Sub-millimeter brightness of early star-forming galaxies
  Based on a three-dimensional model of an early star-forming galaxy, we
explore the evolution of the sub-millimeter brightness. The model galaxy is
employed from an ultra-high-resolution chemodynamic simulation of a primordial
galaxy by Mori & Umemura, where the SFR is ~ 10 Msun/yr at t<0.3 Gyr and
several Msun/yr at t>0.3 Gyr. The former phase well reproduces the observed
properties of LAEs and the latter does LBGs. We solve the three-dimensional
radiative transfer in the clumpy interstellar media in this model galaxy,
taking the size distributions of dust grains into account, and calculate the
dust temperature as a function of galactic evolutionary time. We find that the
clumpiness of interstellar media plays an important role for the sub-millimeter
brightness. In the LAE phase, dust grains are concentrated on clumpy
star-forming regions that are distributed all over the galaxy, and the grains
can effectively absorb UV radiation from stars. As a result, the dust is heated
up to T>35 K. In the LBG phase, the continuous supernovae drive dust grains far
away from star-forming regions. Then, the grains cannot absorb much radiation
from stars, and becomes into a cold state close to the CMB temperature.
Consequently, the dust temperature decreases with the evolutionary time, where
the mass-weighted mean temperature is T=26 K at t=0.1 Gyr and T=21 K at t=1.0
Gyr. By this analysis, it turns out that the sub-millimeter brightness is
higher in the LAE phase than that in the LBG phase, although the dust-to-gas
ratio increases monotonically as a function of time. We derive the spectral
energy distributions by placing the model galaxy at a given redshift. The peak
flux at 850 micron is found to be S_850 ~ 0.2 - 0.9 mJy if the model galaxy is
placed at 6>z>2. This means that ALMA can detect an early star-forming galaxy
with SFR of ~ 10 Msun/yr by less than one hour integration with 16 antennas.
