  We use high-resolution zoom-in cosmological simulations of galaxies of
Romano-Diaz et al., post-processing them with a panchromatic three-dimensional
radiation transfer code to obtain the galaxy UV luminosity function (LF) at z ~
6-12. The galaxies are followed in a rare, heavily overdense region within a ~
5-sigma density peak, which can host high-z quasars, and in an average density
region, down to the stellar mass of M_star ~ 4* 10^7 Msun. We find that the
overdense regions evolve at a substantially accelerated pace --- the most
massive galaxy has grown to M_star ~ 8.4*10^10 Msun by z = 6.3, contains dust
of M_dust~ 4.1*10^8 Msun, and is associated with a very high star formation
rate, SFR ~ 745 Msun/yr.The attained SFR-M_star correlation results in the
specific SFR slowly increasing with M_star. Most of the UV radiation in massive
galaxies is absorbed by the dust, its escape fraction f_esc is low, increasing
slowly with time. Galaxies in the average region have less dust, and agree with
the observed UV LF. The LF of the overdense region is substantially higher, and
contains much brighter galaxies. The massive galaxies are bright in the
infrared (IR) due to the dust thermal emission, with L_IR~ 3.7*10^12 Lsun at z
= 6.3, while L_IR < 10^11 Lsun for the low-mass galaxies. Therefore, ALMA can
probe massive galaxies in the overdense region up to z ~ 10 with a reasonable
integration time. The UV spectral properties of disky galaxies depend
significantly upon the viewing angle.The stellar and dust masses of the most
massive galaxy in the overdense region are comparable to those of the
sub-millimetre galaxy (SMG) found by Riechers et al. at z = 6.3, while the
modelled SFR and the sub-millimetre flux fall slightly below the observed one.
Statistical significance of these similarities and differences will only become
clear with the upcoming ALMA observations.
