A pair of new studies shows that galaxies in the early universe matured more quickly than theorists expected, suggesting the conventional model of galaxy formation needs some serious tweaking.
Scientists have long held that large galaxies like our Milky Way formed as smaller collections of stars merged. The process should take several billion years, and in fact observations show it continues today as our galaxy is still swallowing smaller neighbors.
Until recently, observations of the distant universe have turned up only smaller, immature galaxies, supporting this "hierarchical" model of evolution.
Galaxies bigger than Milk Way
In one new study, Italian astronomers used the European Southern Observatory's Very Large Telescope to discover four galaxies several times more massive than the Milky Way. Each formed more than 12 billion years ago, when the universe was less than 2 billion years old, say the scientists, led by Andrea Cimatti of the Osservatorio Astrofisico di Arcetri in Florence, Italy.
When astronomers peer deeply into space, they also look back in time. An object that is 12 billion light-years away is seen as it existed 12 billion years ago, when its light was emitted.
Most efforts to look that far away and that far back in time spot only the brightest galaxies — those that are in the midst of intense star formation. Such galaxies are generally newly formed and not yet very massive. Mature and more massive galaxies have lower rates of star formation and are not as bright and therefore are harder to find, which probably explains why such objects haven't been detected before in the far corners of the cosmos, experts said.
In a broader survey with the Gemini North Telescope in Mauna Kea, Hawaii, a multinational team examined 300 galaxies dating back 8 billion to 11 billion years. A significant fraction — even among the most distant — were large and mature.
"We expected to find basically zero massive galaxies beyond about 9 billion years ago, because theoretical models predict that massive galaxies form last," said Karl Glazebrook, an astronomer at Johns Hopkins University. "Instead, we found highly developed galaxies that just shouldn't have been there, but are."
Deepening mystery
Both reports — akin to finding chickens in an incubator thought to contain just eggs -- are detailed in the July 8 issue of the journal Nature. They add significant insight to other recent hints that the early universe wasn't as scientists have long imagined.
A separate study in January revealed a "superstructure" of galaxies in the early universe that should also not have had time to form. And a recent Ultra Deep Field survey by the Hubble Space Telescope found a strange overload of star-forming galaxies shortly after the Big Bang and an utter void of galaxies in a slightly more modern epoch.
"There are obviously some aspects of the early lives of galaxies that we don't yet completely understand, Glazebrook said. "Some new ingredient is required to make more stars form earlier in the big galaxies. But what that ingredient is, we don't yet know."
"With this solid confirmation that as far back as 10 billion years ago there were already many old, massive galaxies, it is clear that even the best models can't fully explain the evolution of galaxies," said Gregory Wirth, a researcher at the W.M. Keck Observatory who wrote an analysis of the findings for the journal.