Using ultraviolet vision, scientists have found a smattering of massive baby galaxies in an unexpected place: our own celestial back yard.
"Back yard" is a relative term: The newborn galaxies range in distance from 2 billion to 4 billion light-years away. But until recently, such galaxies had only been spotted in the far reaches of the space-time continuum, looking back to an age 10 billion years ago, when the universe itself was young.
The latest discovery, made by NASA's Galaxy Evolution Explorer, or GALEX, indicates that our aging universe is still giving birth to fresh galaxies, researchers said Tuesday. Until recently, such galaxies were thought to exist only in the distant past, said Tim Heckman, a member of the research team and director of the Center for Astrophysical Sciences at Johns Hopkins University.
"It's almost like looking out the window and seeing a dinosaur walking by," Heckman told reporters during a teleconference.
The findings are to be published in a special GALEX issue of the Astrophysical Journal Letters. GALEX's observations, supplemented by other instruments such as the Hubble Space Telescope, could fill in many of the gaps in our understanding of how galaxies such as our own Milky Way were formed, and why the galactic birth rate has slowed down so dramatically, said Alice Shapley, an astronomer at the University of California at Berkeley.
Baby 'building blocks'
For many astronomers, untangling the origins of galaxies is a "Holy Grail," Heckman said. That's why they look so closely at the faint infant galaxies spotted at the edges of the observable universe.
"The problem is, while these newborn galaxies are much more numerous in the early universe, we can only obtain crude information about them," Shapley said. "This is in terms of the mixtures of stars and gas and dust that they contain, and also what their detailed structure is."
Just last month, Hubble researchers announced the first observations of what appeared to be a baby galaxy in our cosmic neighborhood, 45 million light-years away.
The several dozen galaxies spotted by GALEX are more distant, but also more massive. Thus, they could serve as cosmic "building blocks," eventually coalescing into mature galaxies like the Milky Way, Heckman said.
That process could well take billions of years, but in the shorter term, analyzing the composition and structure of such galaxies will give astronomers a relatively close-up look at cosmic mysteries.
"We have a very good model for the overall structure of our universe — how the universe is shaped, how old it is, what it's made out of and how it's evolved overall," Heckman observed. "We still don't have robust models that tell us how stars form or how galaxies form."
Clues in the ultraviolet
Launched in April 2003, the GALEX satellite has an ultraviolet-sensitive, wide-angle telescope that is designed to document the multibillion-year history of galaxies. GALEX's ultraviolet vision is well-suited to spot newborn galaxies because young stars emit most of their light in ultraviolet wavelengths.
Previous attempts to look for ultraviolet-bright galaxies were unsuccessful because they couldn't look at large swaths of the sky like GALEX can, NASA said. Even in the wide-field GALEX survey, only about 1 out of every 3,000 galaxies was identified as a potential newborn.
Based on their analysis of star-formation rates, the scientists estimated that the nearby newborns were 100 million to a billion years old when the light observed by GALEX began its journey.
The view from a newborn galaxy would look far different from what we see in the Milky Way, said Chris Martin of the California Institute of Technology, principal investigator for GALEX.
"You would see a very large number of quite bright, blue stars ... very hot stars and massive stars that have recently formed," Martin said.
Does all this mean we're in the midst of a galactic baby boom? Not in the least, Heckman said.
"What we're seeing right now is perhaps the last dregs of galaxy birth, the last few stragglers," he said. "We don’t yet know really what's controlling this decline in the formation of galaxies."
Researchers hope that future observations by GALEX and other telescopes will confirm the newborn status of the ultraviolet-bright galaxies — and shed light on those larger questions as well.
"These are needles in a haystack, and this is our first yield," Heckman said. "As we go on and survey the whole sky, we think we'll find some very close-by examples."