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Minicurso III: The interstellar medium in galaxies: Monte Carlo radiation transfer and the impact of ionising radiation in the ISM

Minicurso ministrado pelo Prof. Dr. Bert Vandenbroucke da Ghent University (Belgium).

Resumo: HII regions or photoionisation regions are among the most colourful regions that we can probe with telescopes that operate in the visual part of the spectrum, think e.g. about the beautiful images the Hubble space telescope has produced. The light that reaches us from these regions is mostly emitted at very specific wavelengths from so called emission lines: among the brightest ones are the H-alpha line at 656 nm, the “forbidden” SII line at 673 nm, and the forbidden NII line at 658 nm (these wavelengths correspond to different shades of red). These lines are emitted when electrons in excited states of neutral hydrogen and ionised nitrogen and sulphur deexcite by emitting a photon. The electrons end up in these excited states during collisional recombination, when an ionised hydrogen ion or a double ionised nitrogen/sulphur ion recombines with a free electron that gradually cascades down through the energy levels of the atom. This is only possible in the presence of a sufficiently powerful ionising radiation field that provides a continuous reservoir of ions and free electrons.
Due to the complexity of this whole process – the ionisation of the hydrogen, nitrogen and sulphur atoms by an incident radiation field, and the subsequent emission of emission during collisional recombination – emission lines provide a wealth of information, not only about the composition, temperature and density of the interstellar medium (ISM), but also about the strength and spectral properties of the ionising radiation field and the large scale geometry of the ISM. Unfortunately, this also makes it incredibly challenging to model these emission lines.
During these lectures, I will explain how we can model these processes in detail using 3D Monte Carlo radiation transfer techniques, and apply these techniques to study the structure and properties of the ionised ISM in galaxies.