Galactic "Eyelids" Spotted In Distant Galaxy Merger

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The merging of galaxies is a crucial mechanism involved in driving galaxy evolution over the history of the universe, and it is also responsible for the formation of beautiful and ephemeral structures.

One of these are galactic "eyelids" observed in a pair of merging galaxies by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The observatory saw the waves of dust and gas, which look like an eye, in a spiral galaxy known as IC 2163. The object is slowly crashing into its larger companion, NGC 2207, another spiral galaxy.

These cosmic islands, which are located 114 million light-years from the Milky Way, are colliding at a very shallow angle. This type of collision is not uncommon, but the researchers stress that there are only a few of these objects known in the universe.

The findings will be published in the Astrophysical Journal (pre-print on arXiv).

“Galactic eyelids last only a few tens of millions of years, which is incredibly brief in the lifespan of a galaxy," said lead author Michele Kaufman, formerly of The Ohio State University in Columbus, in a statement. "Finding one in such a newly formed state gives us an exceptional opportunity to study what happens when one galaxy grazes another."

Observations from ALMA show that one eyelid is moving towards the center of the galaxy, with a large difference in the speed of its gas, over 100 kilometers (62 miles) per second between different parts of it.

The fantastic features of the eye in IC 2163. M. Kaufman; B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble Space Telescope

“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 co-author Bruce Elmegreen, a scientist with IBM’s T.J. Watson Research Center in Yorktown Heights, New York.

The peculiar velocities are caused by the strong tidal forces between the galaxies. The intense gravity will compress gas, producing new stars in both galaxies. ALMA is able to measure the effect these forces have on the gas.

“Not only do we find a rapid deceleration of the gas as it moves from the outer to the inner edge of the eyelids, but we also measure that the more rapidly it decelerates, the denser the molecular gas becomes,” said Kaufman. “This direct measurement of compression shows how the encounter between the two galaxies drives gas to pile up, spawn new star clusters and form these dazzling eyelid features.”

This cosmic eyelid is more than just beautiful, as the data collected will help refine our simulations. The researchers are studying this system in even more detail, and they hope to soon understand the precise dynamics of this incredible galaxy merger.