Tomáš, Šoltinský
(2023)
Simulating neutral hydrogen across cosmic time: the 21-cm forest in late reionization models and the global 21-cm signal at cosmic dawn.
PhD thesis, University of Nottingham.
Abstract
The 21-cm line has a potential to be a powerful probe of the intergalactic medium (IGM) during the Cosmic Dawn and the Epoch of Reionization. Various observatories such as LOFAR and SKA aim to observe this line. To aid these observational efforts, I use cosmological simulations drawn from the Sherwood-Relics programme in which a novel hybrid approach of coupling hydrodynamical simulations with radiative transfer is implemented to model the 21-cm forest during the Epoch of Reionization and sky-averaged 21-cm signal during Cosmic Dawn.
Recent Lya line observations can be explained by reionization which is completed at z_R~5.3 (i.e. later than previously thought). In this scenario, large islands of neutral hydrogen are expected to persist in the diffuse IGM until z~6. In this context, I predict the incidence of strong 21-cm forest absorbers (tau_21>10^-2) and find that if the IGM is not pre-heated above ~10^2K, the 21-cm forest should be detectable at redshift as low as z~6. I consider the effect of the pressure smoothing arising from the patchiness of reionization, redshift space distortions, Lya coupling and soft X-ray background pre-heating of the IGM on the observability of the 21-cm forest signal. While the pressure smoothing affects the 21-cm forest signal only modestly, inclusion of redshift space distortion increases the largest tau_21 by up to a factor of ~10. In addition, the soft X-ray background can completely suppress the signal. However, a null detection of strong 21-cm forest absorbers at z~6 in the spectra of ~10 sufficiently radio-bright background sources with SKA1-low and possibly LOFAR can provide informative, model-dependent lower limits on the soft X-ray background at high redshift.
Furthermore, I model the effect of quasar radiation on the ionization and thermal state of the IGM surrounding it. The gas in the proximity of a quasar is ionized enough such that it becomes transparent for Lya photons. The size of this Lya transmission window, the Lya near-zone, has been used to inform us about the quasar optical/UV bright lifetime. The majority of measurements are consistent with lifetimes of ~10^5-10^7yr, however, the smallest Lya near-zones suggest lifetimes shorter than ~10^4yr. Such short lifetimes pose a challenge for the growth of black holes with masses of ~10^9M_solar at z>6. Models of black holes accreting in an obscured phase or quasars with time-varying spectrum (flickering) have been proposed to alleviate this problem. However, it is challenging to discern between young quasars and old quasars driven by episodic accretion with the Lya forest alone. Motivated by this, I model the 21-cm forest absorption in the vicinity of z>6 radio-loud quasars. The distance between a quasar and the closest 21-cm forest absorber with tau_21>10^-2 is sensitive to the heating of the IGM by quasar X-rays. Hence, the extent of the proximate 21-cm forest is sensitive to the integrated quasar lifetime because of a very long gas cooling time. I find that a detection of a strong 21-cm forest absorber within ~3pMpc of a quasar would suggest quasar lifetime of <10^5yr. On the other hand, a measurement of large distance between a quasar and the closest strong 21-cm forest absorber in combination with a small Lya near-zone would be consistent with a quasar that is old and flickering.
Finally, I study the effect of IGM density fluctuations on the amplitude of the cosmic dawn absorption feature in the sky-averaged 21-cm spectrum. The density distribution of the IGM is extracted from Sherwood-Relics simulations. I find that the density fluctuations suppress the signal by 6.4-11.3% depending on the mass resolution of the simulation. Higher mass resolution of the simulation results in larger suppression of the signal. This effect makes the discrepancy between standard theoretical models and the EDGES measurement even larger.
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