Our Universe is expanding, and galaxies are going further and further away from each other. However, sometimes galaxies collide, because of gravitational forces. Eventually, some of these galaxies merge, resulting in a larger and often irregular galaxy. Stars orbiting the wrong way in the heart of their galaxies are probably the only remnants of another cannibalized galaxy.
Just like planets orbit their stars, stars orbit the center of their galaxies. Usually, all of them orbit in the same direction. For a long time, astronomers have noticed that some stars are orbiting the core of their galaxies in the opposite direction of those further out. A suggestion was that these stars were actually coming from another galaxy, swallowed up by a bigger one before they finally formed the one astronomers are observing. Unfortunately, there was no evidence of such a scenario.
New observations by Kaj Kolja Kleineberg of La Laguna University in Tenerife, Spain, and colleagues are now boosting the galactic cannibalism theory. Their results will appear in a paper in Astrophysical Journal Letters.
They examined the stars of an elliptical galaxy called NGC 1700, located 160 million light years away, with its core rotating backwards, relative to the rest of it. The researchers found that the stars in the core are much younger than the ones in the outer regions, which would be impossible if all the stars were born in the same galaxy.
There is also another clue. “Normal” elliptical galaxies (where all the stars are orbiting the same way) usually have stars with high levels of heavy elements in their core. The core stars of NGC 1700 contain only a small fraction of these elements, suggesting once again that a galaxy was once devoured by another, bigger one.