These old galaxies are no longer forming new stars. But the stars inside these galaxies are compacted in a very small area making the size of these SMGs around three times smaller than similar mass galaxies today. They also are not flattened like current spiral galaxies with a center but they are elliptical.
Researchers from the Niels Bohr Institute believe that they might have solved this mystery. They explain that these massive galaxies were formed by colliding galaxies that initiated star formation a few billion years after the Big Bang.
1 to 2 billion years after the Big Bang, they theorize that gas from early galaxies where driven into the center of the galaxy system which ignited to form new stars in the center making it compact. And because of the number of stars formed so quickly, the gas needed to form new stars are used up making it a dead galaxy.
In the image above, an extremely compact dead galaxy is compared to the size of the Milky Way. The two have about the same amounts of stars, which meant that the density of stars in the compact galaxies is more that 10 times higher than the stars in the Milky way.
