Figure 2. left: illustration of Density Wave Theory with galactic matters entering and exiting density waves as well as the expected trail luminosity gradient pattern, the red dashed circle is the corotation radius; right: actual observation of the galactic matters entering and exiting the spiral arm and the trail luminosity gradient in the galaxy J101652.52

From

How Long Can the Spiral Arms of Disc Galaxies Be Optically Observed?

Hongjun Pan

International Journal of Physics. 2024, 12(5), 212-224 doi:10.12691/ijp-12-5-4