Abstract
Patchy cosmic reionization resulted in the ionizing UV background
asynchronous rise across the Universe. The latter might have left imprints
visible in present day observations. Several numerical simulation-based studies
show correlations between reionization time and overdensities and object masses
today. To remove the mass from the study, as it may not be the sole important
parameter, this paper focuses solely on the properties of paired halos within
the same mass range as the Milky Way. For this purpose, it uses CoDaII, a
fully-coupled radiation hydrodynamics reionization simulation of the local
Universe. This simulation holds a halo pair representing the Local Group, in
addition to other pairs, sharing similar mass, mass ratio, distance separation
and isolation criteria but in other environments, alongside isolated halos
within the same mass range. Investigations of the paired halo reionization
histories reveal a wide diversity although always inside-out given our
reionization model. Within this model, halos in a close pair tend to be
reionized at the same time but being in a pair does not bring to an earlier
time their mean reionization. The only significant trend is found between the
total energy at z = 0 of the pairs and their mean reionization time: pairs with
the smallest total energy (bound) are reionized up to 50 Myr earlier than
others (unbound). Above all, this study reveals the variety of reionization
histories undergone by halo pairs similar to the Local Group, that of the Local
Group being far from an average one. In our model, its reionization time is
~625 Myr against 660+/-4 Myr (z~8.25 against 7.87+/-0.02) on average.
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