Sagittarius galaxy

New Delhi, Sept. 14: A dwarf galaxy loaded with mysterious matter has struck the Milky Way twice in the past, creating its iconic spiral arms, and is set to collide again, new research by a Calcutta-born Indian-American scientist and her colleagues has suggested.

The astronomers used telescope data and supercomputer simulations that indicate that the dwarf galaxy named Sagittarius has struck and passed through the Milky Way twice over the past 1.8 billion years. During each collision, the Milky Way cannibalised matter from Sagittarius and acquired its characteristic spiral arms and other structural features, the scientists said. Their findings will appear tomorrow in the journal Nature.

“The strongest telltale sign of a past collision is the stream of stellar debris that wraps around the Milky Way,” said Sukanya Chakrabarti, an astrophysicist and a team member at the Florida Atlantic University who was involved in simulating the interactions between the two galaxies.

Astronomers had assumed that galaxies such as the Milky Way have largely evolved and acquired their spiral shapes on their own, uninfluenced by external forces. “These simulations support the idea that dwarfs can induce changes in the galactic structures,” Chakrabarti told The Telegraph.

Sagittarius is currently about 52,000 light years from the galactic centre on the opposite side of the galaxy from the Sun. The study also suggests that the dwarf galaxy is about to strike the Milky Way again, a process that will bring its core inside the Milky Way in about 10 million years.

“Sagittarius is gravitationally bound to the Milky Way and is being pulled apart by its tidal forces,” said Chris Purcell, a researcher at the University of California, Irvine, and lead author of the research, studying the formation and destruction of galaxies.

Indian-American scientist Sukanya Chakrabarti

Scientists believe 99 per cent of Sagittarius is made up of mysterious dark matter — an unknown form of matter that can be sensed only through its gravitational pull. The simulations suggest that during the first collision with the Milky Way, Sagittarius lost about 75 per cent of its dark matter. This infall of the Sagittarius influenced the architecture of the Milky Way, giving rise to its spiral arms and a ring of stars called the Monoceros Ring around the galaxy.

While such galactic cannibalism can reshape the structure of galaxies, scientists say the scales of galactic interactions are so large that the stellar systems typically pass right through each other, interacting only through gravitation.

“The influence (of the collision) will be negligible on the scale of the Sun and the planets,” said Chakrabarti, who has been looking for gravitational signatures of small, unseen dwarf galaxies.

Chakrabarti was born in Calcutta, but moved to the US with her parents when she was 10.

Even residual matter from the collisions will have little impact on the solar system.

“There is currently some very little dark matter leftover from the Sagittarius satellite passing through the solar neighbourhood, but this is not dramatic enough to make any difference on the scale of the solar system,” Purcell told The Telegraph.

The researchers believe it could take a few billion years more for all the material from Sagittarius to be absorbed by the Milky Way. They have suggested that similar mergers might be shaping structures of galaxies throughout the universe.

Purcell himself ran the simulations, while Chakrabarti worked on the analysis of the ring-like structures and spiral arms that form as a result of the interactions. Astronomers James Bullock, Eric Tollerud and Miguel Rocha at the University of Pittsburgh are co-authors of the study.

Earlier this year, Chakrabarti had shown that dwarf galaxies with dark matter could be studied by analysing disturbances in the outer gas discs of spiral galaxies.

Sagittarius being close to the disc of the Milky Way is hard to see from the Earth and was first observed only in 1994. Since then, surveys have provided a clear picture of what the remnant of Sagittarius looks like today.