Stacking Redshifted 21cm Images of HII Regions Around High Redshift Galaxies as a Probe of Early Reionization. (arXiv:2007.13318v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Davies_J/0/1/0/all/0/1">James E Davies</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Croft_R/0/1/0/all/0/1">Rupert A. Croft</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Di_Matteo_T/0/1/0/all/0/1">Tiziana Di-Matteo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Greig_B/0/1/0/all/0/1">Bradley Greig</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Feng_Y/0/1/0/all/0/1">Yu Feng</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wyithe_S/0/1/0/all/0/1">Stuart Wyithe</a>
A number of current and future experiments aim to detect the reionization of
neutral hydrogen by the first stars and galaxies in the Universe via the
redshifted 21cm line. Using the textsc{BlueTides} simulation, we investigate
the measurement of an textit{average} ionised region towards the beginning of
reionization by stacking redshifted 21cm images around optically identified
bright galaxies using mock observations. We find that with an SKA 1000 hour
observation, assuming perfect foreground subtraction, a $5sigma$ detection of
a stacked HII region can be made with 30 images around some of the brightest
galaxies in textsc{bluetides} (brighter than $M_{UV} < -22.75$) at $z=9$
(corresponding to a neutral fraction of 90.1 % in our model). If the HII
regions are detected with high certainty, we can recover simulated
relationships between the UV magnitude of galaxies and the sizes of the ionised
regions they reside in. These mock observations can also distinguish between
scenarios where the IGM is in net emission or absorption of 21cm photons. Once
21cm foreground contamination is included, we find that even with up to 200
images around these rare, bright galaxies, only a tentative $> 1sigma$
detection will be possible. However, partial foreground subtraction
substantially improves signal-to-noise. For example, we predict that reducing
the area of Fourier space dominated by foregrounds by 50 (80) percent will
allow $> 3sigma$ ($> 5sigma$) detections of ionised regions at $z=9$.
A number of current and future experiments aim to detect the reionization of
neutral hydrogen by the first stars and galaxies in the Universe via the
redshifted 21cm line. Using the textsc{BlueTides} simulation, we investigate
the measurement of an textit{average} ionised region towards the beginning of
reionization by stacking redshifted 21cm images around optically identified
bright galaxies using mock observations. We find that with an SKA 1000 hour
observation, assuming perfect foreground subtraction, a $5sigma$ detection of
a stacked HII region can be made with 30 images around some of the brightest
galaxies in textsc{bluetides} (brighter than $M_{UV} < -22.75$) at $z=9$
(corresponding to a neutral fraction of 90.1 % in our model). If the HII
regions are detected with high certainty, we can recover simulated
relationships between the UV magnitude of galaxies and the sizes of the ionised
regions they reside in. These mock observations can also distinguish between
scenarios where the IGM is in net emission or absorption of 21cm photons. Once
21cm foreground contamination is included, we find that even with up to 200
images around these rare, bright galaxies, only a tentative $> 1sigma$
detection will be possible. However, partial foreground subtraction
substantially improves signal-to-noise. For example, we predict that reducing
the area of Fourier space dominated by foregrounds by 50 (80) percent will
allow $> 3sigma$ ($> 5sigma$) detections of ionised regions at $z=9$.
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