Building Andromeda: Galaxy ended up with stars orbiting at right angles

Image of a galaxy

Enlarge (credit: Amir H. Abolfath

The large galaxies present in the current Universe weren’t
always so big. Evidence indicates that they were built up over
time, largely by collisions with other galaxies. These collisions
have left marks that we can still detect: streams of stars that
were drawn in from the victims of the collisions, and faint dwarf
galaxies that still orbit the larger object that devoured many of
their stars. With enough data, it’s possible to become a galactic
historian and reconstruct the events that brought the modern-day
giants to their present form.

Uncovering some of that history was the goal of a large,
multinational collaboration, spelled out clearly in its name: the
Pan-Andromeda Archaeological Survey. In a paper published on
Wednesday in Nature, the team describes uncovering some of our
nearest galactic neighbor’s violent past. The paper shows that
Andromeda was built in part by two major collisions that have left
clusters of stars occupying two perpendicular orbits. In the
process of writing their paper, the researchers also uncover a bit
of a mystery about an unexpected alignment between some of these
clusters and Andromeda’s satellite galaxies.

Thinking global

The new work focuses on what are called globular clusters, which
are large groups of stars held together by gravity. Unlike other
stars—which shift position relative to each other as they orbit a
galaxy’s center—the stars of a globular cluster stick together
and orbit as a group. As a result, these gravitationally bound
clusters of stars can survive the collisions between galaxies. That
means they can be used as markers to retrace those collisions.

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Source: FS – All – Science – News
Building Andromeda: Galaxy ended up with stars orbiting at right angles