A team of astronomers has used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe a gigantic tsunami of stars and interstellar material that has created striking eyelid-shaped formations in a distant spiral galaxy. Observations of the galaxy containing the eyelid, IC 2163, were made in the wake of a glancing collision with the spiral arms of the neighboring galaxy NGC 2207.

Despite its unfathomable size, the universe is a fairly crowded place, and it is not uncommon for a pair of galaxies to pass in such close proximity to each other that they touch.

In certain circumstances, these interactions can result in the formation of rare eyelid-like structures, created as the powerful influence of one passing galaxy over another sends vast quantities of material rushing toward the galactic center in the form of an enormous, cosmic tsunami.

As this wave sweeps towards the galactic center, it loses momentum, and deposits the material in elegant, eyelid-like structures that move with the spin of the galaxy. This influx of material acts as the catalyst for a furious bout of star formation.

Eyelid formations in spiral galaxies are only thought to last a few tens of millions of years, and while that may sound like an eternity, it is but the blink of an eye in the lifespan of a spiral galaxy. The structure present in IC 2163 is thought to be newly created, and so offers a rare insight into the interior dynamics at play following a close pass between two spiral galaxies.

Annotated ALMA image of carbon monoxide disposition (displayed in orange) accross IC 2163 overlayed on a visible light image captured by the Hubble Space Telescope(Credit: M. Kaufman; B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble Space Telescope)

The team used ALMA's impressive resolution and sensitivity to track the movement of carbon monoxide gas, which represents a component of star formation, through the eyelid structures of IC 2163. It was discovered that gas was moving inward from the outer edge of the eyelid at speeds in excess of 100 km per second (62 mi per second).

As the gas migrated toward the inner edge of the eyelid, and sharply decelerated, its movement was seen to become more erratic. It was discovered that, in areas where the gas decelerated very fast, there was a dense pileup of the gas.

"What we observe in this galaxy is very much like a massive ocean wave barreling toward shore until it interacts with the shallows, causing it to lose momentum and dump all of its water and sand on the beach," said Bruce Elmegreen, a scientist with IBM's T.J. Watson Research Center in New York and co-author on a paper detailing the findings.

In this case, the slowed and compressed streams of gas didn't dump sand, but led to to the creation of the dramatic arcs of star formation that we observe today. Observations regarding the velocity of the inflow of material and its subsequent deceleration appear to support computer-generated models explaining the creation of the ocular formations.

"This evidence for a strong shock in the eyelids is terrific" comments Curtis Stuck, a professor of astrophysics at Iowa State University, and co-author of a paper on the study. "It's all very well to have a theory and simulations suggesting it should be true, but real observational evidence is great,"

The paper on the findings has been published online in the Astrophysical Journal.

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