Since the discovery of the Sagittarius dwarf galaxy, researchers have noticed that some of its younger stars are strikingly similar to stars in the Large Magellanic Cloud, another satellite galaxy that sits just a bit further out in space.
Now a study led by Patrick Cseresnjes of the Paris Observatory shows strong similarities in a certain class of old stars seen in both of these satellite galaxies. Cseresnjes thinks the evidence may point to a common ancestor, a larger galaxy that was ripped apart to form both the Large Magellanic Cloud and the nearer Sagittarius dwarf galaxy, or Sgr as astronomers call it.
The study required rooting out stars that could be clearly identified as belonging to Sgr.
"In the direction of Sagittarius, most of the stars are located in our own galaxy, and there is no easy way to determine their distances," Cseresnjes said in an e-mail interview. "Finding a star of the Sagittarius dwarf galaxy is like picking a needle out of a haystack."
Cseresnjes examined what are called RR Lyrae stars, about which enough is now known to separate them from the intervening thicket of Milky Way stars. The RR Lyrae stars are ancient, more than 10 billion years old, Cseresnjes explained, and so they provide clues about the environments from which they originated.
Most important, these stars vary in brightness. In studying the period of this variation, Cseresnjes found a significant similarity in the distribution of stars with similar periods in both the Sgr and the Large Magellanic Cloud, known as LMC. The fraction of RR Lyraes in a given period range are the same in Sgr and the LMC, he said.
He also compared the variable stars in these two galaxies to those in our other neighboring galaxies.
"There are no two other dwarf galaxies showing such a high similarity," he said.
Cseresnjes figures there are two possible explanations.
"First, Sgr and the LMC could have been part of a larger galaxy which broke up into several pieces after colliding with the Milky Way," he said.
It's unclear how such a collision could leave the two galaxies in their present configuration, however. The orbital plane of one is perpendicular to the other.
"Another possible scenario is that Sgr is a debris of the LMC pulled out after a collision with the Small Magellanic Cloud," Cseresnjes explained. "We know that the SMC and the LMC are strongly interacting, and these two galaxies have probably been bound to each other for a long time."
It's possible, he said, that a close brush pulled a tail out of LMC. This tail would be the Sagittarius dwarf galaxy.
Cseresnjes cautioned that while his observations are solid, scenarios about the history of the two satellite galaxies are highly speculative and need to be mathematically modeled.
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