Probing Cosmic Dawn: Modelling the Assembly History, SEDs, and Dust
  Content of Selected $z\sim9$ Galaxies
  The presence of spectroscopically confirmed Balmer breaks in galaxy spectral
energy distributions (SEDs) at $z>9$ provides one of the best probes of the
assembly history of the first generations of stars in our Universe. Recent
observations of the gravitationally lensed source, MACS 1149_JD1 (JD1),
indicate that significant amounts of star formation likely occurred at
redshifts as high as $z\simeq15$. The inferred stellar mass, dust mass, and
assembly history of JD1, or any other galaxy at these redshifts that exhibits a
strong Balmer break, can provide a strong test of our best theoretical models
from high-resolution cosmological simulations. In this work, we present the
results from a cosmological radiation-hydrodynamics simulation of the region
surrounding a massive Lyman-break galaxy. For two of our most massive systems,
we show that dust preferentially resides in the vicinity of the young stars
thereby increasing the strength of the measured Balmer break such that the
simulated SEDs are consistent with the photometry of JD1 and two other $z>9$
systems (GN-z10-3 and GN-z9-1) that have proposed Balmer breaks at high
redshift. We find strong variations in the shape and luminosity of the SEDs of
galaxies with nearly identical stellar and halo masses, indicating the
importance of morphology, assembly history, and dust distribution in making
inferences on the properties of individual galaxies at high redshifts. Our
results stress the importance that dust may play in modulating the observable
properties of galaxies, even at the extreme redshifts of $z>9$.
