Kavli Affiliate: Mark Vogelsberger
| First 5 Authors: William McClymont, Sandro Tacchella, Aaron Smith, Rahul Kannan, Ewald Puchwein
| Summary:
Characterizing the evolution of the star-forming main sequence (SFMS) at high
redshift is crucial to contextualize the observed extreme properties of
galaxies in the early Universe. We present an analysis of the SFMS and its
scatter in the THESAN-ZOOM simulations, where we find a redshift evolution of
the SFMS normalization scaling as $propto (1+z)^{2.64pm0.03}$, significantly
stronger than is typically inferred from observations. We can reproduce the
flatter observed evolution by filtering out weakly star-forming galaxies,
implying that current observational fits are biased due to a missing population
of lulling galaxies or overestimated star-formation rates. We also explore
star-formation variability using the scatter of galaxies around the SFMS
($sigma_{mathrm{MS}}$). At the population level, the scatter around the SFMS
increases with cosmic time, driven by the increased importance of long-term
environmental effects in regulating star formation at later times. To study
short-term star-formation variability, or ”burstiness”, we isolate the
scatter on timescales shorter than 50 Myr. The short-term scatter is larger at
higher redshift, indicating that star formation is indeed more bursty in the
early Universe. We identify two starburst modes: (i) externally driven, where
rapid large-scale inflows trigger and fuel prolonged, extreme star formation
episodes, and (ii) internally driven, where cyclical ejection and re-accretion
of the interstellar medium in low-mass galaxies drive bursts, even under
relatively steady large-scale inflow. Both modes occur at all redshifts, but
the increased burstiness of galaxies at higher redshift is due to the
increasing prevalence of the more extreme external mode of star formation.
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