The tiniest galaxy known is still in the process of being born. Observing this Lilliputian with the Hubble Space Telescope, astronomers are getting a rare glimpse of how larger galaxies formed early in the history of the universe.
The revelations support a recent model of galaxy formation that holds that the smallest galaxies, rather than the biggest, are last to assemble. At first glance, that model seems to contradict the leading theory of galaxy formation, but this shrinking trend is in fact consistent with the theory, several cosmologists say.
Ground-based images had already shown signs of recent star formation within the dwarf galaxy POX 186. New Hubble images document with unprecedented clarity the asymmetric shape of the galaxy, a burst of star formation at its core, and a stream of newborn stars off to one side. These properties together suggest that “we are seeing two clumps of stars, or subgalactic building blocks . . . coming together to form a single, small galaxy,” says Michael R. Corbin of the Space Telescope Science Institute in Baltimore.
He and William D. Vacca of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, estimate that the building blocks that created POX 186 collided about 100 million years ago. That’s extremely recent compared with the birth of larger galaxies, which most astronomers agree assembled through the merger of hundreds or even thousands of smaller galactic building blocks several billion years ago.
The two clumps that built POX 186 each measured only 300 light-years across, and the galaxy stretches 1,000 light-years, Corbin and Vacca calculate in an upcoming Astrophysical Journal. By comparison, the Milky Way spans 100 thousand light-years.
POX 186 is the smallest member of a galactic group known as blue compact dwarf galaxies. These galaxies tend to reside in the emptier regions of the so-called cosmic web, which refers to the spidery distribution of galaxies in the universe. In contrast, the vast majority of galaxies are bunched along the web’s filaments.
The location of POX 186 could explain its recent formation. The region in which it sits–a giant void some 30 million light-years in length–is so sparsely populated that it took nearly the entire 14-billion-year history of the universe for the few galactic building blocks that reside there to collide.
Likewise, other dwarf galaxies in the cosmic web’s voids may also have just popped into existence.
The notion that small galaxies are the last to assemble, a theory known as downsizing, was first suggested by Len L. Cowie of the University of Hawaii in Honolulu in 1996. Cowie says the new Hubble study fits with the theory “but it’s a bit hard to go much further with just a single object.”
The leading model of galaxy formation, known as the cold-dark-matter model, holds that smaller objects are the first to form in the universe, with more-massive objects forming later, as these smaller entities merge.
Downsizing doesn’t actually contradict the cold-dark-matter model, says Vacca. POX 186 did apparently assemble by the merger of smaller pieces, but because it lies in a void, “there may be nothing else to accrete and so it may remain a small galaxy forever,” he says. “POX 186 may be giving us a glimpse of the early stages in the formation process of all galaxies.”
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