Our Galaxy Keeps Devouring its Smaller Neighbours

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. This discovery came from analysing data from the latest Sloan Digital Sky Survey (SDSS-III) and was announced in a paper released today that connects these new streams with two previously known streams in the Northern Galactic hemisphere. 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. 

Before SDSS-III, Sagittarius was known to have two tails, one in front of and one behind the remnant. Sergey Koposov and colleagues analysed 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. According to Wyn Evans, from the Institute of Astronomy, Cambridge University, Sagittarius is like a beast with four tails. The reasons for such a structure is not entirely clear. However, the Scientists not exclude that two pairs of bright and faint tails are actually “split off” from the different dwarf galaxies. The first dwarf galaxy, left a dull pair of tails, already disappeared, and the dwarf galaxy in the constellation Sagittarius still in the early stages of decomposition.