redshift galaxies, thereby

obtaining direct data on the escape fraction. ALMA,

on the other hand, is primarily used to study the

impact of dust on the escape of ionizing radiation by

observing specific molecular spectral lines, revealing

the influence of dust distribution in galaxies on

radiation propagation.

4 DETERMINATION ANALYSIS

Analysis has shown that there are four main factors

influencing the escape of ionizing radiation. Research

indicates a significant negative correlation between

gas density and escape fraction. Specifically, when

the density of neutral hydrogen in a galaxy increases,

the escape fraction rapidly decreases. This is because

higher gas density leads to more ionizing radiation

being absorbed by neutral hydrogen, reducing the

number of photons that escape. Particularly, when the

density exceeds 100 hydrogen atoms per cubic

centimeter, the escape fraction typically drops below

1%. The results are given in Fig. 1.

Figure 1: Escape Fraction as a Function of Gas Density

(Photo/Picture credit: Original).

IAMPA 2025 - The International Conference on Innovations in Applied Mathematics, Physics, and Astronomy

268

The impact of dust on ionizing radiation is

reflected in two aspects: absorption and scattering.

Dust particles can absorb ionizing radiation, reducing

its energy, or alter the propagation direction of

photons, thereby decreasing the escape fraction.

Studies have shown that when the dust-to-gas mass

ratio exceeds 0.3, the escape fraction significantly

decreases, especially when dust is concentrated in the

core regions of the galaxy, where its impact is even

more pronounced. The typical results are given in Fig.

2. The escape fraction of ionizing radiation shows a

clear evolutionary trend with cosmic time. During

high redshift (z ≈ 8), the escape fraction of galaxies

is typically higher than that of galaxies at low redshift.

This trend is closely related to factors such as lower

metallicity and dust content, thinner galaxy disks, and

stronger star formation feedback as depicted in Fig. 3.

The radiation transfer process has significant

characteristics of spatial and temporal changes as

presented in Fig. 4. On a time scale of tens of millions

of years, the escape fraction of a single galaxy may

fluctuate by 1-2 orders of magnitude. This fluctuation

mainly stems from three physical processes: the

instantaneous voids generated by supernova

explosions, the random distribution of star formation

regions, and the turbulent mixing effect of the

interstellar medium. The study also found that the

amplitude of this fluctuation is inversely proportional

to the mass of the galaxy, which explains why dwarf

galaxies usually exhibit a higher escape fraction.

Figure 2: Escape Fraction vs. Dust Content (Photo/Picture

credit: Original).

Figure 3: Redshift(z) (Photo/Picture credit: Original).

Figure 4: Radiation Transport Simulation (Photo/Picture

credit: Original).

5 LIMITATIONS AND

PROSPECTS

Despite the significant progress made in the study of

galactic ionizing radiation escape, scientists still face

several technical and theoretical challenges. The

research on ionizing radiation escape involves

multiple complex physical processes and a large

number of physical parameters, and the interwoven

influence of these factors makes a comprehensive

understanding difficult. The following will detail the

current limitations of the research and look forward

to future research directions.

Advances in the Study of Ionizing Radiation Escape in Galaxies: Perspectives from Simulations and Observations

269

5.1 The Simulation Challenges of

Small-Scale Physical Processes

Firstly, one of the major challenges faced by current

simulation techniques is the simulation of small-scale

physical processes. The process of ionizing radiation

escape involves very complex gas dynamics, star

formation, supernova feedback, and other processes

within galaxies, which often occur at very small

spatial scales. For instance, the distribution of

hydrogen is not uniform; hydrogen within galaxies

often exists in "clumpy" structures. These clumpy

hydrogen distributions have a significant impact on

the propagation of ionizing radiation. However, due

to the complexity of their structures, existing high-

resolution simulations still struggle to accurately

capture these small-scale physical processes.

Especially in regions with high gas density, ionizing

radiation is easily absorbed or scattered by neutral

hydrogen, leading to systematic errors in simulations.

Furthermore, many current simulation tools, such

as IllustrisTNG and Thesan-1, although capable of

providing high-resolution simulation results, still

have certain limitations when dealing with ultra-

small-scale physical processes. These small-scale

processes have a significant impact on the escape of

ionizing radiation, especially the strong feedback

effects produced by supernova explosions. Such

feedback can alter the density and temperature of gas,

thereby influencing the propagation path of radiation.

However, existing simulations still struggle to

precisely model these short timescale and small

spatial scale dynamic changes.

5.2 Observational Challenges of High-

Redshift Galaxies

Another major limitation stems from the constraints

of observational equipment. Although modern

astronomical devices, e.g., the JWST (James Webb

Space Telescope) and ALMA (Atacama Large

Millimeter/submillimeter Array), have significantly

enhanced our ability to observe distant high-redshift

galaxies, there are still certain challenges in capturing

the escape rate data of extremely distant galaxies.

Observing high-redshift galaxies is quite difficult,

partly because these galaxies are extremely far from

Earth, and the light signals are severely affected by

the redshift effect during transmission, leading to

signal attenuation. This signal attenuation not only

affects the measurement of the escape rate but also

makes it more difficult to obtain high-quality and

high-resolution spectral data.

Especially during the high-redshift period (z ≈ 8

and higher), the escape rate of galaxies is usually high,

but due to the weak spectral signals of these galaxies,

traditional optical telescopes and low-resolution

observation equipment find it difficult to directly

measure their escape rates. Therefore, current

observational techniques still cannot effectively

capture the detailed characteristics of the escaping

photons in these high-redshift galaxies, making data

quality and resolution the bottlenecks restricting

further research in this field.

5.3 Modelling Challenges of Dust

Effects

The role of dust in the escape of ionizing radiation is

a complex physical process. Although one has

relevant theoretical models that can roughly estimate

the impact of dust on the escape rate, the large

uncertainties in the spatial distribution and physical

properties of dust make precise modelling of dust

particularly difficult. The influence of dust is not only

reflected in the absorption of radiation but also in the

scattering effect on the direction of radiation

propagation, especially in the central regions of

galaxies where dust is dense and the scattering effect

is particularly significant. Existing studies mostly

rely on simplified assumptions, such as assuming that

the distribution of dust in galaxies is uniform, but the

actual situation is often much more complex.

Therefore, in the simulation of ionizing radiation

escape, the role of dust still requires further research

and precise modelling.

5.4 Inconsistencies Between Simulation

and Observation Result

Although existing simulation tools have made

significant progress, there are sometimes certain

deviations between these simulation results and

observational data. For instance, while the escape rate

of galaxies is usually predicted to be relatively low in

simulations, some observational results show that the

escape rate of some high-redshift galaxies is

significantly higher than predicted by simulations.

This inconsistency indicates that current theoretical

models and simulation tools may have overlooked

some important physical factors, and perhaps more

observational data are needed to verify these theories.

Additionally, the diversity of simulation results also

suggests that our understanding of the physical

mechanisms of ionizing radiation escape within

galaxies is not yet comprehensive.

IAMPA 2025 - The International Conference on Innovations in Applied Mathematics, Physics, and Astronomy

270

With the continuous advancement of simulation

technology and the advent of next-generation

astronomical equipment, one is expected to overcome

these limitations and achieve more precise and

comprehensive results in future research. Firstly, with

the improvement of computing power, future

simulation tools will be able to better handle the

simulation of small-scale physical processes,

especially the dynamic changes of important factors

such as hydrogen distribution and supernova

feedback. By improving sub-grid models and

conducting higher-resolution simulations, one can

more accurately capture these small-scale effects and

thereby reduce errors in simulations.

Secondly, with the gradual commissioning of

next-generation astronomical observation equipment,

this study will be able to obtain higher-quality

observational data. For instance, the future JWST will

provide more detailed high-redshift galaxy data,

enabling scientists to directly measure the escape rate

and verify the accuracy of theoretical models.

Additionally, radio telescopes like ALMA will

continue to help us study the impact of dust on

ionizing radiation, particularly the role of dust

distribution patterns within galaxies in the high-

redshift era on radiation escape.

In future research, one also needs to pay more

attention to the ionization effect of galaxies on their

surrounding environment during the cosmic

reionization process. By comprehensively utilizing

advanced simulation techniques and observational

data, one will be able to gain a deeper understanding

of the diversity of ionizing radiation escape

mechanisms and further promote the study of galaxy

evolution and the formation of large-scale structures

in the universe.

6 CONCLUSIONS

To sum up, this research summarizes the current

research progress on the escape of ionizing radiation

from galaxies, explores the key factors influencing

the escape rate, and analyses the limitations of current

studies. By integrating simulation tools and

observational data, one has gained a deeper

understanding of the roles of hydrogen ionization

state, gas density, dust content, and other factors in

the escape of ionizing radiation. Although significant

progress has been made in existing research, there are

still some technical and theoretical challenges,

especially in the simulation of small-scale physical

processes, dust impact modelling, and observations of

high-redshift galaxies. Future research will further

refine existing models and verify theoretical

assumptions through more precise observational data.

The study of ionizing radiation escape is not only

crucial for understanding galaxy evolution and the

reionization process of the universe but also provides

important clues for exploring the formation of large-

scale structures in the universe. With the continuous

development of simulation technology and the

advancement of astronomical observation equipment,

one has every reason to believe that the study of

ionizing radiation escape will achieve more in-depth

and accurate results in the future.

REFERENCES

Cullen, F., McLure, R. J., McLeod, D. J., et al., 2023. The

ultraviolet continuum slopes (β) of galaxies at z≃ 8-16

from JWST and ground-based near-infrared imaging.

Monthly Notices of the Royal Astronomical Society,

520(1), 14-23.

Haardt, F., Madau, P., 2012. UV Background and

Reionization. ApJ, 746(2), 125.

Hopkins, P. F., Wetzel, A., Kereš, D., et al., 2018. FIRE-2

simulations: physics versus numerics in galaxy

formation. Monthly Notices of the Royal Astronomical

Society, 480(1), 800-863.

Inoue, A. K., Hashimoto, T., Chihara, H., Koike, C., 2020.

Radiative equilibrium estimates of dust temperature

and mass in high-redshift galaxies. Monthly Notices of

the Royal Astronomical Society, 495(2), 1577-1592.

Naidu, R. P., Forrest, B., Oesch, P. A., Tran, K. V. H.,

Holden, B. P., 2018. A low Lyman Continuum escape

fraction of< 10 per cent for extreme [O iii] emitters in

an overdensity at z∼ 3.5. Monthly Notices of the Royal

Astronomical Society, 478(1), 791-799.

Kannan, R., Springel, V., Hernquist, L., Pakmor, R.,

Delgado, A. M., Hadzhiyska, B., ... & Garaldi, E.

(2023). The MillenniumTNG project: the galaxy

population at z≥ 8. Monthly Notices of the Royal

Astronomical Society, 524(2), 2594-2605.

Osterbrock, D. E., Ferland, G. J., 2006. Astrophysics of

Gaseous Nebulae. Sausalito: University Science Books.

Pillepich, A., Nelson, D., Truong, N., et al., 2021. X-ray

bubbles in the circumgalactic medium of TNG50 Milky

Way-and M31-like galaxies: signposts of supermassive

black hole activity. Monthly Notices of the Royal

Astronomical Society, 508(4), 4667-4695.

Springel, V., White, S. D., Jenkins, A., et al., 2005.

Simulations of the formation, evolution and clustering

of galaxies and quasars. nature, 435(7042), 629-636.

Zhang, M., An, Z., Wang, L., et al., 2021. The regulation

effect of methane and hydrogen on the emission

characteristics of ammonia/air combustion in a model

combustor. International Journal of Hydrogen

Energy, 46(40), 21013-21025.

Advances in the Study of Ionizing Radiation Escape in Galaxies: Perspectives from Simulations and Observations

271