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The presence of spectroscopically confirmed Balmer breaks in galaxy spectralenergy distributions (SEDs) at $z>9$ provides one of the best probes of theassembly history of the first generations of stars in our Universe. Recentobservations of the gravitationally lensed source, MACS 1149_JD1 (JD1),indicate that significant amounts of star formation likely occurred atredshifts as high as $z\simeq15$. The inferred stellar mass, dust mass, andassembly history of JD1, or any other galaxy at these redshifts that exhibits astrong Balmer break, can provide a strong test of our best theoretical modelsfrom high-resolution cosmological simulations. In this work, we present theresults from a cosmological radiation-hydrodynamics simulation of the regionsurrounding a massive Lyman-break galaxy. For two of our most massive systems,we show that dust preferentially resides in the vicinity of the young starsthereby increasing the strength of the measured Balmer break such that thesimulated 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 highredshift. We find strong variations in the shape and luminosity of the SEDs ofgalaxies with nearly identical stellar and halo masses, indicating theimportance of morphology, assembly history, and dust distribution in makinginferences on the properties of individual galaxies at high redshifts. Ourresults stress the importance that dust may play in modulating the observableproperties of galaxies, even at the extreme redshifts of $z>9$.

Probing Cosmic Dawn: Modelling the Assembly History, SEDs, and Dust Content of Selected $z\sim9$ Galaxies