elliptical galaxies illustration
The evolutionary sequence in the growth of massive elliptical galaxies over 13 billion years (credit: NASA, ESA, S. Toft (Niels Bohr Institute), and A. Feild (STScI))

Astronomer David Sanders from the University of Hawaiʻi at Mānoa’s Institute for Astronomy, is one of a group of scientists who have combined observations made with the Hubble Space Telescope, the Spitzer and Herschel infrared space telescopes and ground-based telescopes in Hawaiʻi to assemble a coherent picture of the formation history of the most massive galaxies in the universe, from their initial burst of violent star formation through their appearance as high stellar-density galaxy cores and to their ultimate destiny as giant ellipticals.

This solves a decade-long mystery as to how compact elliptical-shaped galaxies that existed when the universe was only 3 billion years old, or one-quarter of its current age of 13.8 billion years, already had completed star formation. These compact ellipticals have now been definitively linked directly to an earlier population of dusty starburst galaxies that voraciously used up available gas for star formation very quickly. Then they grew slowly through merging as the star formation in them was quenched, and they eventually became giant elliptical galaxies.

“This is the first time anybody has put together a representative spectroscopic sample of ultra-compact, burned-out galaxies with the high quality of infrared imaging of Hubble,” said lead investigator Sune Toft of the Dark Cosmology Center at the Niels Bohr Institute in Copenhagen. Toft is currently on a six-week visit to the UH Mānoa Institute for Astronomy in Honolulu.

“We at last show how these compact galaxies can form, how it happened, and when it happened,” added Toft. “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. 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.”

Even more surprising, said Toft, is that these massive, burned-out galaxies were once extremely compact, compared to similar elliptical galaxies seen today in the nearby universe. This means that stars had to be crammed together 10 to 100 times more densely than seen in galaxies today. “It’s comparable to the densities of stars in globular clusters, but on the larger scale of a galaxy,” said Toft.

Their research, “Submillimeter Galaxies as Progenitors of Compact Quiescent Galaxies,” is being published in the Astrophysical Journal and is also available on the arXiv preprint server.

Read the UH Mānoa Institute for Astronomy news release for more information.

—By Louise Good