GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire), a SIRTF Legacy Science Program, will be a fully sampled, confusion-limited infrared survey of the inner twothirds of the Galactic disk with a pixel resolution of ∼ 1.2 ′′ using the Infrared Array Camera (IRAC ) at 3.6, 4.5, 5.8, and 8.0 µm. The survey will cover Galactic latitudes |b| ≤ 1 • and longitudes |l| = 10 • to 65 • (both sides of the Galactic center). The survey area contains the outer ends of the Galactic bar, the Galactic molecular ring, and the inner spiral arms. The GLIMPSE team will process these data to produce a point source catalog, a point source data archive, and a set of mosaicked images. We summarize our observing strategy, give details of our data products, and summarize some of the principal science questions that will be addressed using GLIMPSE data. Up-to-date documentation, survey progress, and information on complementary datasets are available on the GLIMPSE web site: www.astro.wisc.edu/glimpse.

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The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ∼30 million midinfrared sources toward the inner Galaxy, 10Њ ≤ FlF ≤ 65Њ and FbF ≤ 1Њ, was used to determine the distribution of stars in Galactic longitude, l, latitude, b, and apparent magnitude, m. The counts versus longitude can be approximated by the modified Bessel function N p N 0 (l/l 0 )K 1 (l/l 0 ), where l 0 is insensitive to limiting magnitude, band choice, and side of Galactic center: l 0 p 17Њ-30Њ with a best-fit value in the 4.5 mm band of l 0 p 24Њ ‫ע‬ 4Њ. Modeling the source distribution as an exponential disk yields a radial scale length of H * p 3.9 ‫ע‬ 0.6 kpc. There is a pronounced north-south asymmetry in source counts for FlF Շ 30Њ, with ∼25% more stars in the north. For l p 10Њ-30Њ, there is a strong enhancement of stars of m p 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length R bar p 4.4 ‫ע‬ 0.5 kpc, tilted by f p 44Њ ‫01ע‬Њ to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l p 26Њ-28Њ, 31Њ .5-34Њ, and 306Њ-309Њ are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (m K Ϫm [8.0] 1 3) peak. In these areas, the counts are reduced by extinction due to molecular gas, high diffuse backgrounds associated with star formation, or both.

We present a highly reliable flux-limited census of 18,949 point sources in the Galactic midplane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) I and II surveys of 274 deg 2 of the Galactic midplane, and consist mostly of high-and intermediate-mass young stellar objects (YSOs) and asymptotic giant branch (AGB) stars. The selection criteria were carefully chosen to minimize the effects of positiondependent sensitivity, saturation, and confusion. The distribution of sources on the sky and their location in the Infrared Array Camera and the Multiband Image Photometer for Spitzer 24 μm color-magnitude and colorcolor space are presented. Using this large sample, we find that YSOs and AGB stars can be mostly separated by simple color-magnitude selection criteria into approximately 50%-70% of YSOs and 30%-50% of AGB stars. Planetary nebulae and background galaxies together represent at most 2%-3% of all the red sources. 1004 red sources in the GLIMPSE II region, mostly AGB stars with high mass-loss rates, show significant (0.3 mag) variability at 4.5 and/or 8.0 μm. With over 11,000 likely YSOs and over 7000 likely AGB stars, this is to date the largest uniform census of AGB stars and high-and intermediate-mass YSOs in the Milky Way Galaxy.

Using Infrared Array Camera (IRAC) images at 3.6, 4.5, 5.8, and 8 µm from the GLIMPSE Legacy science program on the Spitzer Space Telescope, we searched for infrared counterparts to the 95 known supernova remnants that are located within galactic longitudes 65 • > |l| > 10 • and latitudes |b| < 1 • . Eighteen infrared counterparts were detected. Many other supernova remnants could have significant infrared emission but are in portions of the Milky Way too confused to allow separation from bright H II regions and pervasive mid-infrared emission from atomic and molecular clouds along the line of sight. Infrared emission from supernova remnants originates from synchrotron emission, shock-heated dust, atomic fine-structure lines, and molecular lines. The detected remnants are G11.G348.5-0.0, and G349.7+0.2. The infrared colors suggest emission from molecular lines (9 remnants), fine-structure lines (3), and PAH (4), or a combination; some remnants feature multiple colors in different regions. None of the remnants are dominated by synchrotron radiation at mid-infrared wavelengths. The IRAC-detected sample emphasizes remnants interacting with relatively dense gas, for which most of the shock cooling occurs through molecular or ionic lines in the mid-infrared.

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A systematic and automated search of the extensive GLIMPSE mid-infrared survey data of the inner Galaxy was carried out to uncover new star clusters. This search has yielded 59 new clusters. Using our automated search algorithm, these clusters were identified as significant localized overdensities in the GLIMPSE point-source catalog (GLMC) and archive (GLMA). Subsequent visual inspection of the GLIMPSE image mosaics confirmed the existence of these clusters plus an additional 33 heavily embedded clusters missed by our detection algorithm, for a total of 92 newly discovered clusters. These previously uncataloged clusters range in type from heavily embedded to fully exposed clusters. More than half of the clusters have memberships exceeding 35 stars, and nearly all the clusters have diameters of 3 0 or less. The Galactic latitude distribution of the clusters reveals that the majority are concentrated toward the Galactic midplane. There is an asymmetry in the number of clusters located above and below the midplane, with more clusters detected below the midplane. We also observe an asymmetry in the number of clusters detected in the northern and southern halves of the Galaxy, with more than twice as many clusters detected in the south.

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