Hubble determines source of cloud of gas approaching Milky Way

Ars Technica : The Smith Cloud contains enough gas to form an estimated 2 million stars, which is exactly what is expected to occur when the cloud collides with the Milky Way 30 million years from now. Its speed of nearly 1 million km per hour has stretched the cloud into a shape 11 000 light-years long and 2500 light-years across. If visible to the naked eye, the cloud would appear to be 30 times the size of the full moon. And now the Hubble Space Telescope has been used to determine that the cloud actually originated in the Milky Way itself. UV spectroscopy of light from distant galaxies revealed a significant amount of sulfur and other heavy elements in the cloud that suggest that it originated from a region of our galaxy 45 000 light years from the galactic core. Based on its speed and trajectory, it appears that the cloud was ejected 70 million years ago. But what ejected it and answers to other questions, such as how it maintained so much mass instead of drifting apart, are still unknown.