The Milky Way is pulling the Sagittarius dwarf galaxy into long streams David Martinez-Delgado (MPIA) & Gabriel Perez (IAC)

It’s poetic justice on a cosmic scale. The same galaxy that the Milky Way is tearing apart may be responsible for a mysterious warp in our galaxy’s disc.

Approximately 50 galaxies orbit our own. The two brightest, the Large and Small Magellanic Clouds, were once thought to be the nearest as well. But in 1994 astronomers spotted a galaxy on the far side of the Milky Way in the constellation Sagittarius that’s only 80,000 light years from Earth – half the distance to the Large Magellanic Cloud.

The Sagittarius dwarf galaxy is so close that the Milky Way’s gravitational pull is tearing it apart, spilling its stars and star clusters into long streams. But now it seems that this victimised galaxy is striking back.


In 1956, radio observations of the Milky Way’s atomic hydrogen gas revealed that the outer disc is warped: parts of it lie above the plane of the galaxy while other parts lie below. The Magellanic Clouds are too far away to warp the disc, and a 2010 estimate of the Sagittarius dwarf’s mass suggested it was too light, only 600 million solar masses. So the source of this warp was a mystery.

Strapping Sagittarius

Now, Simon Gibbons of the University of Cambridge and his colleagues have analysed the motion of stars in the Sagittarius streams to estimate that the galaxy originally possessed 60 billion solar masses.

“This new more massive Sagittarius, if it’s right, could be the cause of the warp,” Gibbons says. Since the 1990s, astronomers have recognised that Sagittarius could warp the disc – if the satellite was massive enough.

David Law of the Space Telescope Science Institute in Baltimore, Maryland – one of the astronomers who did the earlier mass estimate – says the new study uses a more realistic model of the Sagittarius dwarf and does a good job of reproducing its history. In contrast, his work focused on using the path of the Sagittarius stream to probe our galaxy’s dark matter halo.

But the Milky Way may have the last word. In 2011, other astronomers suggested that such a massive Sagittarius would actually add to our galaxy’s beauty by enhancing its spiral structure.

Journal reference: Monthly Notices of the Royal Astronomical Society, submitted. ArXiv: arxiv.org/abs/1607.00803