  We perform 3-D dust radiative transfer (RT) calculations on hydrodynamic
simulations of isolated and merging disk galaxies in order to quantitatively
study the dependence of observed-frame submillimeter (submm) flux density on
galaxy properties. We find that submm flux density and star formation rate
(SFR) are related in dramatically different ways for quiescently star-forming
galaxies and starbursts. Because the stars formed in the merger-induced
starburst do not dominate the bolometric luminosity and the rapid drop in dust
mass and more compact geometry cause a sharp increase in dust temperature
during the burst, starbursts are very inefficient at boosting submm flux
density (e.g., a $\ga16$x boost in SFR yields a $\la 2$x boost in submm flux
density). Moreover, the ratio of submm flux density to SFR differs
significantly between the two modes; thus one cannot assume that the galaxies
with highest submm flux density are necessarily those with the highest
bolometric luminosity or SFR. These results have important consequences for the
bright submillimeter-selected galaxy (SMG) population. Among them are: 1. The
SMG population is heterogeneous. In addition to merger-driven starbursts, there
is a subpopulation of galaxy pairs, where two disks undergoing a major merger
but not yet strongly interacting are blended into one submm source because of
the large ($\ga 15$", or $\sim 130$ kpc at $z = 2$) beam of single-dish submm
telescopes. 2. SMGs must be very massive ($M_{\star} \ga 6 \times 10^{10}
\msun$). 3. The infall phase makes the SMG duty cycle a factor of a few greater
than what is expected for a merger-driven starburst. (Abridged.)
