The Prime Mission

GALEX is currently undertaking the first space UV sky-survey, including both imaging and grism surveys. The prime mission includes an all-sky imaging survey (AIS: 75-95% of the observable sky, subject to bright-star and diffuse Galactic background light limits) (m_AB 20.5), a medium imaging survey (MIS) of 1000 deg²(m_AB 23), a deep imaging survey (DIS) of 100 square degrees (m_AB 25), and a nearby galaxy survey (NGS). Spectroscopic (slitless) grism surveys (R=100-200) are also being undertaken with various depths and sky coverage. Many of the GALEX fields overlap existing and/or planned ground-based and space-based surveys being undertaken in other bands.

All-sky Imaging Survey (AIS): The goal of the AIS is to survey the entire sky subject to a sensitivity of m_AB 20.5, comparable to the POSS II (m_AB = 21 mag) and SDSS spectroscopic (m_AB = 17.6 mag) limits. Several hundred to 1,000 objects are in each 1 deg² field. The AIS is performed in roughly ten 100-second pointed exposures per eclipse (~ 10 deg² per eclipse).

Medium Imaging Survey (MIS): The MIS covers 1000 deg², with extensive overlap of the Sloan Digital Sky Survey. MIS exposures are a single eclipse, typically 1500 seconds, with sensitivity m_AB 23, net several thousand objects, and are well-matched to SDSS photometric limits.

Bianchi et al. [1] have made the first characterization of the objects detected in the GALEX AIS and DIS surveys by cross-comparing objects already discovered and classified on the Sloan Digital Sky Survey in regions of overlap. Emphasis was put on discovering quasars and for the 75 and 92 square degrees studied in the MIS and AIS overlap with SDSS they report 1,736 and 222 z 2 QSO candidates, for the two GALEX survey depths, respectively. This significantly increases the number of fainter candidates, and moderately augments the candidate list for brighter objects.

Deep Imaging Survey (DIS): The DIS consists of 20 orbit (30 ksec, m_AB 25) exposures, over 80 deg², located in regions where major multiwavelength efforts are already underway. DIS regions have low extinction, low zodiacal and diffuse galactic backgrounds, contiguous pointings of 10 deg² to obtain large cosmic volumes, and minimal bright stars. An Ultra DIS of 200 ksec, m_AB ~ 26 mag is also in progress in four fields.

Nearby Galaxies Survey (NGS): The NGS targets well-resolved nearby galaxies for 1-2 eclipses. Surface brightness limits are m_AB ~ 27.5 mag arcsec^-2. The 200 targets are a diverse selection of galaxy types and environments, and include most galaxies from the Spitzer IR Nearby Galaxy Survey (SINGS). Figure 1 shows the NGS observation of the M81 / M82 system.

Figure 1. GALEX Composite NUV/FUV Image of M81 (bottom) and M82 (top). Note the verey different morphology of M82 as compared to its optical image. The scattering of light by dust ejected by the starburst is especially prominent in the blue light detected by GALEX. On the other hand, the bulge of M81 is almost invisible in the GALEX bands, leaving only the complex array of star-forming filaments detailing the spiral structure of this galaxy. The irregular object to the east of M81 is Holmberg IX. Another interacting galaxy in the M81-M82 system, NGC 3077 is outside the field of view to the east and may never be imaged by GALEX because of a nearby bright star.