Heavyweights spotted in the early universe: newfound massive galaxies may force theorist to revisit leading formation model.

[ILLUSTRATION OMITTED]

Peering into the center of five of the youngest known clusters of galaxies in the universe, astronomers recently found several full-grown, cigar-chomping adults among the myriad of toddlers. The remote galaxies hail from a time when the 13.7-billionyear-old cosmos was less than 5 billion years old. Yet measurements reveal that the bodies are just as massive as galaxies like the modern-day Milky Way, which took at least 10 billion years to mature.

The findings appear to call into question the leading theory of galaxy formation, known as the dark matter model, at least as it applies to the dense regions where galaxies congregate into clusters, says Chris Collins of the Liverpool John Moores University in England. He and his colleagues used the infrared Subaru telescope atop Hawaii's Mauna Kea to observe the galaxies, and the team describes the findings in the April 2 Nature.

"No doubt the theorists will want to say that tweaking [the model] in very dense regions will suffice, but I think the problem could be more general than that," Collins says.

The highly successful model holds that the gravity of a proposed, invisible material known as cold dark matter draws together gas and stars to form galaxies. Because of dark matter's properties, the model always builds tiny, lightweight galaxies first, merging these small-fry to make bigger bodies. Indeed, simulations suggest that having formed in the young universe, the galaxies the team examined should have attained only about 20 percent of the weight actually observed.

In the dense environment of a cluster, galaxy formation is predicted to occur more quickly. Nonetheless, there doesn't seem to have been enough time, some 4 billion to 5 billion years after the Big Bang, for the five massive galaxies to have formed by the merging of smaller galaxies, according to the model. The findings suggest that some massive galaxies formed wholesale, rather than cannibalizing their neighbors to build up stars and gas little by little.

"These observations are certainly surprising," comments theorist Gus Evrard of the University of Michigan in Ann Arbor. Although more data and even larger-scale simulations are needed to determine whether the observations and theory are truly inconsistent, "the difference between nature's brightest cluster galaxies and the simulated sample is quite striking," he says.

Evrard collaborates on the Millennium Simulation, an international effort that combines the largest supercomputer simulation of the growth of dark matter ever attempted with new techniques for tracking the evolution of the visible universe. Collins' team directly compared its observations of the five galaxies with the masses of galaxies predicted by this simulation at about one-third the universe's current age.

"Our result is strong evidence that, for reasons we as yet do not understand, the process of galaxy assembly at early times was much more rapid and efficient than the [dark matter model] in the simulations would have us believe," Collins says. Although the dark matter scenario for galaxy formation accurately predicts many features over a wide range of cosmic history, "it seems that in these extreme cluster environments, something else is needed."

In the dense regions examined by Collins' team, the simulations do predict extremely rapid growth. But even in these regions, the masses of the galaxies are much heavier than the model allows.

Over the past few years, other astronomers have peered even further back in time and also found some monster galaxies among the newborns (SN: 10/8/05, p. 235). These heavyweights, although less massive than the ones found by Collins and his colleagues, would have had much less time to bulk up and could have put even tighter limits on models of galaxy formation. However, theorists argue that dark matter models allow a few statistical oddballs. Modelers explain away this handful of early massive galaxies as extremely rare objects that happened to be in the densest dark matter regions, Collins says.

[GRAPHIC OMITTED]

In contrast, massive galaxies don't seem rare in clusters. And the rapid growth rate of galaxies in clusters is already included in the Millennium Simulation.

One reason that the dark matter model may fail to produce massive galaxies rapidly is that in high-density regions gas that is gravitationally snared by a young galaxy would be compressed quickly and heated. Hot gas, which cannot form stars, would likely loiter in the halo of the young galaxy rather than sinking toward the center to add to the system's mass.

A report in the Jan. 22 Nature by Avishai Dekel of the Hebrew University of Jerusalem and his colleagues offers a possible solution to this cosmic conundrum (SN: 3/22/08, p. 186).

His team's high-resolution simulations show that some gas funnels toward the center of the galaxy before the gas heats up and can therefore make stars. That would mean that galaxies could have bulked up more efficiently in the past.

"Dekel's stuff points to the underlying difficulties of forming large galaxies quickly and suggests a nice possible way out, but even here it may not be the last word," Collins says. "I think our data will stimulate more theoretical work."

In their models, theorists could also attempt to ramp up the rate at which gas turns into stars in the brightest members of galaxy clusters, Evrard suggests. However, he cautions that it could be difficult to fatten up only the brightest members while leaving neighboring galaxies svelte. "The unintended consequence could be gigantic galaxies in today's universe that aren't seen, and they certainly would be easy to see," he says.

Speaking of the distance to the observed galaxies, Evrard says, "The observers have laid out a 10 billion light-year tightrope and challenged the theorists to balance on it. It may not be easy."

Back Story | IN LEADING THEORY, COALESCENCE BUILDS GALAXIES

[ILLUSTRATION OMITTED]

1. Fluctuations in the early universe cause dark matter to collapse, forming halos. This computer simulation shows dark matter wells about 150 million years after the Big Bang. Soon after the wells form, visible matter, such as gas, will begin to fall into them.

[ILLUSTRATION OMITTED]

2. As densities within those wells become high enough, stars begin to form. White crosses (within red areas) denote stars forming in halos around 220 million years after the Big Bang. As matter continues to fall in, the stars and halos coalesce.

[ILLUSTRATION OMITTED]

3. Galaxies form as dark matter pulls in more stars and gas. In this simulation, the galaxy takes on a more detailed structure around 440 million years after the Big Bang. Galaxies grow and merge until jets and winds resist the buildup of matter, slowing accumulation.