  In this paper, we demonstrate that the inference of galaxy stellar masses via
spectral energy distribution (SED) fitting techniques for galaxies formed in
the first billion years after the Big Bang carries fundamental uncertainties
owing to the loss of star formation history (SFH) information from the very
first episodes of star formation in the integrated spectra of galaxies. While
this early star formation can contribute substantially to the total stellar
mass of high-redshift systems, ongoing star formation at the time of detection
outshines the residual light from earlier bursts, hampering the determination
of accurate stellar masses. As a result, order of magnitude uncertainties in
stellar masses can be expected. We demonstrate this potential problem via
direct numerical simulation of galaxy formation in a cosmological context. In
detail, we carry out two cosmological simulations with significantly different
stellar feedback models which span a significant range in star formation
history burstiness. We compute the mock SEDs for these model galaxies at z=7
via 3D dust radiative transfer calculations, and then backwards fit these SEDs
with Prospector SED fitting software. The uncertainties in derived stellar
masses that we find for z>7 galaxies motivate the development of new techniques
and/or star formation history priors to model early Universe star formation.
