Kavli Affiliate: Mark Vogelsberger
| First 5 Authors: Jessica Y. -C. Yeh, Aaron Smith, Rahul Kannan, Enrico Garaldi, Mark Vogelsberger
| Summary:
A fundamental requirement for reionizing the Universe is that a sufficient
fraction of the ionizing photons emitted by galaxies successfully escapes into
the intergalactic medium. However, due to the scarcity of high-redshift
observational data, the sources driving reionization remain uncertain. In this
work we calculate the ionizing escape fractions ($f_{rm esc}$) of
reionization-era galaxies from the state-of-the-art THESAN simulations, which
combine an accurate radiation-hydrodynamic solver AREPO-RT with the well-tested
IllustrisTNG galaxy formation model to self-consistently simulate both
small-scale galaxy physics and large-scale reionization throughout a large
patch of the universe ($L_{rm box} = 95.5,rm cMpc$). This allows the
formation of numerous massive haloes ($M_{rm halo} gtrsim 10^{10},{rm
M_{odot}}$), which are often statistically underrepresented in previous
studies but are believed to be important to achieve rapid reionization. We find
that low-mass galaxies ($M_{rm stars} lesssim 10^7,{rm M_{odot}}$) are the
main drivers of reionization above $z gtrsim 7$, while high-mass galaxies
($M_{rm stars} gtrsim 10^8,{rm M_{odot}}$) dominate the escaped ionizing
photon budget at lower redshifts. The variation in halo escape fractions
decreases for higher-mass haloes, which can be understood from the more settled
galactic structure, SFR stability, and fraction of sightlines within each halo
significantly contributing to the escaped flux. We show that dust is capable of
reducing the escape fractions of massive galaxies, but the impact on the global
$f_{rm esc}$ depends on the dust model. Finally, AGN are unimportant for
reionization in THESAN and their escape fractions are lower than stellar ones
due to being located near the centres of galaxy gravitational potential wells.
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