Research Byte

Published in the RSAA Lunations
Vol1 Issue24 1–31 January 2022

 Emission-lines produced from nebula encode the clues of galaxy formation and evolution, as well as the ionization mechanisms of the Universe. Nebula modeling sits at the core of understanding the nebular emission-line behaviors, including emission-line fluxes, equivalent widths and line ratios. Nebula models created from a reliable photoionization code provide the accurate interpretation from the observational emission-line data to the theoretical physical quantities.

A successful photoionization code needs a comprehensive consideration of the physical conditions in nebula, like cooling and heating processes, ionization and recombination of atoms. The physical conditions of nebula are so complex that most nebula models assume a simple nebular geometry. Spherical and plane-parallel are two major simplified geometries applied by most nebula models. Researches based on these simplified nebula models unveiled many myths of the Universe, like disentangling the ionizing sources (stars, shocks and active galactic nuclei) in galaxies, tracing the heavy element abundance (oxygen and nitrogen) evolution across cosmic time, and estimating the ionizing photons escaping into the circumgalactic medium to ionize the Universe.

However, as the advent of the new observational technique with fine-resolution, complex nebular geometry becomes a significant factor in nebula modeling. We used the WiFeS instrument on the ANU 2.3m telescope to observe the highly spatially- resolved HII regions in the Large Magellanic Cloud and the Small Magellanic Cloud, finding the complex internal structures of HII regions that cannot be interpreted by either spherical or plane-parallel nebula models.

In order to provide the accurate nebula models in the realistic environment, we developed a photoionization code to model nebula with arbitrary geometries. The Messenger Interface Monte-Carlo Mappings V (M^3) is the photoionization code that we implement the Monte-Carlo radiative transfer technique in the well known

MAPPINGS V photoionization code. With M^3, we obtained nebula models with complex geometries and a comprehensive consideration of physical processes. M^3 is a promising tool for understanding the emission-line behaviors in the era of next generation observations and telescopes, like SDSS-V/LVM and JWST.

Yifei Jin

_{Figure1: Nebula with fractal geometry created by M^3}