Spectral Energy Distribution of the First Galaxies: Contribution from
  Pre-Main-Sequence Stars
  One of the major goals of next-generation space-borne and ground-based
telescopes is to detect and characterize the first galaxies that were in place
in the first few hundred million years after the big bang. We study the
spectral energy distribution (SED) of the first galaxies and discuss the
prospects for detection and identification. We consider very young star-forming
galaxies at $z=15$ and incorporate the contribution from pre-main-sequence
(PMS) stars. Unlike in the present-day galaxies, primordial protostars are not
embedded in dusty gas clouds, and hence the light from them can be visible at a
wide range of wavelengths. We use MESA code to follow the PMS evolution and use
the BT-Settl model to calculate the SED of individual PMS stars. We show that
PMS stars contribute to boost the flux in the mid-infrared, and that the galaxy
SED at very early evolutionary phases is overall redder than at later phases.
The infrared flux contribution is comparable to that caused by emission lines
powered by massive stars. We argue that the contribution from PMS stars is
important for characterizing young galaxies in the early Universe and also for
target selection with future deep galaxy surveys.
