New observations with the Atacama Large Millimeter/submillimeter Array (ALMA) push back the epoch of massive-galaxy formation even further by identifying a pair of giant star-forming galaxies seen when the Universe was only 780 million years old, or about 5% its current age. The research appears in the journal Nature.

Artist’s impression of a pair of galaxies from the early Universe. Image credit: NRAO / AUI / NSF / D. Berry.

Artist’s impression of a pair of galaxies from the early Universe. Image credit: NRAO / AUI / NSF / D. Berry.

The galaxies, collectively known as SPT-S J031132-5823.4 (SPT0311-58 for short), were originally identified as a single luminous source by the South Pole Telescope in Antarctica.

These first observations indicated that the object was very distant and glowing brightly in infrared light, meaning that it was extremely dusty and likely going through a burst of star formation.

Subsequent ALMA observations revealed the distance and dual nature of SPT0311-58, clearly resolving the pair of interacting galaxies.

The larger of the pair is forming stars at a rate of 2,900 solar masses per year, contains 270 billion solar masses of gas and 2.5 billion solar masses of dust.

It is the most gargantuan galaxy ever seen inhabiting the Universe during the first billion years following the Big Bang.

Its rapid star formation is probably triggered by its companion galaxy at a projected separation of 26,100 light-years.

The companion is no lightweight either, comprising roughly 40 billion solar masses of gas and dust.

“Either of these galaxies on its own would be extreme, and here you have two of them together,” said Dr. Chris Hayward, a researcher at the Center for Computational Astrophysics at the Flatiron Institute.

“The two galaxies are in such close proximity that they will shortly merge to form the largest galaxy ever observed at that period in cosmic history,” the astronomers said.

A composite image showing ALMA data (red) of the two galaxies of SPT-S J031132-5823.4. These galaxies are shown over a background from the NASA/ESA Hubble Space Telescope (blue and green). The ALMA data show the two galaxies’ dusty glow. The image of the galaxy on the right is distorted by gravitational lensing. The nearer foreground lensing galaxy is the green object between the two galaxies imaged by ALMA. Image credit: ALMA / ESO / NAOJ / NRAO / Marrone et al / B. Saxton / AUI / NSF / NASA / ESA / Hubble.

A composite image showing ALMA data (red) of the two galaxies of SPT-S J031132-5823.4. These galaxies are shown over a background from the NASA/ESA Hubble Space Telescope (blue and green). The ALMA data show the two galaxies’ dusty glow. The image of the galaxy on the right is distorted by gravitational lensing. The nearer foreground lensing galaxy is the green object between the two galaxies imaged by ALMA. Image credit: ALMA / ESO / NAOJ / NRAO / Marrone et al / B. Saxton / AUI / NSF / NASA / ESA / Hubble.

Based on the amount of gas in the two galaxies and the typical ratio of dark to ordinary matter in galaxies, Dr. Hayward and co-authors estimated the extent of the dark matter halo containing the two galaxies.

“The halo’s estimated mass of 1 trillion times the Sun’s mass is conservative, and the actual number may be larger,” Dr. Hayward said.

This discovery provides new details about the emergence of large galaxies and the role that dark matter plays in assembling the most massive structures in the Universe.

“The colossal collection of dark and ordinary matter isn’t just a record-setting curiosity,” said Dr. Dan Marrone, from the University of Arizona in Tucson.

“The merging galaxies will now serve as a testbed for astronomers to study how massive, star-forming galaxies evolved during the earliest stages of the Universe. There’s still more work to do.”

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D.P. Marrone et al. Galaxy growth in a massive halo in the first billion years of cosmic history. Nature, published online December 6, 2017; doi: 10.1038/nature24629

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