Theoretically Modelling Photoionized Regions with Fractal Geometry in Three Dimensions

TL;DR

This paper develops a 3D fractal photoionization model of turbulent interstellar medium regions, revealing complex ionization structures and emphasizing the importance of geometry and turbulence for accurate modeling of nebulae and high-redshift galaxies.

Contribution

It introduces a novel 3D fractal photoionization model incorporating turbulence, improving understanding of nebular inhomogeneity and emission-line sensitivity.

Findings

Turbulence causes significant inhomogeneity in electron temperature and density.

Complex nebular geometry affects emission-line diagnostics.

Fractal models better represent high-redshift turbulent ISM regions.

Abstract

We create a photoionization model embedded in the turbulent ISM by using the state-of-the-art Messenger Monte-Carlo MAPPINGS~V code (M$^3$) in conjunction with the CMFGEN stellar atmosphere model. We show that the turbulent ISM causes the inhomogeneity of electron temperature and density within the nebula. The fluctuation in the turbulent ISM creates complex ionization structures seen in nearby nebulae. The inhomogeneous density distribution within the nebula creates a significant scatter on the spatially-resolved standard optical diagnostic diagrams, which cannot be represented by the spherical constant density photoionization model. We analyze the dependence of different optical emission lines on the complexity of nebular geometry, finding that the emission-lines residing on the nebular boundary are highly sensitive to the complexity of nebular geometry, while the emission-lines produced throughout the nebula are sensitive to the density distribution of the ISM within the nebula. Our fractal photoionization model demonstrates that a complex nebular geometry is required for accurate modeling of HII regions and emission-line galaxies, especially for the high-redshift galaxies, where the ISM is highly turbulent based on the increasing observational evidence.