Simulating the 21-cm signal from the Epoch of Reionization

Abstract

In this thesis, the 21-cm signal from the epoch of reionization is simulated using a combination of N-Body techniques and semi-analytical models. Images, line-of-sight spectra and power spectra of the simulated 21-cm signal are presented. The N-Body code, GADGET-2, is used to obtain the distribution of dark matter and a friends-of-friends algorithm is used to identify dark matter halos to construct halo merger trees. We ran seven simulations with periodic boxes of volumes ranging from (10h(^-1)Mpc)(^3) to (140h(^-1)Mpc)(^3), and particle masses ranging from 6.46 x lO(^4)h(^-1)M(_ʘ) to 1.42 X 10(^9) h(^-1)M(_ʘ). These merger trees are used with the GALFORM semi-analytical model of galaxy formation to predict the locations of galaxies and their ionizing luminosities within the halos. We find that halos in a broad range of masses contribute significantly to the total ionizing emissivity of each simulation. The effect of suppressing gas cooling due to reionization in low mass halos in GALFORM was also investigated. For a redshift of reionization, z(_reion) = 15, this significantly reduces the number of ionizing photons produced at z < 15. This results in a prolonged period of reionization as the Universe makes the transition from neutral to fully ionized. Next, the 21-cm signal and ionization state of the hydrogen is calculated for the simulation box with a volume of (20h(^-1)Mpc)(^3) and a particle mass of 5.17 x 10(^5)h(^-1)M(_ʘ). The hydrogen is assumed to follow the dark matter, and the 21-cm differential brightness temperature is calculated from the distribution of neutral hydrogen. High resolution images and spectra of the 21-cm signal are generated from these neutral hydrogen density fields. A toy model is first investigated, randomly distributed ionized spheres of constant radii are generated to investigate the effect ionized regions have on the 21-cm power spectra. We find that this increases power on the scales of the size of the ionized spheres, but decreases power on small scales. The amount of change in power is sensitive to the overall ionized fraction of the simulation box, increasing the ionized fraction increases the power on the scales of the spheres while decreases the power on small scales. Next, the GALFORM model is investigated. The ionizing luminosities of GALFORM galaxies are used to calculate cosmological Strömgren spheres representing ionized regions. These spheres are "painted" around the location of the galaxies on the density field. We find that the power spectra lack any obvious features due to the range of sizes of the GALFORM ionized regions. However, the power spectra is sensitive to the ionized fraction of the simulation box. The slope of the power spectra decreases as ionized fraction increases. Finally, the temperature field is smoothed over a bandwidth of 200 kHz and a 2-D Gaussian beam with a FWHM of 3 arcmin to emulate the LOFAR telescope beam response. We find that only the largest features, such as large ionized regions, are still identifiable after smoothing. LOFAR should be able to detect the early stages of reionization, but may have difficulties during later stages when much of the brightest 21-cm signal is removed by reionization.