Observational tests of reionization


Our second key goal is to understand the inhomogeneous progression (topology) of the reionization process and devise observational strategies to detect this. 

1. We have recently published a paper entitled "Probing the fluctuating ultraviolet background using the Hubble Frontier Fields" (Choudhury & Dayal, 2019, MNRAS, 482, 19). In this work, we propose a proof-of-concept calculation for constraining the fluctuating ultra-violet (UV) background during reionization by constraining the faint-end slope (alpha) of the UV luminosity function (UV LF) in different volumes of the Universe. Because of patchy reionization, different volumes will experience different amount of photo-heating which should lead to a scatter in the measured alpha. Although current data is not sufficient to constrain alpha in different fields, we expect that, in the near future, observations of the six lensed Hubble Frontier Fields with the James Webb Space Telescope (JWST) will offer an ideal test of our concept.

The dependence of the faint-end slope (alpha) of the UV LF on the critical halo mass (Mcrit) below which galaxies are suppressed by reionization and the volume filling fraction of neutral hydrogen (QHI) for different redshifts. The black dashed curves in the three panels in the top row denote the allowed 1-sigma ranges in alpha obtained from the available observational data. Once alpha is measured with sufficient accuracy in different patches of the sky using JWST observations of the Hubble frontier Fields, and assuming Mcrit does not vary across fields, the scatter in the value of alpha from these fields would provide an ideal and direct test of patchy UV feedback at high-z.

2. Given the complexity of the problem, we are also using another approach to test the reionization topology using a class of galaxies called Lyman Alpha Emitters (LAEs) detected by means of their Lyman Alpha emission line. The sensitivity of this line to neutral hydrogen results in LAEs preferentially lying in ionized regions as we have shown in another work  

"Spectroscopic Investigation of a Reionized Galaxy Overdensity at z = 7" (Castellano et al. 2018, ApJ, 863, 3). We have been working on building a framework for combining LAE data with the neutral hydrogen (21cm) data expected from forthcoming facilities such as the Square Kilometre Array (SKA) as part of the SKA Epoch of Reionization synergy group. Our work focuses on using the LAE-21cm cross-correlation to shed light on the ionization state of the intergalactic medium at different redshifts. In our paper, entitled "Survey parameters for detecting 21-cm-Ly α emitter cross-correlations with the Square Kilometre Array" (Hutter, Trott & Dayal, 2018, MNRAS, 479, 129), we present survey strategies to optimise such a LAE-21cm cross-correlation. The key issue is that while the uncertainties in the 21cm signal detection are reduced by larger survey volumes, the shot noise arising from the finite number of LAEs decreases with increasing Lyman Alpha luminosity detection limits. At z~6.6, we find SILVERRUSH type surveys, with a field of view of 21 deg2 and survey luminosity limits of  >=7.9 × 1042 erg/s will be optimal to distinguish between an IGM that is 50 , 25 , and 10 per cent neutral.











21cm-LAE cross correlation function at at radius r=3.6/h cMpc for survey Lyman Alpha luminosity limits corresponding to 10^41-42 (faint), 42-43 (intermediate), 10^>43 erg/s (bright LAEs) at z~6.6. Orange, green and blue lines represent and IGM that is 50%, 20% and 10% neutral, respectively. The shaded regions show the cross correlation function uncertainties as a function of the survey volume of the SKA and LAE observations. As shown, at z~6.6, surveys with a field of view of 21 deg2 and survey luminosity limits of  >=7.9 × 1042 erg/s will be optimal to distinguish between an IGM that is 50 , 25 , and 10 per cent neutral. Naturally, smaller survey volumes can be compensated by probing to fainter Lyman Alpha luminosities.