The Milky Way is surrounded by a host of small galaxies, many only recently discovered. These have been thought to be satellites orbiting the main mass of the galaxy, like more distant versions of the globular clusters that dot the space around the galactic disk. However, measurements of forty dwarf galaxies’ speed of movement have revealed that most are not in orbit around the Milky Way – some may become gravitationally captured, while others will pass on like ships in the night.
Residents of the southern hemisphere have known about two dwarf galaxies for millennia, even if they didn’t recognize what exactly they were. Now known as the Large and Small Magellanic clouds, these conglomerations of billions of stars were recognized for what they were more than a century ago. A few other nearby dwarf galaxies were found in the 1930s, but in the last thirty years we’ve recognized dozens of even smaller examples.
Nevertheless, the relationship of these objects to the Milky Way has been uncertain. It is only now, thanks to the extraordinary precision with which the Gaia Telescope can detect stellar movements, that astronomers have been able to fill that gap. The movements of forty nearby dwarf galaxies have been reported in the Astrophysical Journal – and they are much faster than most people expected.
The energies of these dwarf galaxies far exceed those globular clusters known to orbit the Milky Way, even where the masses are similar.
Establishing whether an object is in orbit around another requires knowing its speed and the mass of the thing that may be orbited. Gaia provides the first, but there’s actually considerable uncertainty about the Milky Way’s mass. At the lower end of past estimates, the Milky Way’s gravitational force is not strong enough to hold onto the majority of dwarf galaxies in the study. Even using higher estimates, however, the authors found most have not been here long-term.
"Some of them will be captured by the Milky Way and will become satellites," First author François Hammer of the Observatoire de Paris said in a statement. Others will be pulled apart by tidal forces emanating from a galaxy as large as ours. The fate of many, however, can’t be determined without more precise estimates of the Milky Way’s mass.
The smallest dwarf galaxies contain just a thousand stars – far smaller than globular clusters. The two categories are distinguished in part by the amount of dark matter they contain. Dwarf galaxies need abundant dark matter to survive for a long time near a galaxy as large as the Milky Way without getting ripped apart. If they haven’t been around for long, it’s possible the galaxies included in the study have less dark matter than previously thought.
“Thanks in large part to Gaia, it is now obvious that the history of the Milky Way is far more storied than astronomers had previously understood,” said Gaia project scientist Timo Prusti who was not an author of the study.
Perhaps we should have expected this, following the discoveries that the Magellanic Clouds and the Sagittarius Dwarf Galaxy are not long-term satellites. The Sagittarius Dwarf Galaxy is slowly being devoured by the Milky Way after getting too close, but has only been under its sway for 4-5 billion years. Nevertheless, the discovery raises the questions: where do all these tiny cities of stars come from, and what were they doing before they got so close to the Milky Way?