NASA’s Swift Maps Magellanic Clouds With Ultraviolet
June 4, 2013

NASA’s Swift Maps Magellanic Clouds With Ultraviolet

April Flowers for — Your Universe Online

Using NASA´s Swift satellite, astronomers from NASA and Pennsylvania State University have created the most detailed ultraviolet surveys to-date of the two closest major galaxies, the Large and Small Magellanic Clouds.

"We took thousands of images and assembled them into seamless portraits of the main body of each galaxy, resulting in the highest-resolution surveys of the Magellanic Clouds at ultraviolet wavelengths," said Stefan Immler. Immler originally proposed the program and led NASA's contribution from the Goddard Space Flight Center.

At the 222nd American Astronomical Society meeting this week, Immler presented a 160-megapixel mosaic image of the Large Magellanic Cloud (LMC) and a 57-megapixel mosaic image of the Small Magellanic Cloud (SMC). These images reveal approximately one million ultraviolet sources in the LMC and another 250,000 in the SMC. Light ranging from 1,600 to 3,300 angstroms, which is a range of UV wavelengths largely blocked by Earth's atmosphere, was included in the images as well.

"Prior to these images, there were relatively few UV observations of these galaxies, and none at high resolution across such wide areas, so this project fills in a major missing piece of the scientific puzzle," said Michael Siegel, lead scientist for Swift's Ultraviolet/Optical Telescope (UVOT) at the Swift Mission Operations Center.

The LMC lies approximately 163,000 light-years from Earth, while the SMC is about 200,000 light-years away. They orbit each other, as well as orbiting our Milky Way galaxy, and are both smaller than our own galaxy. The LMC is one-tenth the size, containing only one percent of the mass of the Milky Way. The SMC, on the other hand, is half the size and contains two-thirds the mass.

Because they are so close to us, relatively speaking, both galaxies loom large in our sky and extend far beyond the UVOT's field of view. Thousands of images were needed in order to cover both galaxies in three ultraviolet colors centered at wavelengths of 1,928 angstroms, 2,246 angstroms, and 2,600 angstroms.

Astronomers were able to suppress the light of normal stars like our Sun by viewing in the ultraviolet wavelength. These stars are not very bright at such higher energies, allowing the ultraviolet to provide a clearer picture of the hottest stars and star-formation regions. UVOT is the only telescope currently able to produce such high-resolution, wide-field multicolor surveys in the ultraviolet range. The wide-field imaging capabilities of the Swift satellite provide a powerful complement to the deeper, but much narrower-field imaging power of NASA's Hubble Space Telescope.

UVOT acquired 2,200 snapshots for a cumulative exposure of 5.4 days to create the 160-megapixel LMC mosaic, while the 57-megapixel SMC image was comprised of 656 individual images with a total exposure of 1.8 days. Both images, however, have an angular resolution of 2.5 arc seconds, which is a measure of their sharpness. Separated by this angle, which is equivalent to the size of a dime seen from one mile away, sources are visible as distinct objects.

"With these mosaics, we can study how stars are born and evolve across each galaxy in a single view, something that's very difficult to accomplish for our own galaxy because of our location inside it," Immler said.

Named after Ferdinand Magellan, the Portuguese explorer who in 1519 led an expedition to sail around the world, both the LMC and the SMC are visible from the Southern Hemisphere as faint, glowing patches in the night sky.