The Galaxy Evolution Explorer (GALEX): an All Sky Ultraviolet Survey

We are preparing a mission to perform imaging and spectroscopic surveys of the sky in the Ultraviolet. GALEX (The GALaxy Evolution EXplorer) will fill in the crucial missing piece of the spectrum so that our knowledge of the sky from 100 micron to 10 A is complete. The Palomar Sky Survey has served as the fundamental resource for research in optical astronomy for over thirty years, and the IRAS, EUVE, and Rosat satellites provided similar databases for the infrared, extreme ultraviolet, and X-ray regimes. GALEX will provide a catalog with an order of magnitude more sources than any of these experiments, and will produce an unprecedented statistically powerful database of UV images and spectra of nearby and distant galaxies, linked to a multiwavelength archive. GALEX is approved as a NASA Small Explorer (SMEX) mission, to fly in 2001. GALEX will perform a series of spectroscopic and imaging surveys in the space ultraviolet band (1300-3000A ), that will map the history and probe the causes of star formation over the 0<z<2 range, 80% of the life of the Universe, a period of dramatic evolution, when most stars, elements, and galaxy disks were formed. GALEX will provide a direct measurement of each the redshift (using metal lines and the Lyman break), extinction (using the UV spectral slope), and star formation rate (UV luminosity) in 100,000 galaxies, with efficient slitless grism spectroscopy. It will determine whether rapid evolution has occured in the star formation rate, or in the extinction or initial mass function. The GALEX database will furnish a direct link between rest frame UV properties of galaxies seen locally to those that dominate the faint blue galaxy population. As such, it will provide a framework for the interpretation of the deepest HST images and those that may be obtained by the NGST.