NGC 2217, is a galaxy in transition – Astronomy Now

Image: ESA/Hubble and NASA/J. Dalcanton.

NGC 2217, located in the constellation Canis Major and about 65 million light-years away, is a galaxy in transition. It appears as a ring galaxy with a central bar in an oval ring surrounded by two tightly coiled spiral arms further out. There appears to be empty space between the inner oval and the outer ring, although in reality this space will contain faint stars and gas. Such galactic structures often form after a head-on collision with another, smaller galaxy.

The outer ring is still forming stars, but the inner oval is red and dusty. Star formation appears to have stopped there, and the light is increasingly dominated by interstellar dust reflecting starlight. Galaxies with this smeared, dusty appearance are known as the lenticular galaxies that NGC 2217 appears to be transforming into. However, if you look closely, you will notice two things about the core of the galaxy. One is that the galaxy has not one bar, but two, with the second, inner bar hidden in the center of the outer bar. These are what are known as “nested bars” where one is embedded within the other. The longer outer bar formed first, funneling gas from the outer regions of the galaxy into the center where that gas formed stars, and in this case those stars and the dumped gas formed the base of an inner bar. And the second thing to notice about NGC 2217’s core is that the inner bar has a blue hue, indicating hot, young stars still being born there.

Nested barred galaxies could be of enormous importance for the overall evolution of galaxies and the intergalactic medium. Dual bars are thought to direct gas to the center of a galaxy more efficiently than just a single bar. This gas not only forms stars, but also feeds the supermassive black hole that lurks at the center of a galaxy. When one of these black holes becomes overfed, it can spew some of its meal back into space in the form of radiation called feedback. This feedback can change a galaxy forever, blowing all of its star-forming material into the intergalactic medium.

Double-bar galaxies have only really been known since the late 1990s, when galaxies like the Hubble Space Telescope that took this image were finally able to resolve the smaller inner bars. Today, about 50 nearby spiral galaxies are known to have nested bars, and it is estimated that up to 30 percent of all barred spirals have double bars. Image: ESA/Hubble and NASA/J. Dalcanton. NGC 2217, is a galaxy in transition – Astronomy Now

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