How does the Milky Way’s history compare with that of its peers?
It’s hard to figure these things out for other, more distant galaxies, because we don’t have as much detailed information about them and their stars. However, there are more and more studies searching for streams and substructures in other galaxies, and we are learning about the importance of the merging process in general. And there are studies of how common interacting pairs of galaxies are, which identify objects that are currently or about to merge. Theoretical models of the growth of galaxies in the expanding universe are also guiding us as to where to look, or how to identify, clues of the merger history of a galaxy.
Some astronomers state that the Milky Way seems to be quiescent compared to other galaxies, since models tend to predict more mergers, on average, but I believe a more detailed assessment is necessary to establish this firmly.
That’s particularly because we only reconstructed the history up to 10 billion years ago, and we don’t know what happened before that. We need more data of many more, fainter stars — particularly their chemical compositions, which is known for a very small number of halo stars in the Milky Way — to figure this out.
What role has dark matter played in your research on the history of the Milky Way?
I think you always have to take into account that there is dark matter around these galaxies, including around the Milky Way. If there is a merger, dark matter makes the merger happen on a faster timescale, because there’s so much more mass in dark matter than in the stars alone.
At the moment, we have not yet used any of the information we have gathered recently about the ancient merger history of the Milky Way to try to estimate how much dark matter there is in and around it or how it’s distributed, but we will in the near future. For example, if you’re convinced that certain stars came from the same object, and they’re located in different regions of the galaxy, that can be used to calculate the gravitational pull of the Milky Way and the dark matter distribution in it.
What have you learned from the Gaia space telescope that wasn’t known before?
Measuring the motions of stars on the sky is extremely challenging. Before Gaia, we had the measurements of about 2 million stars nearby, from a mission called Hipparcos in the ’90s. Now it’s 2 billion. Then there’s the volume: The volume of space we can measure the motions in is a factor of 100 in radius larger now. And it’s a factor of 1,000 more precise. It’s a vast amount of data of exceedingly high quality.
It’s just been completely transformational. This research would not have been possible without Gaia. It has changed the way we understand the Milky Way. For example, we’ve also realized that we cannot consider the Milky Way as an isolated system. People used to think of galaxies as “island universes,” separated from the environment around them. That is an important change in how we approach the problem of determining the distribution of mass throughout the galaxy. In the past it was often assumed that the galaxy was in equilibrium and wasn’t really changing much. Now we have the data showing us that that’s an oversimplification, since the motions of the stars near the sun are revealing the imprints of the pull of neighboring galaxies, which themselves are being pulled in by the Milky Way.