The Milky Way Galaxy continues to devour its small neighboring dwarf galaxies, and the evidence is spread out across the sky.
A team of astronomers led by Sergey Koposov and Vasily Belokurov of the University of Cambridge recently discovered two streams of stars in the southern galactic hemisphere that were torn off the Sagittarius Dwarf Galaxy.
“We have long known that when small dwarf galaxies fall into bigger galaxies, elongated streams, or tails, of stars are pulled out of the dwarf by the enormous tidal field,” said Sergey Koposov. The Sagittarius Dwarf Galaxy used to be one of the brightest of the Milky Way satellites. Its disrupted remnant now lies on the other side of the galaxy, breaking up as it is crushed and stretched by huge tidal forces. It is so small that it has lost half of its stars and all its gas over the last billion years.
Before SDSS-III, Sagittarius was known to have two tails, one in front of and one behind the remnant. Previous SDSS imaging had already found the Sagittarius tidal tail in the northern galactic sky in 2006 and revealed that one of the tails was forked into two.
“That was an amazing discovery,” said Vasily Belokurov, “but the remaining piece of the puzzle, the structure in the south, was missing until now.”
Sergey Koposov and colleagues analyzed density maps of over 13 million stars in the latest release of Sloan Digital Sky Survey data, including the crucial coverage of the southern galactic sky. The new data show that the Sagittarius stream in the south is also split into two, a fatter and brighter stream alongside a thinner and fainter stream. This brighter stream is more enriched with iron and other metals than its dimmer companion. Because each generation of stars makes and distributes more metals into the next generation, the Cambridge astronomers concluded that the brighter stream is younger than the older fainter one.
“Sagittarius is like a beast with four tails,” observed Wyn Evans, from the Institute of Astronomy, University of Cambridge.
The wagging of the four tails of Sagittarius will shed new light on both the structure and formation of the Milky Way.