Galaxy Mega Merger Spotted By Herschel And Keck
May 23, 2013

Mega Merger: Witnessing The Making Of A Giant Galaxy

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April Flowers for - Your Universe Online

Around 11 billion years ago, two hungry young galaxies collided and they are now forming a massive galaxy approximately 10 times the size of the Milky Way. The new mega-galaxy, HXMM01, was discovered as part of a UC-Irvine led project conducted with the W.M. Keck Observatory, the Herschel space observatory, and other research facilities around the world.

It is extremely rare to capture the creation of this type of large, short-lived star body. Rare enough that the scientists equate it to finding the missing link between winged dinosaurs and modern birds. HXMM01 is the brightest, most luminous and most gas-rich submillimeter-bright galaxy merger known. The scientists relied heavily on data from Keck Observatory´s NIRC2 fitted with the laser guide star adaptive optics (LGSAO) system and Herschel´s far-infrared detectors.

As fast as it forms, HXMM01 is fading away, a victim of its own cataclysmic birth. The two parent galaxies are gobbling up huge amounts of hydrogen, emptying that corner of the galaxy of the gas needed for star formation.

“These galaxies entered a feeding frenzy that would quickly exhaust the food supply in the following hundreds of millions years and lead to the new galaxy´s slow starvation for the rest of its life,” said Hai Fu, a UC Irvine postdoctoral scholar.

One of the mysteries of astronomy might be answered with this discovery. When our Universe was 3 to 4 billion years old, it was populated by large reddish elliptical-shaped galaxies made up of old stars. It has been a question whether such galaxies built up slowly over time through the acquisition of smaller galaxies, or formed more rapidly through violent collisions between two larger galaxies. The findings of this study suggest the massive merger theory is the correct one. Scientists have also speculated about why such galaxies stop producing stars so early in their life cycle. Some theorized that giant black holes in the heart of the galaxies blew strong winds that expelled the gas. Cosmologist Asantha Cooray, from UC Irvine, said that the found definitive proof that cosmic mergers and the resulting highly efficient consumption of gas for stars are causing the quick burnout.

"We're looking at a younger phase in the life of these galaxies -- an adolescent burst of activity that won't last very long," said Fu.

"These merging galaxies are bursting with new stars and completely hidden by dust," said Cooray. "Without Herschel's far-infrared detectors, we wouldn't have been able to see through the dust to the action taking place behind."

Initially, the new galaxy was spotted by UC Irvine postdoctoral scholar Julie Wardlow, who noticed “an amazing, bright blob” recorded by Herschel. HXMM01 was found in images of the so-called cold cosmos, which are areas where gas and dust come together to form stars. “Herschel captured carpets of galaxies, and this one really stood out.”

More than a dozen ground based observatories, including the Keck, obtained follow-up views at a variety of wavelengths.

“The NIRC2/LGSAO image has revealed the existing stellar population of this pair of galaxies,” Fu said. “The radiation captured by Keck tells us how many stars have already been formed in the system at the observed epoch. These data told us the constituents of the galaxy pair: they are each made of half gas and half stars, which indicates they are nascent galaxies in formation.”

The velocity difference between the two galaxies is only 300 km/s according to the NIRSPEC spectra, indicating that the two galaxies are soon to merge instead of just flying by each other. The spectra also revealed that high-velocity winds are driven by the intense star formation in both galaxies, creating a violent environment.

The findings of this study have been published in a recent issue of Nature.