A submillimeter galaxy (SMG, or submillimeter-selected galaxy or dusty star-forming galaxy, i.e., DSFG) is a galaxy viewed mostly through submillimeter radiation. These include high redshift galaxies. Specifically, the 850 um flux density is generally greater than 3-5 millijanskies. They are very luminous because of star formation rather than because they are active galactic nucleus galaxies. The spectrum typically seen is black-body radiation from dust grains and emission lines from the gas in the interstellar medium.
An SMG is typically a massive starburst high-redshifted elliptical galaxy.