Astronomers have for the first time gotten a glimpse of the earliest stages of massive galaxy construction. The building site, dubbed "Sparky," is a developing galaxy containing a dense core that is blazing with the light of millions of newborn stars which are forming at a ferocious rate. The discovery was made possible through combining observations from NASA's Hubble and Spitzer space telescopes, the European Space Agency's Herschel Space Observatory, and the W.M. Keck Observatory in Hawaii.
Because the infant galaxy is so far away, it is seen as it appeared 11 billion years ago, just 3 billion years after the birth of the universe in the big bang. Astronomers think the compact galaxy will continue to grow, possibly becoming a giant elliptical galaxy, a gas-deficient assemblage of ancient stars theorized to develop from the inside out, with a compact core marking its beginnings.
"We really hadn't seen a formation process that could create things that are this dense," explained Erica Nelson of Yale University in New Haven, Connecticut, lead author of the science paper announcing the results. "We suspect that this core-formation process is a phenomenon unique to the early universe because the early universe, as a whole, was more compact. Today, the universe is so diffuse that it cannot create such objects anymore."
The research team's paper appears in the August 27 issue of the journal Nature.
Although only a fraction of the size of the Milky Way, the tiny powerhouse galaxy already contains about twice as many stars as our galaxy, all crammed into a region only 6,000 light-years across. The Milky Way is about 100,000 light-years across. The barely visible galaxy may be representative of a much larger population of similar objects that are obscured by dust.
"They're very extreme environments," Nelson said. "It's like a medieval cauldron forging stars. There's a lot of turbulence, and it's bubbling. If you were in there, the night sky would be bright with young stars, and there would be a lot of dust, gas, and remnants of exploding stars. To actually see this happening is fascinating."
Alongside determining the galaxy's size from the Hubble images, the team dug into archival far-infrared images from the Spitzer and Herschel telescopes. The analysis allowed them to see how fast the young galaxy is churning out stars. Sparky is producing roughly 300 stars per year. By comparison, the Milky Way produces roughly 10 stars per year.
Astronomers believe that this frenzied star formation occurred because the galactic center is forming deep inside a gravitational well of dark matter, an invisible form of matter that makes up the scaffolding upon which galaxies formed in the early universe. A torrent of gas is flowing into this well at the galaxy's core, sparking waves of star birth.
The sheer amount of gas and dust within an extreme star-forming region like this may explain why these compact galaxies have eluded astronomers until now. Bursts of star formation create dust, which builds up within the forming galaxy and can block some starlight. Sparky was only barely visible, and it required the infrared capabilities of Hubble's Wide Field Camera 3, Spitzer, and Herschel to reveal the developing galaxy.
The observations indicate that the galaxy had been furiously making stars for more than a billion years (at the time the light we now observe began its long journey). But the galaxy didn't keep up this frenetic pace for very long, the researchers suggested. Eventually, the galaxy probably stopped forming stars in the packed core. Smaller galaxies then might have merged with the growing galaxy, making it expand outward in size over the next 10 billion years, possibly becoming similar to one of the mammoth, sedate elliptical galaxies seen today.
"I think our discovery settles the question of whether this mode of building galaxies actually happened or not," said team member Pieter van Dokkum of Yale University. "The question now is, how often did this occur? We suspect there are other galaxies like this that are even fainter in near-infrared wavelengths. We think they'll be brighter at longer wavelengths, and so it will really be up to future infrared telescopes such as NASA's James Webb Space Telescope to find more of these objects."
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