Astronomers revise galaxy-formation models with the discovery that early galaxies could have grown fat - fast.

Slurping up cold streams of star fuel, some of the Universe's first galaxies got fat quickly, new observations suggest. The findings could overturn existing models for the formation and evolution of galaxies that predict their slow and steady growth through mergers.

Researchers using the Subaru telescope in Hawaii have identified five distant galaxy clusters that formed five billion years after the Big Bang. They calculated the mass of the biggest galaxy in each of the clusters and found, to their surprise, that the ancient galaxies were roughly as big as the biggest galaxies in equivalent clusters in today's Universe.

The ancient galaxies should have been much smaller, at only a fifth of today's mass, based on galaxy-formation models that predict slow, protracted growth. "That was the reason for the surprise - that it disagrees so radically with what the predictions told us we should be seeing," says Chris Collins of Liverpool John Moores University in Birkenhead, UK. Collins and his colleagues publish the work today in Nature¹.

The work suggests that an earlier modelling result may have correctly posited a mechanism - a cold stream of star-nourishing hydrogen gas - by which these first massive galaxies grew so rapaciously2. Taken together, the two results suggest that early galaxies grew quickly through injections of gas, rather than slowly through mergers. "We have a whole different story now about how galaxies form," says Avishai Dekel of the Hebrew University in Israel and first author of the earlier paper.

Hierarchy undermined

For years, astronomers have relied on a hierarchical model of galaxy formation. It explained how small chunks of stars were born in dense regions where cold dark matter clustered. These galactic building blocks, through mergers, would stack up and form the large massive galaxies that are seen today. In recent years, astronomers have tried to refine the models by adding in the actual gas mechanics of star formation.

But adding the gas mechanics slowed down the rate of galaxy growth seen in the models. That's because, as gas cools and falls in to the star-birthing centre of a galaxy, it's heated up. Shock waves are produced that expand outwards to agitate and heat up new gas on the periphery, preventing it from collapsing and falling in to form new stars. "It's a mechanism that quenches or stops star formation," says Dekel. So the models predicted that, to reach the massive galaxy sizes seen today, galaxies would have to steal their stars through mergers - a slow process - rather than growing their own.

But Dekel's models suggested a way to turbocharge the early galaxies. They found that veins of cold gas, clinging to filaments of dark matter, could pierce the hot gas shell of a growing galaxy and fuel its continued growth. That would allow galaxies to reach massive proportions quickly. So some astronomers are seeing the new observations as evidence of the existence of these cold gas streams. "This is another important piece in the puzzle of how big galaxies are assembled," says Avi Loeb, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was not affiliated with either study.

Adapt or die?

It's not yet certain how much of a readjustment the hierarchical model will need if the observations hold up. Dekel says that the basic model, describing the way dark matter clumps and merges, is fine; only the gas mechanics need to be adjusted for his cold streams. But Collins says the underlying models of dark-matter mergers could have problems. "I think the problem could be more general than just needing a tweaking."

But both agree that more work needs to be done, and an obvious next step is to search for filaments of cold gas streaming into early galaxies. And Loeb says they may already have been spotted. Since 2000, astronomers have wondered about the origin of strange, glowing blobs of hydrogen gas in distant, ancient corners of the Universe. Loeb says that they share many characteristics with the putative cold-gas streams³. Now that the idea is catching on, many astronomers are requesting time on bigger telescopes that can make finer pictures of these 10-billion-year-old objects. Loeb thinks the blobs will resolve into filaments.

And then, he says, astronomers will have a new tool to gauge the growth of a galaxy. They'll also get to see the very first ones while they are still in their nurseries. "This could give you a direct image of how a galaxy is being made."

NASA/JPL-Caltech

http://www.nature.com/news/2009/090401/full/news.2009.225.html