Astronomers observe ancient merger of two huge galaxies

Dust and 12.7-billion-year distance makes these galaxies impossible to observe in visible light.
By Laurel Kornfeld | Nov 16, 2017
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope successfully observed a merger of two massive, extremely luminous galaxies that occurred 12.7 billion years ago.

The merger is in the process of producing an elliptical galaxy known as ADFS-27, which may someday become the core of a galactic cluster. It also generated high levels of star formation, which had begun earlier when the galaxies sideswiped one another before fully coming together.

Located in the direction of the Dorado constellation, the galaxies merged when the universe was only one billion years old, which was also the time the first galaxies began forming.

First seen with the European Space Agency's (ESA) Herschel Space Observatory, ADFS-27 showed up as a red dot in the southern sky. When followup observations were conducted with ALMA, scientists confirmed their suspicion that the galaxy looked faint because it was both extremely distant and very bright.

With ALMA, astronomers were able to measure the distance of ADFS-27 as well as resolve it into its two precursor galaxies.

"Finding just one hyper-luminous starburst galaxy is remarkable in itself. Finding two of these rare galaxies in such close proximity is truly astounding," emphasized Dominik Riechers of Cornell University, who is lead author of a paper on the findings published in the journal Astrophysics of Galaxies.

"Considering their extreme distance from Earth and the frenetic star-forming activity inside each, it's possible we may be witnessing the most intense galaxy merger known to date."

ADFS-27 is estimated by scientists to have 50 times more star-forming gas than the Milky Way and is generating stars about 1,000 times as fast as our galaxy, Riechers said.

Many of these are massive blue stars. Even though they are extremely luminous, the region of the merging galaxies has so much dust that their light is obscured.

However, the process in which the dust absorbs starlight generates heat, which can be seen in infrared wavelengths.

Due to the universe's constant expansion, these infrared wavelengths are shifted to even longer millimeter and submillimeter wavelengths through the Doppler effect.

Completion of the merger between the galaxies, which are about separated by about 30,000 light years, will take up to several hundred million years.

The researchers hope that observations of ADFS-27 with the James Webb Space Telescope, which will launch in 2019, will be combined with ALMA data, leading to better insight into these rare phenomena.


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