Beginning at a tree trunk’s dense core and moving out to the soft bark, the passage of time is marked by concentric rings, revealing chapters of the tree’s history.
Galaxies outlive trees by billions of years, making their growth impossible to see. But like biologists, astronomers can read the rings in a galaxy’s disk to unravel its past. Using data from NASA’s Wide-field Infrared Survey Explorer (WISE) and Galaxy Evolution Explorer (GALEX), scientists have acquired more evidence for the “inside-out” theory of galaxy growth, showing that bursts of star formation in central regions were followed 1 to 2 billion years later by star birth in the outer fringes.
The discovery may solve a mystery of elderly galaxies. The galaxies in the study, known as “red and dead” for their red color and lack of new star births, have a surprising amount of ultraviolet light emanating from the outer regions. Often, ultraviolet light is generated by hot, young stars, but these galaxies were considered too old to host such a young population.
The solution to the puzzle is likely hot, old stars. Sara Petty of Virginia Tech University in Blacksburg, Virginia, lead author of a paper appearing in the October 2013 issue of Astronomical Journal, and colleagues used a new multi-wavelength approach to show that the unexplained ultraviolet light appears to be coming from a late phase in the lives of older stars, when they blow off their outer layers and heat up.
In the image above of a galaxy called NGC 3377, infrared light from WISE is colored red and ultraviolet light from GALEX is green and blue. The center of the galaxy appears white, where all three wavelengths of light are present and add up. The outside of the galaxy is mostly ultraviolet light, and thus contains more blue and green. The dots in the picture are stars located in the foreground.
Researchers sponsored by Yonsei University in South Korea and the Centre National d’Etudes Spatiales in France collaborated on the GALEX mission.