(Sub)millimetre interferometric imaging of a sample of COSMOS/AzTEC
  submillimetre galaxies IV. Physical properties derived from spectral energy
  distributions
  We characterise the physical nature of a 1.1 mm-selected, flux-limited, and
interferometrically followed up sample of SMGs in COSMOS. We used the MAGPHYS
code to fit the multiwavelength (UV-radio) SEDs of 16 of the target SMGs. We
also constructed the pure radio SEDs of our SMGs using three different radio
bands (325 MHz, 1.4 GHz, and 3 GHz). Moreover, since two SMGs in our sample,
AzTEC1 and AzTEC3, benefit from previous CO line observations, we studied their
properties in more detail. We found that 63% of our target SMGs lie above the
galaxy main-sequence by more than a factor of 3, and hence are starbursts. The
3 GHz radio sizes we have previously measured for the target SMGs were compared
with the present stellar mass estimates, and we found that the z>3 SMGs are
fairly consistent with the mass-size relationship of z~2 compact, quiescent
galaxies (cQGs). The median IR-radio correlation parameter is found to be
q=2.27, which is lower than measured locally (median q=2.64). AzTEC1 is found
to have a sub-Eddington SFR surface density (by a factor of 2.6), while AzTEC3
appears to be an Eddington-limited starburster. The gas reservoir in these two
high-z SMGs would be exhausted in only ~86 and 19 Myr at the current SFR,
respectively. A comparison of the MAGPHYS-based properties of our SMGs with
those of equally bright ALESS SMGs suggests that the two populations share
fairly similar physical characteristics, including the q parameter. A hint of
negative correlation is found between the 3 GHz size and the level of
starburstiness, and hence cosmic-ray electrons in more compact starbursts might
be more susceptible to free-free absorption. Some of the derived low and high q
values (compared to the local median) could be the result of a specific
merger/post-starburst phase of galaxy evolution. Overall, our results support
the scenario where z>3 SMGs evolve into today's giant ellipticals.
