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Heavyweight galaxies in the young universe

Newfound massive galaxies may force theorists to revisit formation model

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4:00pm, April 1, 2009
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Peering into the center of five of the youngest clusters of galaxies known 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-billion-year-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 billions 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, an astronomer at 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. Due to the properties of dark matter, the model always builds tiny, lightweight galaxies first, merging these small-fry to make bigger bodies. Indeed, dark matter simulations suggest that at such a young age, the galaxies the team examined should have attained only 20 percent of the weight that the astronomers 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 building up stars and gas little by little as they cannibalized their neighbors.

“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 adds.

Evrard is a collaborator 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 with the masses of galaxies predicted by this simulation when the universe was about one-third its 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 predict extremely rapid growth. But even in these regions the masses of the galaxies were much heavier than the model allowed.

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 the handful of early massive galaxies as extremely rare objects that happened to be in the densest dark matter regions, Collins says.

In contrast, galaxy clusters aren’t rare. Moreover, 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 at high densities, nearby gas gravitationally snared by a young galaxy would be compressed quickly and heated. Hot gas cannot form stars and in addition 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 bulk 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.”

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