We determine and tabulate A [λ] /A K , the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25µm ≤ λ ≤ 8.0µm along two lines of sight: l = 42 • and l = 284 • . The first is a relatively quiescent and unremarkable region; the second contains the giant H II region RCW 49 as well as a "field" region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3-8µm by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A [λ] − A K )/(A J − A K ) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g. A J /A K , is required to calculate A [λ] /A K from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sightlines, and fit the reddening as a function of magnitude (distance) to determine A J /kpc, A K /kpc, and A J /A K . Our values of A [λ] /A K show a flattening across the 3-8µm wavelength range, roughly consistent with the Lutz et al. (1996) extinction measurements derived for the sightline toward the Galactic center.

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.

ABSTRACT. A brief description is given of the GLIMPSE surveys, including the areas surveyed, sensitivity limits, and products. The primary motivations for this review are to describe some of the main scientific results enabled by the GLIMPSE surveys and to note potential future applications of the GLIMPSE catalogs and images. In particular, we discuss contributions to our understanding of star formation and early evolution, the interstellar medium, galactic structure, and evolved stars. Infrared dark clouds (IRDCs), young stellar objects (YSOs), and infrared bubbles/H II regions are discussed in some detail. A probable triggered star formation associated with expanding infrared bubbles is briefly mentioned. The distribution and morphologies of dust and polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium are discussed. Examples are shown from GLIMPSE images of bow shocks, pillars (elephant trunks), and instabilities in massive star-formation regions. The infrared extinction law of diffuse interstellar dust is discussed. The large-scale structure of the Galaxy has been traced by red-clump giants using the GLIMPSE point-source catalog to reveal the radius and orientation of the central bar, the stellar radial scale length, an obvious increase in star counts toward the tangency to the Scutum-Centaurus spiral arm, the lack of an obvious tangency from star counts toward the Sagittarius spiral arm, and a sharp increase in star counts toward the nuclear bulge. Recent results on evolved stars and some serendipitous discoveries are mentioned. More than 70 refereed papers have been published based on GLIMPSE data as of 2008 November.

A visual examination of the images from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) has revealed 322 partial and closed rings that we propose represent partially or fully enclosed three-dimensional bubbles. We argue that the bubbles are primarily formed by hot young stars in massive star formation regions. We have found an average of about 1.5 bubbles per square degree. About 25% of the bubbles coincide with known radio H ii regions, and about 13% enclose known star clusters. It appears that B4-B9 stars (too cool to produce detectable radio H ii regions) probably produce about three-quarters of the bubbles in our sample, and the remainder are produced by young O-B3 stars that produce detectable radio H ii regions. Some of the bubbles may be the outer edges of H ii regions where PAH spectral features are excited and may not be dynamically formed by stellar winds. Only three of the bubbles are identified as known SNRs. No bubbles coincide with known planetary nebulae or W-R stars in the GLIMPSE survey area. The bubbles are small. The distribution of angular diameters peaks between 1 0 and 3 0 with over 98% having angular diameters less than 10 0 and 88% less than 4 0. Almost 90% have shell thicknesses between 0.2 and 0.4 of their outer radii. Bubble shell thickness increases approximately linearly with shell radius. The eccentricities are rather large, peaking between 0.6 and 0.7; about 65% have eccentricities between 0.55 and 0.85.

Using images from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), we have identified more than 300 extended 4.5 μm sources (Extended Green Objects (EGOs), for the common coding of the [4.5] band as green in three-color composite InfraRed Array Camera images). We present a catalog of these EGOs, including integrated flux density measurements at 3.6, 4.5, 5.8, 8.0, and 24 μm from GLIMPSE and the Multiband Imaging Photometer for Spitzer Galactic Plane Survey. The average angular separation between a source in our sample and the nearest IRAS point source is greater than 1 . The majority of EGOs are associated with infrared dark clouds (IRDCs), and where high-resolution 6.7 GHz CH 3 OH maser surveys overlap the GLIMPSE coverage, EGOs and 6.7 GHz CH 3 OH masers are strongly correlated. Extended 4.5 μm emission is thought to trace shocked molecular gas in protostellar outflows; the association of EGOs with IRDCs and 6.7 GHz CH 3 OH masers suggests that the extended 4.5 μm emission may pinpoint outflows specifically from massive protostars. The mid-IR colors of EGOs lie in regions of color-color space occupied by young protostars still embedded in infalling envelopes.

We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; $7 ; 7) using the IRAC (3.6, 4.5, 5.8, and 8 m) and MIPS (24, 70, and 160 m) instruments on board the Spitzer Space Telescope in the Surveying the Agents of a Galaxy's Evolution (SAGE) survey, these agents being the interstellar medium (ISM) and stars in the LMC. This paper provides an overview of the SAGE Legacy project, including observing strategy, data processing, and initial results. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities >1:2 ; 10 21 H cm À2 permits detailed studies of dust processes in the ISM. SAGE's point-source sensitivity enables a complete census of newly formed stars with masses >3 M that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass-loss rates >1 ; 10 À8 M yr À1 will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by 3 months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are nonproprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point-source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data for a region near N79 and N83. The MIPS 70 and 160 m images of the diffuse dust emission of the N79/N83 region reveal a similar distribution to the gas emissions, especially the H i 21 cm emission. The measured point-source sensitivity for the epoch 1 data is consistent with expectations for the survey. The point-source counts are highest for the IRAC 3.6 m band and decrease dramatically toward longer wavelengths, A

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.