The two largest satellite galaxies of the Milky Way – known as the Magellanic Clouds – appear to be connected by a bridge stretching across 43,000 light years, scientists have discovered.
Researchers from the University of Cambridge in the UK concentrated on the area around the Magellanic Clouds and picked out pulsating stars of a particular type: the RR Lyrae, very old and chemically un-evolved.
As these stars have been around since the earliest days of the Clouds’ existence, they offer an insight into the pair’s history. Studying the Large and Small Magellanic Clouds (LMC and SMC respectively) has always been difficult as they sprawl out over a large area. Around the Milky Way, the clouds are the brightest, and largest, examples of dwarf satellite galaxies.
Known to humanity since the dawn of history the Magellanic Clouds have remained an enigma to date. Even though the clouds have been a constant fixture of the heavens, astronomers have only recently had the chance to study them in any detail.
Whether the clouds fit the conventional theory of galaxy formation or not depends critically on their mass and the time of their first approach to the Milky Way. The researchers clues that could help answer both of these questions. The RR Lyrae stars were used to trace the extent of the Large Magellanic Cloud. The LMC was found to possess a fuzzy low-luminosity ‘halo’ stretching as far as 20 degrees from its centre.
The LMC would only be able to hold on to the stars at such large distances if it was substantially bigger than previously thought, totalling perhaps as much as a tenth of the mass of the entire Milky Way. “Having marked the locations of the RR Lyrae on the sky, we were surprised to see a narrow bridge-like structure connecting the two clouds,” said Vasily Belokurov, from Cambridge.
“We believe that at least in part this ‘bridge’ is composed of stars stripped from the Small Cloud by the Large. The rest may actually be the LMC stars pulled from it by the Milky Way,” said Belokurov. The researchers believe the RR Lyrae bridge will help to clarify the history of the interaction between the clouds and our galaxy.
“We have compared the shape and the exact position of the stellar bridge to the computer simulations of the Magellanic Clouds as they approach the Milky Way,” said Denis Erkal, from Cambridge. “Many of the stars in the bridge appear to have been removed from the SMC in the most recent interaction, some 200 million years ago, when the dwarf galaxies passed relatively close by each other,” Erkal said.