* Astronomy

A new image from NASA's Galaxy Evolution Explorer shows that a galaxy once thought to be rather plain and old is actually endowed with a gorgeous set of young spiral arms.
The unusual galaxy, called NGC 4625, is a remarkable find because it is relatively nearby. Until now, astronomers had thought that this kind of youthful glow in galaxies was a thing of the past.


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Position(2000): RA 12 41 52.84 Dec +41 16 25.9
Image credit: NASA/JPL-Caltech/Carnegie Observatories/DSS

"This galaxy is an amazing surprise. We are practically up-close and personal with a galaxy undergoing an evolutionary stage that was thought to occur only at the dawn of the universe, in very young and faraway galaxies" - Dr. Armando Gil de Paz of the Carnegie Observatories, Pasadena, lead author of a paper appearing in the July issue of Astrophysical Journal Letters.

The Galaxy Evolution Explorer is an orbiting space telescope that makes observations at ultraviolet wavelengths to measure the history of star formation in the universe 80 percent of the way back to the Big Bang.
This image highlights the hidden spiral arms (blue) that were discovered around the nearby galaxy NGC 4625.
The image is composed of ultraviolet and visible-light pictures, from the Galaxy Evolution Explorer and the California Institute of Technology's Digitized Sky Survey, respectively. Near-ultraviolet light is coloured green; far-ultraviolet light is coloured blue; and optical light is coloured red.
As the image demonstrates, the lengthy spiral arms are nearly invisible when viewed in optical light while bright in ultraviolet. This is because they are bustling with hot, newborn stars that radiate primarily ultraviolet light.
The youthful arms are also very long, stretching out to a distance four times the size of the galaxy's core. They are part of the largest ultraviolet galactic disk discovered so far.
NGC 4625 is located 31 million light-years away in the constellation Canes Venatici. It is the closest galaxy ever seen with such a young halo of arms. The armless companion galaxy seen below NGC 4625 is called NGC 4618. Astronomers do not know why it lacks arms but speculate that it may have triggered the development of arms in NGC 4625. Compared to our Milky Way galaxy, NGC 4625 is slightly smaller both in size and mass.
However, the fact that the disk of this galaxy is forming stars very actively has led astronomers to think that NGC 4625 might evolve into a more massive and mature galaxy resembling our own.



Previous visible-light images of NGC 4625 showed only an oval-shaped ball of light, with very faint hints of a halo of spiral arms. These arms were finally revealed to the ultraviolet eyes of the Galaxy Evolution Explorer. Their intense brightness indicates that the arms are teeming with hot, newborn stars, which shine primarily with ultraviolet light.

"The stars in the arms are about one billion years old, while the stars in the body are about ten times older" - Gil de Paz.

NGC 4625's spiral arms are very lengthy, extending four times beyond the size of the core of the galaxy. They represent the largest ultraviolet galactic disk discovered so far.
Also of interest in the new Galaxy Evolution Explorer image is a nearby companion galaxy, which looks very similar to NGC 4625, yet has no arms. How could this galactic duo have turned out so differently? Astronomers do not know, but some theories hold that the presence of the armless galaxy was required for NGC 4625 to grow a set.

"We know that interactions between galaxies can spur the creation of stars, but it is not clear why only one galaxy ended up with arms" - Dr. Chris Martin of the California Institute of Technology in Pasadena, Calif, principal investigator for the Galaxy Evolution Explorer.

Previous studies of the gas distribution around the two galaxies indicate that NGC 4625 might have developed in a more dynamically stable environment, while the armless galaxy grew up in a more chaotic and turbulent setting.

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