Abstract
Lyman-alpha (Lyα) emission is a powerful tracer of the circumgalactic medium (CGM) surrounding galaxies, while also tracing regions of star formation. Observations reveal extended Lyα halos around galaxies, but interpreting these observations is challenging because Lyα photons undergo resonant scattering in neutral hydrogen gas. Because of this, the observed emission can become spatially and spectrally extended far from the original Lyα production sites. Modeling Lyα emission in simulations therefore requires both Lyα radiative transfer and the ionization state of hydrogen in the interstellar medium (ISM) and CGM. Many cosmological simulations do however not include on-the-fly ionizing radiative transfer, which leads to inaccurate distributions of neutral and ionized hydrogen. This incorrect distribution affects the Lyα luminosities, spectra, and surface brightness profiles, making the simulations less accurate.
In this thesis, we analyze galaxy halos from the Romulus25 [Michael Tremmel et al. 2017] cosmological simulation, which have been post-processed by ionizing radiative transfer code RAMSES-RT [Rosdahl et al. 2013]. The post-processing recomputes both the thermal and ionized state of the gas using stellar radiation and the ultraviolet background (UVB). The analysis in this thesis is done through radiative transfer code RASCAS [Michel-Dansac et al. 2020], to generate both intrinsic and scattered Lyα emission, before and after radiative transfer post-processing. The luminosity maps, spectra and surface brightness profiles are compared to observations from the Lyman Alpha Reference Sample (LARS).
We find that ionizing radiative transfer post-processing significantly alters the neutral and ionized hydrogen distribution in the gas. Dense gas cools towards 104 K, and can become more neutral through self-shielding and recombination. Lower density gas in the CGM remains ionized. Lyα emission images of recombination and collisional excitation emission reveal how post-processing affects the emission origin of Lyα photons in the ISM and CGM. Comparing with LARS reveals we are missing emission in the central part of our surface brightness profiles, likely due to unresolved ISM structure and insufficiently dense star-forming regions.
We also find that the resonant scattering of Lyα emission spatially propagates and re-distributes Lyα emission into more extended halos, away from their production sites. In the process of scattering, observed Lyα spectra broaden due to photons having to shift into neighboring wavelengths to escape high optical depth HI regions.
Supervisor: Sijing Shen, Professor at Institute of Theoretical Astrophysics, UiO
Intern. assessor: Tiago Pereira, Professor at Institute of Theoretical Astrophysics, UiO
Extern. assessor: Jens Melinder, Senior researcher at Stockholm University