A rare meeting of two gas-rich galaxies, noticed a retired Hershel, points to a long-standing solution to the problem: how to build massive and extinguished galaxy in the early universe?

Most large galaxies fall into one of two main categories: spiral, like our own Milky Way, are filled with gas and are actively forming stars, or elliptical, populated by old red stars and cold with a depleted stock of rare gas and star formation.

For a long time it was believed that the large elliptical galaxies are visible in the universe today formed gradually over a long time by the gravitational absorption sets of dwarf galaxies. Theory states that the gas in these galaxies has been gradually transformed into a cool low-mass stars, so that today they have exhausted all the star-forming material, leaving them red and dead.

The discovery of the last decade, according to which a massive elliptical galaxies were able to form only during the first 3-4 billion years of the universe's history, put a riddle, since short on space standards, the term of these galaxies quickly formed a huge number of stars, then "shut down."

One idea is that the merger of two spiral galaxies, which resulted in a huge elliptical galaxy is formed, resulting in a massive burst of star formation that quickly drains the gas material. Using data Hershel, astronomers have caught the beginning of the process between two massive galaxies in the early universe, when she was only 3 billion years.

Galactic couple was originally defined in the data Hershel as one bright source called HXMM01. Subsequent observations have shown that it is actually two galaxies, each of which boasts a stellar mass equal to about 100 billion suns or the equivalent amount of gas. Galaxy bridge were connected gas, which pointed to the process of their merger.

"It is a monstrous system of interacting galaxies is the most efficient factory for the creation of the stars ever found in the early universe," says Hai Fu of the University of California, Irvine, USA, who led the study.

"The system HXMM01 is unusual not only because of its large mass and intense star formation, but also because it provides the key to an intermediate stage of the integration process, providing valuable information that will help us to adjust the model of formation and evolution of galaxies," adds co-author Asantha Cooray, also from the University of California, Irvine.

Start merger triggered star formation at a phenomenal rate, equal to about 2,000 Sun-like stars in the year. For comparison, the Milky Way today has time to produce the equivalent of a sun-like stars in the year.
Furthermore, the efficiency with which gas is converted to the star, about ten times higher than in normal visible galaxies in which stars are formed at a much slower pace.

Such a high rate of star formation is not sustainable because the reserves of gas contained in the system HXMM01, will be quickly exhausted, leading to the extinction of future star formation and stellar population aging, thereby leaving only the cold and the red low-mass stars.

The team of Dr. Fu believes that the conversion of all the gas in the stars take about 200 million years, and the merger process will be completed within a billion years. Eventually formed a massive (about 400 billion solar masses) and dead red elliptical galaxy.

"We are very well caught this extreme system at such a critical transition period. This shows that the fusion of rich gas and actively forming stars in the Galaxy is the most likely mechanism for the formation of the most massive elliptical galaxies observed in the early universe," says Seb Oliver from the University of Sussex , UK and principal investigator of the main program HerMES.

"This finding underlines the importance of an extensive survey of the sky, which was completed Hershel. In this case, the discovery of an unusual source HXMM01 may indicate a solution to the puzzle of how to form and develop the most massive galaxies when the Universe was still young, "adds Göran Pilbratt, project scientist Hershel.

The video: simulation of merging galaxies, according to the observations of HXMM01.