ATLANTA, GEORGIA--The standard picture of how massive galaxies form is wrong. Astronomers assumed they grew over time through mergers of smaller pieces. If so, fat galaxies should have been rare in the early universe. But new results from the Gemini North telescope on Mauna Kea, Hawaii, indicate that galactic heavyweights were about 100 times more numerous than expected. "These results make theorists very, very uncomfortable," says team member Roberto Abraham of the University of Toronto.
The results of the Gemini Deep Deep Survey were announced here 5 January at a meeting of the American Astronomical Society. Led by Sandra Savaglio and Karl Glazebrook of Johns Hopkins University in Baltimore, Maryland, the team studied a few hundred galaxies at distances of some 10 billion light-years, looking back to a time when the universe was only about 4 billion years old. Surprisingly, many galaxies turned out to be not only larger and more massive than our own Milky Way, but more mature, judging by their colors and abundance of heavy elements. That indicates that they were born a mere 1 billion or 2 billion years after the big bang.
Astronomers have never been able to study normal galaxies in much detail in this early epoch of the universe. Spotting them is akin to finding a firefly's glow in broad daylight. The faraway galaxies' infrared signal gets lost in the much brighter infrared glow of Earth's atmosphere, says Glazebrook, who helped develop a novel technique to subtract the background and to study the faint galaxies spectroscopically. This enabled the team to deduce their distances, masses, and ages, albeit by using exposure times of 30 hours or so.
"The big surprise is that we're finding too many massive galaxies that formed too early," says team member Patrick McCarthy of the Carnegie Observatories in Pasadena, California. "There are some major aspects about the early lives of galaxies that we just don't understand." One possible solution, according to McCarthy, is that giant black holes played a more crucial role in "seeding" the formation of massive galaxies. Theorist Virginia Trimble of the University of Maryland, College Park, is confident that she and her colleagues will find a solution to the conundrum: "There's no need to lose any sleep over it."