  Three-dimensional panchromatic dust radiative transfer calculations are
performed on a set of 198 galaxies of stellar masses in the range
5x10^8-3x10^10 Msun from a cosmological hydrodynamic simulation (resolved at
29pc/h) at z=7. In a companion paper (Kimm & Cen), the stellar mass and UV
luminosity functions, and UV-optical and FUV-NUV colors are shown to be in good
agreement with observations, if an SMC-type dust extinction curve is adopted.
Here we make useful predictions, self-consistently, of the infrared properties
of these z=7 simulated galaxies that can be confronted with upcoming ALMA data.
Our findings are as follows. (1) The effective radius in the rest-frame MIPS 70
micron band is in the range of 80-400pc proper for z=7 galaxies with
L_FIR=10^{11.3-12}Lsun. (2) The median of the peak wavelength of the
far-infrared (FIR) spectral energy distribution is in the range of 45-60
micron, depending on the dust-to-metal ratio. (3) For star formation rate in
the range 3-100 Msun/yr the median FIR to bolometric luminosity ratio is
60-90%. (4) The FIR luminosity function displays a power law in the high end
with a slope of -3.1 +- 0.4, instead of the usual exponential decline.
