Infalling gas creates a galaxy’s ‘eyelids’ 
New discoveries

Infalling gas creates a galaxy’s ‘eyelids’ 

Read time: 3 minutes

Many galaxies are beautiful spirals, but IC 2163 is different. This galaxy looks a bit like a huge eye. The spiral arms are like giant eyelids, surrounding a central pupil. Using ALMA, astronomers have now discovered what’s going on here. 

IC 2163 is not alone. Close to it is another spiral galaxy, known as NGC 2207. The two galaxies have experienced a grazing collision, like two people brushing against each other in a busy shopping mall. The gravity of NGC 2207 has sent clouds of cold gas in IC 2163 streaming toward its center. 

Where the gas piles up, new clusters of stars are born. Apparently, these clusters line up in huge arcs, giving the galaxy its eye-like appearance. At least, that has always been the theory. Now, the ALMA observations have confirmed this general idea. 

The gas clouds in IC 2163 contain carbon monoxide molecules. These molecules emit radiation at millimeter wavelengths that ALMA can observe. By studying the carbon monoxide radiation, astronomers were able to clock the motion of the gas. 

It turns out that the gas in the outer parts of the eyelids is streaming inward with velocities of over 100 kilometers per second. Closer to the galaxy’s center, the gas is slowed down and starts to pile up, forming clusters of stars in the process. Eventually, the gas and the newborn stars take part in the general rotation of the galaxy. 

Grazing collisions between galaxies occur quite often. Still, galaxies with eyelids like IC 2163 are rare. That’s probably because the eyelid features are short-lived – just a few tens of millions of years. We’re just lucky to observe IC 2163 during its eyelid phase. 


IC 2163 and NGC 2207 are two spiral galaxies at a distance of 114 million light-years in the constellation Canis Major, the Big Dog. In the relatively recent past, they experienced a grazing collision. Thanks to their mutual gravity, both galaxies were slightly distorted, and clouds of cold interstellar gas started to flow toward the center of IC 2163. In the future, the two galaxies may actually collide and merge into one. 


The ALMA observations of IC 2163 were carried out by a team of astronomers from the United States, France, and the United Kingdom. The team was led by Michele Kaufman, a former physics professor at the University of Ohio in Columbus. Michele is now retired. The results of the ALMA observations have been published in The Astrophysical Journal.