Astronomers combining the power of the Hubble Space Telescope, Spitzer and Herschel infrared space telescopes, and ground-based telescopes have assembled 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 Sune Toft of the Dark Cosmology Center at the Niels Bohr Institute in Copenhagen.
"We at last show how these compact galaxies can form, how it happened, and when it happened," Toft added. "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.
In tying together an evolutionary sequence for these compact massive galaxies, Toft identified their progenitors as highly dust-obscured galaxies undergoing rapid star formation at rates that are thousands of times faster than in our Milky Way galaxy. Starbursts in these galaxies are likely ignited when two gas-rich galaxies collided. These galaxies are so dusty that they are almost invisible at optical wavelengths, but are bright at submillimeter wavelengths, where they were first identified nearly two decades ago by the SCUBA (Submillimeter Common-User Bolometer Array) camera on the James Clerk Maxwell Telescope in Hawaii.
Toft's team assembled, for the first time, representative samples of the two galaxy populations using the rich dataset in Hubble's COSMOS (Cosmic Evolution Survey) program.
They constructed the first representative sample of compact quiescent galaxies with accurate sizes and distances (spectroscopic redshifts) measured from the Hubble Space Telescope's CANDELS (Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey) and 3D-HST programs. 3D-HST is a near-infrared Hubble spectroscopic survey to study the physical processes that shape galaxies in the distant universe. The astronomers combined these data with observations from the Subaru telescope in Hawaii and NASA's Spitzer Space Telescope. This allowed for accurate stellar age estimates, from which they concluded that galaxies formed in intense starbursts 1 billion to 2 billion years earlier, in the very early universe.
The team then made the first representative sample of the most distant submillimeter galaxies using the rich COSMOS data from the Hubble, Spitzer, and Herschel space telescopes, and ground-based telescopes such as Subaru, the James Clerk Maxwell Telescope, and the Submillimeter Array. This multi-spectral information, stretching from optical light through submillimeter wavelengths, yielded a full suite of information about the sizes, stellar masses, star-formation rates, dust content, and precise distances of the dust-enshrouded galaxies present early on in the universe.
When Toft's team compared the samples of these two galaxy populations, they discovered a link between the compact elliptical galaxies and the submillimeter galaxies observed 1 billion to 2 billion years earlier. The observations show that the violent starburst activity in the earlier galaxies had the same characteristics that would have been predicted for progenitors to the compact elliptical galaxies. The team also calculated that the intense starburst activity only lasted about 40 million years before the interstellar gas supply was exhausted.
Dark Cosmology Center, Niels Bohr Institute, Copenhagen, Denmark