A team of astronomers from the United States, France and the UK has observed a tsunami of stars and gas that is crashing midway through the disk of the spiral galaxy IC 2163. This colossal wave of material – which was triggered when IC 2163 recently sideswiped another spiral galaxy, NGC 2207 – produced a giant ocular (eye-shaped) structure.
This image, a composite of data from ALMA and the NASA/ESA Hubble Space Telescope, shows a very rare cosmic sight: a pair of interacting galaxies that have taken on an ocular structure. Image credit: ALMA / ESO / NAOJ / NRAO / M. Kaufman.
As the name suggests, some types of grazing encounters between galaxies create shapes that resemble the human eye.
While galaxy collisions of this type are not uncommon, only a few galaxies with ocular structures have been observed.
The paucity of these features is likely due to their very ephemeral nature — ocular structures like these tend to only last for several tens of millions of years, which is merely the blink of an eye in a galactic lifetime.
“Galactic eyelids last only a few tens of millions of years, which is incredibly brief in the lifespan of a galaxy,” said lead author Dr. Michele Kaufman, an astronomer formerly with the Ohio State University in Columbus.
“Finding one in such a newly formed state gives us an exceptional opportunity to study what happens when one galaxy grazes another.”
Dazzling eyelid-like features bursting with stars in the spiral galaxy IC 2163 formed from a tsunami of stars and gas triggered by a glancing collision with the spiral galaxy NGC 2207 (a portion of its spiral arm is shown on right side of image). ALMA image of carbon monoxide (orange), which revealed motion of the gas in these features, is shown on top of Hubble image (blue) of IC 2163. Image credit: M. Kaufman / B. Saxton / NRAO / AUI / NSF / ALMA / ESO / NAOJ / NASA / ESA / Hubble Team.
IC 2163 and NGC 2207 reside around 114 million light-years from us in the direction of the constellation Canis Major.
These galaxies brushed past each other — scraping the edges of their outer spiral arms — in what is likely the first encounter of an eventual merger.
Using remarkable sensitivity and resolution of the Atacama Large Millimeter/submillimeter Array (ALMA), Dr. Kaufman and co-authors made the most detailed measurements ever of the motion of carbon monoxide gas in IC 2163’s narrow eyelid features.
The data reveal that the gas in the outer portion of the galaxy’s eyelids is racing inward at speeds in excess of 224,000 mph (100 km a second).
This gas, however, quickly decelerates and its motion becomes more chaotic, eventually changing trajectory and aligning itself with the rotation of the galaxy rather than continuing its pell-mell rush toward the center.
“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 Dr. Bruce Elmegreen, from IBM’s T.J. Watson Research Center.
“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,” Dr. Kaufman added.
“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.”
The research has been accepted for publication in the Astrophysical Journal (arXiv.org preprint).
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Michele Kaufman et al. 2016. Ocular Shock Front in the Colliding Galaxy IC 2163. ApJ, accepted for publication; arXiv: 1608.02130
This article is based on a press-release issued by the National Radio Astronomy Observatory.
