Studies of the intergalactic medium during the Epoch of Reionization : Understanding observational probes with simulations

• The first billion years of the Universe is a unique era, marked by the formation of the first stars, galaxies, and accreting black holes, which release ionising radiation into the intergalactic medium (IGM). As a result, these luminous sources initiate a period during which the cold and dense IGM, primarily consisting of neutral hydrogen (HI), is heated and ionised. We refer to this era as the Epoch of Reionization (EoR). The EoR is a global phase transition that is not trivial to observe or model computationally. It is a multi-scale event that evolves with time and depends on the nature of the astrophysical processes that govern the formation of stars and galaxies, as well as the fundamental cosmology that defines the properties of the large-scale IGM. While various measurements of cosmic reionization exist, presently they are too few to constrain the entirety of the process. However, observations from the James Webb Space Telescope and the Square Kilometre Array (SKA), among others, will provide new insight into the process. Particularly, the SKA will observe the power spectrum (PS) of the 21 cm signal from the EoR, which originates from the hyperfine transition of neutral hydrogen atoms HI in the IGM that can emit 21 cm photons. In Paper I, we investigate the evolution of the 21 cm PS across the EoR by perturbing the signal and studying its composing terms. We highlight the importance higher-order terms play in shaping the PS on large scales and quantify its evolution. Crucially, we find a characteristic length scale within the 21 cm PS, determined by the mean free path ionising photons travel in the IGM (MFP). Hence, the 21 cm PS has two regimes. We show that the large-scale signal is a biased version of the cosmological density field, and the small-scale PS is determined by the astrophysics of reionization. In Paper II, we use the decomposition of the 21 cm PS and relate it to the PS of the free electron density field. Thus, we analytically connect the 21 cm observable to a probe of the free electron density field. Such a probe is the patchy kinetic Sunyaev-Zel'dovich effect (pkSZ), observed as a foreground to the primary cosmic microwave background temperature anisotropies on small scales. The pkSZ is an integrated probe sensitive to the duration of the EoR and the characteristic size of ionised bubbles. We construct a forecast study of both probes. We show that inferences from 21 cm PS from the SKA can be verified when combined with the pkSZ observation, as each data set is influenced by different systematics. In Paper III, we focus on the modelling of the MFP within large-scale simulations, focusing on the end of reionization (EndEoR). The MFP of ionising photons is inferred from quasar data and depends on several factors. In the post-EoR era, it depends on the distribution and evolution of Lyman Limit systems (LLS), small-scale absorbers that are typically not resolved in large-scale simulations. We investigate the assumptions needed to accurately model the LLS in simulations, and we study their impact on the observables at the EndEoR. We find that LLS modelling has a profound impact on the duration of the final stages of the EoR, the shape of the 21 cm PS as well as other observables of the ionised IGM inferred from quasar spectra, such as the Ultraviolet Background of ionising photons, the effective optical depth of Lyman alpha photons, and the MFP of ionising photons.

Subject headings

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Keyword

cosmology

theory

large-scale structure of Universe

reionization

first stars

first galaxies

Astronomy

astronomi

Publication and Content Type

vet (subject category)

dok (subject category)