ALMA finds best evidence for galactic merger in distant protocluster AzTEC-3

An artist's impression of the protocluster observed by ALMA. 

It shows the central starburst galaxy AzTEC-3 along with its labeled cohorts of smaller, less active galaxies. 

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. 

Credit: B. Saxton (NRAO/AUI/NSF)

Nestled among a triplet of young galaxies more than 12.5 billion light-years away is a cosmic powerhouse: a galaxy that is producing stars nearly 1,000 times faster than our own Milky Way.

This energetic starburst galaxy, known as 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.

An international team of astronomers observed these remarkable objects with the Atacama Large Millimeter/submillimeter Array (ALMA).

"The ALMA data reveal 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 Dominik Riechers, an astronomer and assistant professor at Cornell University in Ithaca, New York, and lead author on a paper published today (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."

Combined data from Japan's Subaru telescope and ALMA of the AzTEC-3 region; the circled regions are members of this protocluster, which were previously highlighted by Subaru. 

The ALMA data are highlighted with arrows. 

Credit: Subaru/NASA/JPL, P. Capak (SSC/Caltech); ALMA (NRAO/ESO/NAOJ); B. Saxton (NRAO/AUI/NSF)

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.

More information: Dominik A. Riechers et al. 2014 ApJ 796 84: iopscience.iop.org/0004-637X/796/2/84/article