Cosmological Reionisation

Here we investigate how LyC-opaque systems present in the intergalactic medium at z≈3 can distort the spectral shape of a uniform UV background (UVB) through radiative transfer (RT) effects. With this aim in mind, we perform a multi-frequency RT simulation through a cosmic volume of 10h−1~cMpc scale polluted by metals, and self-consistently derive the ions of all the species. The UVB spatial fluctuations are traced by the ratio of HeII  and HI  column density, η, and the ratio of CIV  and SiIV  optical depths, ζ. We find that: (i) η spatially fluctuates through over-dense systems (Δ) with statistically significant deviations δη>25\% in 18\% of the volume ; (ii) same fluctuations in ζ are also present in 34\% of the enriched domain (only 8\% of the total volume) and derive from a combination of RT induced effects and in-homogeneous metal enrichment, both effective in systems with Δ>1.5.

The detection of ionized bubbles around quasars in redshifted 21-cm maps is possibly one of the most direct future probes of reionization. We consider two models for the growth of spherical ionized bubbles to study the apparent shapes of the bubbles in redshifted 21-cm maps, taking into account the finite light travel time (FLTT) across the bubble. We find that the FLTT, whose effect is particularly pronounced for large bubbles, causes the bubble's image to continue to grow well after it's actual growth is over. There are two distinct FLTT distortions in the bubble's image: (i) its apparent center is shifted along the line of sight (LOS) towards the observer from the quasar; (ii) it's shape is anisotropic along the LOS. The bubble initially appears elongated along the LOS. This is reversed in the later stages of growth where the bubble appears compressed. The FLTT distortions are expected to have an impact on matched filter bubble detection where it is most convenient to use a spherical template for the filter. We find that the best matched spherical filter gives a reasonably good estimate of the size and the shift in the center of the anisotropic image. The mismatch between the spherical filter and the anisotropic image causes a 10 - 20% degradation in the SNR relative to that of a spherical bubble. We conclude that a spherical filter is adequate for bubble detection. The FLTT distortions do not effect the lower limits for bubble detection with 1000 hr of GMRT observations. The smallest spherical filter for which a detection is possible has comoving radii 24 Mpc and 33 Mpc for a 3-sigma and 5-sigma detection respectively, assuming a neutral fraction 0.6 at z \sim 8.