Mystery of ultra-compact, burned-out galaxies solved

Astronomers using NASA's Hubble Space Telescope and Europe's Herschel Space Observatory have pieced together the evolutionary sequence of compact elliptical galaxies, which erupted and burned out when the universe was only 3 billion years old.

Sune Toft of the Dark Cosmology Center at the Niels Bohr Institute in Copenhagen, who is the leader of this study, said that they at last show how these compact galaxies can form, how it happened, and when it happened. This basically is the missing piece in the understanding of how the most massive galaxies formed, and how they evolved into the giant ellipticals of today.

He said that this had been a great mystery for many years because just 3 billion years after the big bang we see that half of the most massive galaxies have already completed their star formation.

Through the research, astronomers have determined the compact ellipticals voraciously consumed the gas available for star formation, to the point they could not create new stars, and then merged with smaller galaxies to form giant ellipticals.

The stars in the burned-out galaxies were packed 10 to 100 times more densely than in equally massive elliptical galaxies seen in the nearby universe today, and that surprised astronomers, according to Toft.

Toft's team assembled representative samples of two galaxy populations using the rich dataset in Hubble's COSMOS (Cosmic Evolution Survey) program.

One group of galaxies is the compact ellipticals. The other group contains galaxies that are highly obscured with dust and undergoing rapid star formation at rates thousands of times faster than observed in the Milky Way. Starbursts in these dusty galaxies likely were ignited when two gas-rich galaxies collided.

The results have been published online in The Astrophysical Journal.