Contact Armando Gil de Paz at 626-304-0273,; or
Barry Madore at 626-304-0247,,

For images and information about the Galaxy Evolution Explorer on the Internet, visit


Pasadena, CA – A nearby spiral galaxy, similar to our own Milky Way, has been discovered to have a disk observed in the ultraviolet (UV) part of the spectrum that is several times larger than in the visible spectrum. The finding suggests that the outer parts of the disk are mostly formed by very young blue stars. The radius of extended UV disk is 28,000 light-years, four times the optical radius. “We are excited about this particular galaxy,” commented Armando Gil de Paz, lead author of the study from the Carnegie Observatories. “Not only is it the most extended UV galaxy disk found so far, the galaxy appears to be in its most active star-formation stage. Surprisingly, it has properties typical of galaxies found farther away, when the universe was much younger.”

The discovery opens a new window on viewing how stars and disks originally formed in galaxies including our own Milky Way. The paper is published in the Astrophysical Journal Letters this July.

Ultraviolet observations can reveal structures and substances that are invisible at other wavelengths. Using NASA’s Galaxy Evolution Explorer (GALEX), the researchers honed in on a nearby spiral galaxy named NGC 4625, some 31 million light-years away in the constellation Canes Venatici. GALEX is a space ultraviolet telescope mission launched in April 2003, which is mapping the history of star formation during the period when galaxy evolution was in its heyday.

“With this galaxy we may be seeing in detail what happened in our own Milky Way when it was created over 10 billion years ago,” commented Barry Madore, co-author at Carnegie. “What we are witnessing in this disk is a time when most of the stars and the elements that they produce first arrived on the scene. We had expected to be able only to see such activity farther away and at times when the universe as a whole was young, not in a galaxy that's so very close.”

The astronomers simultaneously used both far UV and near UV bands for their observations and coupled their findings with ground-based optical observations. Their results suggest that the active star formation in the extended disk may be a result of dust and gas swirled into a productive stellar nursery from the disk’s interaction with another nearby galaxy, NGC 4618, and possibly with a newly discovered other galaxy, NGC 4625A. They found that the extended UV disk consists of a few fragmented spiral arms in the inner area with a hint of a larger arm toward the outside. The ground-based optical images corroborated these observations and found the companion galaxy NGC 4625A. The extended UV galaxy contains very blue colors—the telltale sign for infant stars that are less than 1 billion years old.

“It's a spectacular opportunity to see the conditions in which spiral galaxies like our own Milky Way formed,” remarked Samuel Boissier, Carnegie co-author. “This galaxy will provide a fundamental test for the models of galaxy formation.”

Information about the Carnegie Observatories is at
More information about NASA’s GALEX, a partnership among the U.S., South Korea and France, can be found at

The Carnegie Observatories is part of the Carnegie Institution (, which has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.

Caltech leads the Galaxy Evolution Explorer mission and is responsible for science operations and data analysis. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the mission and built the science instrument.

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