We give an overview of the Galaxy Evolution Explorer (GALEX), a NASA Explorer Mission launched on 2003 April 28. GALEX is performing the first space UV sky survey, including imaging and grism surveys in two bands (1350-1750 and 1750-2750 galaxy survey. Spectroscopic (slitless) grism surveys ( ) are underway with various depths and sky R p 100-200 coverage. Many targets overlap existing or planned surveys in other bands. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the relationship of the UV and global star formation rate in local galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the redshift range 0 ! z ! and probe the physical drivers of star formation in galaxies. The GALEX mission includes a guest investigator 2 program, supporting the wide variety of programs made possible by the first UV sky survey.

We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX ). These releases have identical pipeline calibrations that are significantly improved over the GR1 data release. GALEX continues to survey the sky in the far-ultraviolet (FUV, $154 nm) and near-ultraviolet (NUV, $232 nm) bands, providing simultaneous imaging with a pair of photon-counting, microchannel plate, delay line readout detectors. These 1.25 field of view detectors are well suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations, and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 m AB in the FUV and NUV, respectively. We have detected a long-term drift of order 1% FUV and 6% NUVover the mission. Astrometric precision is of order 0.5 00 rms in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.

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The Wide-field Infrared Survey Explorer has surveyed the entire sky at four infrared wavelengths with greatly improved sensitivity and spatial resolution compared to its predecessors, the Infrared Astronomical Satellite and the Cosmic

We have studied ∼ 2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range 0 < z < 0.11. Combining GALEX U V photometry with corollary optical data from the SDSS, we find that, at a 95 percent confidence level, at least ∼ 30 percent of galaxies in this sample have U V to optical colours consistent with some recent star formation within the last Gyr. In particular, galaxies with a N U V − r colour less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to N U V flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semi-analytical ΛCDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have ∼ 1 to 3 percent of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ∼ 300 to 500 Myrs. We also find that 'monolithically' evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colours (N U V − r < 5.5) seen in a significant fraction (∼ 30 percent) of our observed sample.