Wednesday, 24 December 2014

Image of the Day: A Colossal Galaxy Almost as Old as the Universe


“AzTEC-3 is a massive galaxy that already contains billions of stars at this early epoch, but has the potential to form many more by present day. It produces a thousand times more stars each year than what our own Milky Way galaxy produces,” says Dominik Riechers, Cornell assistant professor of astronomy. “Think about a star factory that puts out 50 billion objects the mass of our sun.

Our Milky Way galaxy forms one star the mass of Earth’s own sun each year. Massive AzTEC-3, the second-most-distant one of its kind known to humanity, produces about five of our suns each Earth day, churning out a total of 1,800 solar masses annually. Such ancient massive star-bursting galaxies can be found by astronomers using modern, mountaintop telescopes like the National Science Foundation-funded Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile. This exceptional galaxy, which at present day is only slightly younger than the 13.8 billion-year-old universe, is named after the AzTEC-millimeter-wave camera on the James Clerk Maxwell Telescope – through which it was initially found.

“These things are just humongous,” said Dominik Riechers, Cornell assistant professor of astronomy. “That’s why we call them ‘monsters.’ Essentially, thanks to telescopes like the ALMA, we’re looking back in time to the childhood, the toddler years of the universe, and are trying to discern how these galaxies form.”

Squinting close to the beginning of time, Riechers, discovered an association of gas-rich galaxies near the infancy of cosmic time. It’s an early epoch – some 12.7 billion years ago – telling a tale that revolves around an exceptionally dusty galaxy called AzTEC-3.

The NASA image at the top of the page shows the formation of a galaxy during the first 2bn years of the universe. The supercomputer-generated image displays hydrogen gas in grey, young stars appearing in blue, and older stars as red. The simulation reveals that gas flows into galaxies along filaments akin to cosmic bendy, or swirly, straws.

AzTEC-3, together with its gang of calmer galaxies may represent the best evidence yet that large galaxies grow from the merger of smaller ones in the early Universe, a process known as hierarchical merging.

New ALMA (Atacama Large Millimeter/submillimeter Array) data reveals that AzTEC-3 is a very compact, highly disturbed galaxy that is bursting with new stars at close to its theoretically predicted maximum limit and is surrounded by a population of more normal, but also actively star-forming galaxies," said Riechers, lead author of a new paper published today on Nov. 10 in the Astrophysical Journal. "This particular grouping of galaxies represents an important milestone in the evolution of our Universe: the formation of a galaxy cluster and the early assemblage of large, mature galaxies."

In the early Universe, starburst galaxies like AzTEC-3 were forming new stars at a monstrous pace fueled by the enormous quantities of star-forming material they devoured and by merging with other adolescent galaxies. Over billions of years, these mergers continued, eventually producing the large galaxies and clusters of galaxies we see in the Universe today.

Evidence for this hierarchical model of galaxy evolution has been mounting, but these latest ALMA data show a strikingly clear picture of the all-important first steps along this process when the Universe was only 8 percent of its current age.

"One of the primary science goals of ALMA is the detection and detailed study of galaxies throughout cosmic time," said Chris Carilli, an astronomer with the National Radio Astronomy Observatory in Socorro, New Mexico. "These new observations help us put the pieces together by showing the first steps of a galaxy merger in the early Universe."

AzTEC-3, which is located in the direction of the constellation Sextans, is what astronomers refer to as a submillimeter galaxy, since it shines brightly in that portion of the spectrum, but is remarkably dim at optical and infrared wavelengths. This is due to light from its stars being absorbed by dust in the star-forming environments of the galaxy and then re-emitted by the dust at far-infrared wavelengths. As this light travels across the cosmos, it becomes stretched due to the expansion of the Universe, so by the time it arrives at Earth, the far-infrared light has shifted to the submillimeter/millimeter portion of the spectrum.

New ALMA observations suggest that AzTEC-3 recently merged with another young galaxy and that the whole system represents the first steps toward forming a galaxy cluster. ALMA, with its remarkable sensitivity and high resolving power, was able to observe this system at these wavelengths in unprecedented detail. It also was able to study, for the first time, the star-forming gas in three additional, extremely distant members of an emerging galactic protocluster.

The ALMA data revealed that the three smaller, more normal galaxies are indeed producing stars from their gas at a relatively calm and steady pace. Unlike its neighbors, however, AzTEC-3 is burning through star-forming fuel at breakneck speed. Indeed, AzTEC-3 appears to form more new stars each day than our Milky Way galaxy forms in an entire year -- outpacing the normal galaxies in its proximity by about a factor of 100.

The researchers also observed very little rotation in AzTEC-3's dust and gas -- suggesting that something had disrupted its motion. Taken together, these two characteristics are strong indications that AzTEC-3 recently merged with another galaxy.

"AzTEC-3 is currently undergoing an extreme, but short-lived event," said Riechers. "This is perhaps the most violent phase in its evolution, leading to a star formation activity level that is very rare at its cosmic epoch."

New ALMA observations suggest that AzTEC-3 recently merged with another young galaxy and that the whole system represents the first steps toward forming a galaxy cluster. The astronomers believe that AzTEC-3 and the other nearby galaxies appear to be part of the same system, but are not yet gravitationally bound into a clearly defined cluster. This is why the astronomers refer to them collectively as a protocluster.

The starburst galaxy was originally observed with and named after the AzTEC millimeter-wavelength camera, which was installed at the time on the James Clerk Maxwell Telescope, a single-dish radio telescope located on Mauna Kea, Hawaii. Only with ALMA has it become possible to understand the nature of this exceptional galaxy and those in its immediate environment.

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