Stripped and cast out, the universe’s loneliest galaxies
elliptical galaxies have always been a conundrum. They look like a
galaxy stripped bare: as if a normal elliptical galaxy—the sort that is a
featureless mass of stars without a spiral structure—has had all its
outer stars removed, leaving just the dense core of stars at its center.
These rare systems—only a few tens were known until recently—were
thought to have had their outer coats of stars ripped away by the
gravity of other, larger galaxies as they passed nearby, a theory
supported by the fact that they were usually found in the centers of
large clusters of galaxies. But in 2013, an isolated compact elliptical
galaxy was found, far from any predator galaxies able to rob it of its
coat. So how was it created? To find out, astronomers scoured publicly
available astronomy databases. They found 195 compact ellipticals; most
were in galaxy clusters, but 11 were free fliers, the team reports online today in Science.
What’s more, these galaxies had properties just like the others, so
they should have had a common origin. The researchers conclude that
these outliers were originally in clusters like the others, but after
having their outer stars stripped away while orbiting a larger galaxy
(orbit shown in this simulation), they had a close encounter with a
third galaxy (approaching from bottom) whose gravity flung them out of
the cluster like a slingshot. Such a process is known to occur in
planetary systems when close encounters can cast a planet into deep
space, and within galaxies when a star can get ejected, but these lonely
compact galaxies are the result of slingshots on a supergalactic scale.
Using ESO’s Very Large Telescope, an international team of astronomers has discovered a stunning rare case of a triple merger of galaxies. This system, which astronomers have dubbed ‘The Bird’ - although it also bears resemblance with a cosmic Tinker Bell - is composed of two massive spiral galaxies and a third irregular galaxy.