Imagine picking up a tiny baby and finding that he’s as heavy as a full-grown adult.
A group of astronomers found themselves in a similar situation recently when they observed nine galaxies that hail from a time when the 13.7-billion-year-old universe was less than 3 billion years old. Images taken by the Hubble Space Telescope and Keck Observatory atop Hawaii’s Mauna Kea revealed that the galaxies, already known to be surprisingly massive, were small, measuring only about 5,000 light-years across — one-twentieth the diameter of the Milky Way.
The small size combined with the enormous heft of these young galaxies poses a mystery, says Pieter van Dokkum of Yale University. Most young galaxies are both small and lightweight. Van Dokkum thought that the few youngsters his team had found that were heavy — bodies that despite their youth had quickly matured, produced a huge number of stars and then stopped — would also be large.
“We expected that these galaxies would be more or less the same size as those in today’s universe,” he says. But instead of being precocious in both size and weight, “it turns out that some are tiny.”
The finding suggests that these massive galaxies do not evolve in isolation over billions of years, as theorists had assumed, notes van Dokkum. “Galaxies can’t get bigger by themselves,” but must undergo a string of collisions with other galaxies to puff up their size, growing as big as large, elliptical galaxies seen in the cosmos today, he says. Many collisions would be required to enlarge these galaxies to adult proportions, van Dokkum says. He and his colleagues report their findings in the April 10 Astrophysical Journal Letters.
“This is a fascinating result which raises some profound questions as to how galaxies might grow to form the giant galaxies we see today,” says Richard Ellis of the California Institute of Technology in Pasadena. “The puzzle is that many seem to have already attained large masses at early times and yet are physically very small.”
A review of images taken by the Sloan Digital Sky Survey of nearby reaches of the universe might turn up some tiny, but massive galaxies — possible leftovers from this early, puzzling population, van Dokkum suggests. Tiny but hefty galaxies appear pointlike and might have been overlooked, mistaken for stars rather than small galaxies, he notes.
Ellis of Caltech cites two possible way of explaining the data. “It might be that these galaxies are not the precursors of the typical large galaxies we see today. Maybe they grow into much rarer objects with super-large masses, such as the central ‘monster’ galaxies in clusters.”
Or, he notes, “perhaps this result is telling us that our inferences, for example, about galaxy masses, are more uncertain and overestimated. We rely on infrared data to get the masses of distant galaxies and conceivably there are greater uncertainties here than we would like to admit.
Either way, this is a potentially very important development.”