A Collision of Galaxies

Scanning the universe for rare anomalies, UC Irvine astronomy researcher Hai Fu and his research team recently published their findings of a rare collision of two spiral galaxies in the journal, Nature. The collision, which was observed during the course of the HerMES project conducted by Herschel, provided a rare observation of the formation of gas-poor elliptical galaxies from spiral mergers that occurred 11 billion years ago.

“We are looking at the galaxies at a snapshot 11 billion years ago, when the universe was just 3 billion years old,” Fu said. “From this study, we have learned how the most massive elliptical galaxies were formed.”

Due to the length of time that light takes to travel from the distances of these far away galaxies, researchers are barely now able to observe formations that happened millions of years ago.

Most massive galaxies observed by the project are subdivided into two general categories: Spirals and ellipticals. Spiral galaxies, rich in gases necessary for star formation, are actively creating stars. Ellipticals in contrast, which are gas-poor, are largely populated by old, dying red stars.

Utilizing the world’s largest astronomical telescope, the HerMES project first detected the colliding spiral galaxies as an unusually bright object dating back to the ancient epochs of the universe. The source of the illumination was named “HXMM01.”

“Its tremendous star formation rate leads to its extreme luminosity and therefore, its brightness,” Fu said. “We see these two galaxies forming stars at a phenomenal rate — about 2000 stars like the Sun per year — and the conversion of gas into stars is more efficient than in normal galaxies by an order of magnitude.”

The merger of the two young spiral galaxies, involved in intense starbursts, represents a rare occurrence in that most galaxy mergers are not of this kind. HXMM01 exceeds the scale of all previously documented mergers of its kind — in terms of size and rate of star formation. Due to the exceptional conditions of this galaxy’s formation, it provides a glimpse into the process of creating an elliptical galaxy.

“Because gas is converted to stars 10 times more efficient in this kind of mergers, all of the gas in both galaxies will be consumed very quickly, leaving no fuel for future star formation,” Fu said. “The merger will create an elliptical galaxy because the collision will randomize the orbits of the stars, so that the resulting galaxy will look more elliptical.”

Due to the rarity of these collisions in the course of universal time, this discovery may likely be the only glimpse researchers may have at seeing spiral galaxies in this early phase of collision — forming elliptical galaxies in an intensely violent process.

“We were very lucky to catch this extreme system in such a critical transitional phase,” Seb Oliver, the Principal Investigator of the HerMES Key Programme, within which the data from the HerMES project has been collected, said. “It shows that the merger of gas-rich and actively star forming galaxies is a possible mechanism to form the most massive ellipticals that are observed in the young universe.”