Milky Way still feasting on dwarf galaxies

The Sagittarius dwarf galaxy used to be one of the brightest of the Milky Way satellites.

Washington: The Milky Way galaxy is continuing to devour its small neighbouring dwarf galaxies, the evidence of which is spread across the sky, a new study has found.

A team of astronomers led by Sergey Koposov and Vasily Belokurov of Cambridge University recently discovered two streams of stars in the Southern Galactic hemisphere that were torn off the Sagittarius dwarf galaxy y analysing data from the latest Sloan Digital Sky Survey (SDSS-III).

“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,” Koposov said.

The Sagittarius dwarf galaxy used to be one of the brightest of the Milky Way satellites, and 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.

Koposov and his 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 shows 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.

Since 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,” Wyn Evans, from the Institute of Astronomy, Cambridge University, said.

The mechanism that caused the splitting of the tidal tails still remains to be a secret, and scientists believe that perhaps the Sagittarius dwarf galaxy was once a part of a binary galactic system, similar to the present day Large and Small Magellanic Clouds.

Each of these could have produced a leading and trailing tail on falling into the Milky Way Galaxy, yielding four in all.

“Perhaps the Sagittarius dwarf galaxy has suffered an encounter with an object in the game of Galactic billiards.Maybe a collision with a massive clump of dark matter, or even another satellite galaxy, has split each of the streams into two,” Geraint Lewis, co-author of the study, said.

A final theory suggests that, just as meteors have spread into different streams through evolution in the Solar system, debris from Sagittarius may have spread into different streams at different points in time. Different epochs may suffer different amounts of precession in the Galaxy, causing the split streams.

“I have been running hundreds of simulations of the disruption of the Sagittarius dwarf and this idea looks very plausible,” Jorge Penarrubia, who was also involved in the study, said.


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