Kavli Affiliate: Mark Vogelsberger
| First 5 Authors: Yu Zhao, Yu Zhao, , ,
| Summary:
The timing of cosmic reionization across Local Group (LG) analogues provides
insights into their early histories and surrounding large-scale structure.
Using the radiation-hydrodynamic simulation THESAN-1 and its dark matter-only
counterpart THESAN-DARK-1, we track the reionization histories of all haloes,
including 224 LG analogues within the proximity of any of the 20 Virgo-like
clusters with halo masses above 10^14 Msun at z=0 and their environments. The
statistically controlled samples quantify how the reionization redshift
(z_reion) correlates with halo mass, local overdensity, and present-day pair
properties. Even at fixed mass, haloes in denser regions ionize earlier, and
increasing the overdensity smoothing scale systematically suppresses
small-scale structure, including local variations and environmental gradients
in z_reion. Virgo-like clusters accelerate reionization in their surroundings
out to ~5-10 cMpc, beyond which local overdensity again becomes the dominant
factor. Within LG pairs, reionization timing offsets reach up to ~150 Myr and
correlate with present-day halo separation, reflecting sensitivity to
large-scale structure rather than mass ratio in driving asynchronous
reionization. The results support an extreme inside-out picture where clustered
sources rapidly ionize their immediate neighborhoods, while lower-density
regions self-ionize later and voids wait for external homogenization. These
links between environment and reionization timing explain the influence of
protoclusters and help interpret fossil records in LG dwarfs around the Milky
Way. For Milky Way analogues, we find a reionization redshift as early (late)
as z_reion = 12.7^+2.0_-1.7 (8.88^+0.66_-0.70) when considered on 125
ckpc (500 ckpc) scales, with LG analogues following an inside-out reionization
picture.
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