Heavyweights Spotted in the Early Universe: Newfound Massive Galaxies May Force Theorist to Revisit Leading Formation Model
Cowen, Ron, Science News
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. …