Not all galaxies are like the spindle-like galaxies, which represent another – very rare – kind of galaxy, shaped like a cigar, which rotates along its long axis, like a spindle.
Using the CALIFA survey (an astronomical project to map 600 galaxies with imaging spectroscopy), Athanasia Tsatsi (Max Planck Institute for Astronomy) and her colleagues, found that these slender galaxies, which rotate along their longest axis, are much more common than previously thought.
The team discovered eight of such cosmic spindles, almost doubling the total known number of them (from 12 to 20). Cosmic spindles are considerably less rare than astronomers had thought!
Now it is important to determine how these unusual galaxies come into existence.
In general, galaxies grow when they merge with other galaxies. However, to make a cosmic spindle, two large disk galaxies need to collide at right angles, as shown in this animation:
One of the spiral galaxies develops a marked elongated structure (a “bar,” to use the technical term) before the merger, which gives the resulting elliptical galaxy its cigar-like (prolate) shape. The stars of the second spiral galaxy end up orbiting around the bar of the first companion.
Together they form a cigar-shaped elliptical galaxy that rotates like a spindle around its long axis.
As the galaxies begin to interact via gravitational attraction, one of them forms a bar: an elongated structure near the center. That bar becomes the cigar-like shape of the merged galaxy, while the orbiting stars of the other galaxy imbue the merged galaxy with its overall sense of rotation.