Kaplinghat. (Courtesy: University of California, Irvine)

New Delhi, Aug. 30: In the orbital dance of 18 dwarfs in the neighbourhood of the Milky Way, Indian astrophysicist Manoj Kaplinghat may have discovered what it takes to make a galaxy — any galaxy.

Kaplinghat, an assistant professor at the University of California, Irvine, and his colleagues have studied a group of small, satellite galaxies close to the Milky Way and found that each one has a mass of about 10 million suns.

The findings, presented this week in the journal Nature, suggest that this mass represents the minimum required for the formation of galaxies. The 18 are among 23 dwarf galaxies within a distance of about 500,000 light years — each spewing light 1,000 times to 10 million times the amount of light from the sun.

“It’s fascinating — they all look so different from each other in visible light, but have almost the same mass,” said Kaplinghat, who had studied engineering physics at the Indian Institute of Technology, Mumbai, during the early 1990s.

Scientists aren’t sure why these galaxies have the same mass of 10 million suns.

“It’s possible the 10-million mass limit is a basic building block for galaxies,” Kaplinghat told The Telegraph over the phone. “You need this minimum mass for matter to begin clumping to make a galaxy. A mass lower than this might be unable to hold on to the material that makes up stars.”

Researchers believe the galaxies are among the earliest objects to have formed at an early age of the universe. “Understanding what we’ve found will almost certainly lead to a better understanding of galaxy formation,” Kaplinghat said.

The faintest of these 18 dwarfs could also provide a new window into the mysterious dark matter — something optically invisible — detected only through its gravitational effect, but about five times more abundant in the universe than ordinary matter.

“We’re excited because some of these galaxies are virtually invisible but contain a tremendous amount of dark matter,” said James Bullock, director of the Centre for Cosmology at the University of California, Irvine.

The researchers used telescopes in Hawaii and Chile to analyse starlight from the galaxies, determined the speed of the stars and then calculated the mass of each of the galaxies.

Astronomers believe dark matter — one of cosmology’s biggest mysteries — governs the growth of structures and galactic patterns in the universe. Without dark matter, galaxies such as the Milky Way would not have formed.

The dark matter in the 18 dwarf galaxies varies, but some have 10,000 times more dark matter than ordinary matter, which means the stars. “They would serve as an ideal laboratory to find clues about the microscopic properties of dark matter,” Kaplinghat said.

“By knowing this minimum galaxy mass, we can better understand how dark matter behaves, which is essential to learning how our universe and life as we know it came to be,” said Louis Strigari, lead author of the study.

Scientists hope to spot signatures of dark matter particles in underground experiments at the Large Hadron Collider in Europe within the next decade. The collider will accelerate subatomic particles — nuclei — and smash them into each other to mimic conditions just after the Big Bang. From these collisions, a dark matter particle is expected to emerge for the first time in a laboratory.