The spiral galaxy, NGC 4622 (also called Backward galaxy), lies approximately 111 million light years away from Earth in the constellation Centaurus. NGC 4622 is an example of a galaxy with leading spiral arms. Each spiral arm winds away from the center of the galaxy and ends at an outermost tip that "points" in a certain direction (away from the arm). Spiral arms were thought to always trail, meaning that the outermost tip of every spiral arm points away from the direction of the disk's orbital rotation. This is true of the inner spiral arm of NGC 4622 but not of its outer spiral arms. The outer arms of NGC 4622 are instead leading spiral arms, meaning the tips of the spiral arms point towards the direction of disk rotation. This may be the result of a gravitational interaction between NGC 4622 and another galaxy or the result of a merger between NGC 4622 and a smaller object.
NGC 4622 also has a single inner trailing spiral arm. Although it was originally suspected that the inner spiral arm was a leading arm, the observations that established that the outer arms were leading also established that the inner arm was trailing.
These results were met with skepticism in part because they contradicted conventional wisdom with one quote being “so you’re the backward astronomers who found the backward galaxy.” The fact that a pair of arms could lead was not easy to accept. Astronomical objections centered on the fact that dust reddening and cloud silhouettes were used to determine that the outer arms lead. The galaxy disk is tilted only 19 degrees from face-on making near to far-side effects of dust hard to discern and because clumpy dust clouds might be concentrated on one side of the disk, creating misleading results.
A simulation of the NGC 4622 arm pairs, exhibiting the Fourier transform m = 2.
In response, the “backward astronomers” determined NGC 4622's spiral arm sense with a method independent of the previous work. The new Fourier component method is actually assisted by the small tilt, and dust reddening and cloud silhouettes are not used in the latest analysis. The Fourier component method reveals two new weak arms in the inner disk winding opposite the outer strong clockwise pair. Thus the galaxy must have a pair of arms winding in the opposite direction from most galaxies. Analysis of a color-age star formation angle sequence of the Fourier components establishes that the strong outer pair is the leading pair.
While the presence of backward arms in a galaxy may seem like an inconvenient truth to many, two independent methods now indicate that NGC 4622's arms do indeed behave in a very unusual fashion, with the outer arms winding outward in the same direction the disk turns.
A Fourier component image of the arm pairs is shown with one of the pair of arms marked for the newly discovered inner CCW pair (black dots) as well as one of the already known (CW) outer pair (white dots).