A sweeping census of galaxies dating back to when the universe was just 2.5 billion years old shows an early and puzzling divide between two main types -- the relatively flat, pancake-shaped galaxies that continue to pop out new stars and their spherical cousins which are filled with old stars that have remained virtually unchanged through the eons.
Whatever shut down star formation in those galaxies is unknown, but the new study, based on the single largest Hubble Space Telescope project, indicates it happened early and fast.
“For the first time we have access to very large samples of distant galaxies, observed with data that is very high quality,” astronomer Mauro Giavalisco, with the University of Massachusetts, told Discovery News.
“It’s pretty conclusive that the diversification of galaxies into this spheroidal-type and the disk-type is a process that started very early. It was already there in place at the time when the universe was a mere 2.5 billion years old. It's something that happened very quickly,” he said.
It was astronomer Edwin Hubble, for whom the space telescope is named, who first classified galaxies into two main types.
Scientists later fleshed out Hubble’s 1926 landmark work with additional details showing the two basic types of galaxies not only had different structures, but also different stellar populations.
Computer models have pretty much nailed down how big clouds of dust and gas can collapse under their own weight and pancake themselves into disks. These structures eventually feed a central region that ends up looking like a bulge. Our Milky Way galaxy is an example.
More puzzling is what happens in the other types of galaxies, the spheroids, or elliptical galaxies.