In the outer reaches of galaxies, stars don’t move quite how they should. Is this deviation due to mysterious dark matter? Or is something else at work? In a recent study, scientists turn to elliptical galaxies in search of new clues.
Weirdness in Galactic Fringes
Two rotating galaxies are shown with their rotation curves in this animated gif. Based on the distribution of visible matter, we would expect to see inner stars moving fast and outer stars moving slowly, producing a rotation curve like what’s seen on the left. Instead, we see stars moving with the same speed, producing the flat rotation curve seen on the right. [Ingo Berg]Usually, things that orbit do so in predictable ways. In our solar system, for instance, planets orbit the Sun following predictable laws of gravitation: close-in planets speed along quickly, whereas planets farther out move more slowly.
You might think that galaxies would work the same way. Since most of a galaxy’s visible mass is concentrated at its center — just like most of our solar system’s mass (the Sun) is at its center — we would expect stars near the center of a galaxy to orbit quickly, and stars in the galaxy’s ...