NASAのインスタグラム(nasa) - 11月18日 13時26分

A cosmic search for a missing arm… This image shows a dwarf galaxy, located about 30 million light-years away in the constellation of Canes Venatici (The Hunting Dogs). Taken by the Hubble Space Telescope (@NASAHubble), the picture reveals the single major spiral arm of the galaxy, which gives it an asymmetric appearance.

But why is there only one such spiral arm, when spiral galaxies normally have at least two? Observations in the ultraviolet provided the first hint: in ultraviolet light the disk of the galaxy appears four times larger than on the image depicted here. An indication that there are a large number of very young and hot stars forming in the outer regions of the galaxy – only visible in the ultraviolet.

At first, astronomers assumed that this high star formation rate was being triggered by the interaction with another, nearby dwarf galaxy. They speculated this galactic neighbor may be the culprit, causing it to lose all but one spiral arm. In 2004 astronomers found proof for this claim. The gas in the outermost regions of the neighboring dwarf galaxy has been strongly affected by the galaxy in this image.

Credit: ESA/Hubble & NASA

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