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.

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.

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We report 269 mid-infrared bubbles within 10 of the Galactic center from visual inspection of the Spitzer GLIMPSE II Legacy Science program images. The surface density of bubbles is $5 deg À2 or about 3 times that detected in longitudes jlj ¼ 10 Y65 , because the inner 10 of longitude were more thoroughly searched for small bubbles. There is a gradient in the number of bubbles with longitude with an increase of about a factor of 2 from 2 to 10 ; this is probably the result of several factors, including decreasing diffuse background brightness, confusion, and opacity with longitude. Bubble eccentricities are typically between 0.6 and 0.8, and >50% show evidence for blowouts, which we suggest result from local density fluctuations of the ISM and/or anisotropic stellar winds and radiation fields. The fraction of bubbles identified with H ii regions and clusters is only about half that found at jlj > 10 . This is largely a result of the much smaller angular diameter of bubbles cataloged in the inner Galaxy than in the outer Galaxy. At least 12% of the bubbles have morphologies suggestive of triggered star formation. Most of the bubbles that show evidence for triggered star formation in the inner Galaxy have not formed secondary bubbles; it is postulated that this may be because they are too young to have had time for this to occur.

We combine diffuse emission photometry from GLIMPSE and several other Galactic plane surveys covering near-IR through radio wavelengths to synthesize a global spectral energy distribution (SED) for the M17 complex. By balancing the integrated flux in the SED with the total bolometric luminosity of all known O and early B stars in the ionizing cluster, we estimate a distance to M17 of 1:6 þ0:3 À0:1 kpc. At this distance, the observed total flux in the SED corresponds to a luminosity of 2:4 AE 0:3 ; 10 6 L . We find that the SED from the H ii region peaks at shorter wavelengths and has a qualitatively different shape than the SED from the photodissociation region ( PDR). We find that polycyclic aromatic hydrocarbons ( PAHs) are destroyed over a short distance or edge at the boundary of the H ii region. We demonstrate that this PAH destruction edge can be located easily using GLIMPSE band-ratio images and confirm this using Spitzer IRS spectra. We investigate the relative roles of extreme ultraviolet (EUV ) and X-ray photons in the destruction of PAHs, concluding that X-rays are not an important PAH destruction mechanism in M17 or, by extension, in any other Galactic H ii region. Our results support the hypothesis that PAHs are destroyed by EUV photons within H ii regions. PAHs dominate the mid-IR emission in the neutral PDR beyond the ionized gas.