The large galaxies present in the present Universe were not always so large. Evidence suggests that they have formed over time, mostly collisions with other galaxies. These collisions have left traces that we can still detect: streams of stars drawn from collision victims and faint dwarf galaxies that still bypass the larger object that devours many of their stars. With enough data, you can become a galaxy historian and reconstruct the events that have brought today's giants to their present appearance.
The disclosure of part of this story was the goal of a large multinational collaboration, clearly spelled out in its title: Pan-Andromeda Archaeological Survey. In a report released Wednesday in Nature, the team describes the discovery of some of the violent pasts of our closest galactic neighbors. The article shows that Andromeda was built in part by two large collisions that left clusters of stars occupying two perpendicular orbits. In the process of writing their work, the researchers also reveal a bit of mystery about the unexpected alignment of some of these clusters and Andromeda satellite galaxies.
New work focuses on what is called ball clusters, which are large groups of stars held by gravity. Unlike other stars ̵
This is what the researchers behind the Pan-Andromeda Archeological Survey decided to do. First, they eliminated clusters near the core of Andromeda, since these clusters took the form of their orbits with more frequent interaction with other bodies there. At that time, the researchers recovered 92 globular clusters in the galaxy's halo, rotating at least 25,000 parsecs from its nucleus. They were depicted to determine their motion relative to Earth by Doppler displacement. This information can be converted to local cluster traffic for the rest of Andromeda.
The analysis showed that there are two distinct groups. One cluster group was associated with previously identified structures within Andromeda during its orbits. The second group rotating in a plane 90 ° away from this one does not seem to be associated with anything in the galaxy itself. It is noteworthy that none of these aircraft corresponds to the Andromeda disk drive.
So what's going on here? Andromeda's structures provide a hint. With the exception of basic functions such as spiral arms, structures like these are eventually overlooked because their constituent stars and other material are not gravitationally connected to each other. Thus, structures like these must be the product of a relatively recent event – an event such as a galaxy merger. "They clearly represent the fragments of one or more growths that have probably occurred recently," the authors write, "to ensure that the underlying structures are still coherent."
This, in their view, may be related to a union with a relatively large galaxy that happened about a billion years ago ("recent" means something else in astronomy.)
Old and odd
This obviously means that the second group is much older and comes from the earlier unions that built Andromeda. As the mass of globular clusters tends to be a constant fraction of the total galactic mass, the authors were able to estimate the mass of the galaxy swallowed by Andromeda. This analysis shows that Andromeda swallowed a galaxy that was well in excess of 10% of its own mass at a time when it was much smaller than it is now.
But there was something strange about this particular result. The orbit of the older group of globular clusters is roughly lined by the orbits of many Andromeda satellite galaxies. This makes sense if we assume that these spherical clusters were devoid of some satellites. But it makes less sense if you think that satellites should not be in the same plane for long, since the gravitational interactions between themselves and the halo of the Andromeda dark matter must distort them from the plane they began.
The authors at a loss explain this part of their results. There may have been a better merger orientation that has persisted for billions of years throughout Andromeda history. Or perhaps some additional effect is affecting the orbits of satellite galaxies or globular clusters. But for now, there is simply no obvious explanation for this alignment.
This indicates that archeology reveals interesting secrets, even when digging into the past of an entire galaxy.
Nature, 2019. DOI: 10.1038 / s41586-019-1597-1 (About DOI).