Showing papers by "Brian Babler published in 2007"

The Bubbling Galactic Disk. II. The Inner 20

Abstract: 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-2 or about 3 times that detected in longitudes |l| = 10°-65°, 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 |l| > 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.

A multiwavelength study of m17: the spectral energy distribution and pah emission morphology of a massive star formation region

Abstract: 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 kpc. At this distance, the observed total flux in the SED corresponds to a luminosity of 2.4 ± 0.3 × 106 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.

Embedded Star Formation in the Eagle Nebula with Spitzer GLIMPSE

Abstract: We present new Spitzer photometry of the Eagle Nebula (M16, containing the optical cluster NGC 6611) combined with near-infrared photometry from 2MASS. We use dust radiative transfer models, mid-infrared and near-infrared color-color analysis, and mid-infrared spectral indices to analyze point-source spectral energy distributions, select candidate YSOs, and constrain their mass and evolutionary state. Comparison of the different protostellar selection methods shows that mid-infrared methods are consistent, but as has been known for some time, near-infrared-only analysis misses some young objects. We reveal more than 400 protostellar candidates, including one massive YSO that has not been previously highlighted. The YSO distribution supports a picture of distributed low-level star formation, with no strong evidence of triggered star formation in the pillars. We confirm the youth of NGC 6611 by a large fraction of infrared excess sources and reveal a younger cluster of YSOs in the nearby molecular cloud. Analysis of the YSO clustering properties shows a possible imprint of the molecular cloud's Jeans length. Multiwavelength mid-IR imaging thus allows us to analyze the protostellar population, to measure the dust temperature and column density, and to relate these in a consistent picture of star formation in M16.

Molecular Outflows and a Mid-Infrared Census of the Massive Star Formation Region Associated with IRAS 18507+0121

Abstract: We have observed the central region of the infrared-dark cloud filament associated with IRAS 18507+0121 at millimeter wavelengths in CO(J = 1-0), ^(13)CO(J = 1-0), and C^(18)O(J = 1-0) line emission and with Spitzer at mid-infrared wavelengths. Five massive outflows from two cloud cores were discovered. Three outflows are centered on or near an ultracompact (UC) H II region (G34.4+0.23), while the remaining two outflows originate from the millimeter core G34.4+0.23 MM. Modeling of the spectral energy distributions of the mid-infrared sources identified 31 young stellar objects in the filament with a combined stellar mass of ~127 ± 27 M_☉. An additional 22 sources were identified as probable cluster members based on the presence of strong 24 μm emission. The total star formation efficiency in the G34.4 cloud filament is estimated to be ~7%, while the massive and intermediate-mass star formation efficiency in the entire cloud filament is estimated to be roughly 2%. A comparison of the gravitational binding energy with the outflow kinetic energy suggests that the compact core containing G34.4+0.23 MM is being destroyed by its molecular outflows, whereas the outflows associated with the more massive core surrounding the G34.4 UC H II region are not likely to totally disrupt the cloud. In addition, a qualitative evaluation of the region appears to suggest that stars in this region may have formed in two stages: first lower mass stars formed and then, a few Myr later, the more massive stars began to form.

Spitzer IRAC observations of newly discovered planetary nebulae from the MACQUARIE-AAO-Strasbourg Hα planetary nebula project

Abstract: We compare H?, radio continuum, and Spitzer Space Telescope images of 58 planetary nebulae (PNe) recently discovered by the Macquarie-AAO-Strasbourg H? PN Project (MASH) of the SuperCOSMOS H? Survey. Using Infrared Array Camera (IRAC) data, we examine the relationships between optical and MIR morphologies from 3.6 to 8.0 ?m and explore the ratio of mid-infrared (MIR) to radio nebular fluxes, which is a valuable discriminant between thermal and nonthermal emission. MASH emphasizes late evolutionary stages of PNe compared with previous catalogs, enabling study of the changes in MIR and radio flux that attend the aging process. Spatially integrated fluxes are constructed for all MASH PNe observed by the GLIMPSE Legacy Project, using the H? morphologies to define the areas of MIR and radio continuum emission observed by the Midcourse Space Experiment (MSX), IRAC, the Molonglo Observatory Synthesis Telescope, and the Very Large Array (VLA). The ratio of IRAC 8.0 ?m to MSX 8.3 ?m fluxes provides an assessment of the absolute diffuse calibration of IRAC at 8.0 ?m. We independently confirm the aperture correction factor to be applied to IRAC at 8.0 ?m to align it with the diffuse calibration of MSX. The result is in accord with the recommendations of the Spitzer Science Center and with our results from a parallel study of H II regions in the MIR and radio. However, these PNe probe the diffuse calibration of IRAC on a spatial scale of 9''-77'', as opposed to the many-arcminute scale from the H II regions' study.

Absolute diffuse calibration of IRAC through mid-infrared and radio study of H ii regions

Abstract: We investigate the diffuse absolute calibration of the Infrared Array Camera (IRAC )o n the Spitzer Space Telescope (SST) at 8.0 μm using a sample of 43 H II regions with a wide range of morphologies near l = 312 ◦ . For each region we carefully measure sky-subtracted, pointsource-subtracted, areally integrated IRAC 8.0-μm fluxes and compare these with Midcourse Space eXperiment (MSX) 8.3-μm images at two different spatial resolutions, and with radio continuum maps. We determine an accurate median ratio of IRAC 8.0-μm/MSX 8.3-μm fluxes, of 1.55 ± 0.15. From robust spectral energy distributions of these regions we conclude that the present 8.0-μm diffuse calibration of the SST is 36 per cent too high compared with the MSX validated calibration, perhaps due to scattered light inside the camera. This is an independent confirmation of the result derived for the diffuse calibration of IRAC by the Spitzer Science Centre (SSC). From regression analyses we find that 843-MHz radio fluxes of H II regions and mid-infrared (MIR) fluxes are linearly related for MSX at 8.3 μm and Spitzer at 8.0 μm, confirming the earlier MSX result by Cohen & Green. The median ratio of MIR/843-MHz diffuse continuum fluxes is 600× smaller in non-thermal than thermal regions, making it a sharp discriminant. The ratios are largely independent of morphology up to a size of ∼24 arcmin. We provide homogeneous radio and MIR morphologies for all sources. MIR morphology is not uniquely related to radio structure. Compact regions may have MIR filaments and/or diffuse haloes, perhaps infrared counterparts to weakly ionized radio haloes found around compact H II regions. We offer two IRAC colour‐colour plots as quantitative diagnostics of diffuse H II regions.

Spitzer IRAC observations of newly-discovered planetary nebulae from the Macquarie-AAO-Strasbourg H-alpha Planetary Nebula Project

Abstract: We compare H-alpha, radio continuum, and Spitzer Space Telescope (SST) images of 58 planetary nebulae (PNe) recently discovered by the Macquarie-AAO-Strasbo- urg H-alpha PN Project (MASH) of the SuperCOSMOS H-alpha Survey. Using InfraRed Array Camera (IRAC) data we define the IR colors of PNe and demonstrate good isolation between these colors and those of many other types of astronomical object. The only substantive contamination of PNe in the color-color plane we illustrate is due to YSOs. However, this ambiguity is readily resolved by the unique optical characteristics of PNe and their environs. We also examine the relationships between optical and MIR morphologies from 3.6 to 8.0um and explore the ratio of mid-infrared (MIR) to radio nebular fluxes, which is a valuable discriminant between thermal and nonthermal emission. MASH emphasizes late evolutionary stages of PNe compared with previous catalogs, enabling study of the changes in MIR and radio flux that attend the aging process. Spatially integrated MIR energy distributions were constructed for all MASH PNe observed by the GLIMPSE Legacy Project, using the H-alpha morphologies to establish the dimensions for the calculations of the Midcourse Space Experiment (MSX), IRAC, and radio continuum (from the Molonglo Observatory Synthesis Telescope and the Very Large Array) flux densities. The ratio of IRAC 8.0-um to MSX 8.3-um flux densities provides a measure of the absolute diffuse calibration of IRAC at 8.0 um. We independently confirm the aperture correction factor to be applied to IRAC at 8.0um to align it with the diffuse calibration of MSX. The result agrees with the recommendations of the Spitzer Science Center and with results from a parallel study of HII regions. These PNe probe the diffuse calibration of IRAC on a spatial scale of 9-77 arcsec.

The frequency of mid-infrared excess sources in galactic surveys

Abstract: We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 μm mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSE surveys. Of these, 183 are either OB stars earlier than B8 in which the excess plausibly arises from a thermal bremsstrahlung component or evolved stars in which the excess may be explained by an atmospheric dust component. The remaining 47 stars have spectral classifications B8 or later and appear to be main-sequence or late pre-main-sequence objects harboring circumstellar disks. Six of the 47 stars exhibit multiple signatures characteristic of pre-main-sequence circumstellar disks, including emission lines, near-IR K-band excesses, and X-ray emission. Approximately one-third of the remaining 41 sources have emission lines suggesting relative youth. We modeled the excesses in 26 stars having two or more measurements in excess of the expected photospheres as single-component blackbodies. We determine probable disk temperatures and fractional IR luminosities in the range 191 K < T < 787 K and 3.9 × 10-4 < LIR/L* < 2.7 × 10-1. The majority of our modeled sample (14 stars) have 10-3 < LIR/L* < 10-2 and are consistent with either transition disks or massive debris disks. These objects have fractional IR luminosities and temperatures between those of β Pic-type debris disk systems (LIR/L* ≤ 10-3) and Class II pre-main-sequence systems (LIR/L* 10-1). We estimate a lower limit on the fraction of Tycho-2 Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses to be 1.0% ± 0.3%.

Infrared Point-Source Variability between the Spitzer and Midcourse Space Experiment Surveys of the Galactic Midplane

Abstract: We present a list of 552 sources with suspected variability, based on a comparison of mid-infrared photometry from the GLIMPSE I and Midcourse Space Experiment (MSX) surveys, which were carried out nearly a decade apart. We were careful to address issues such as the difference in resolution and sensitivity between the two surveys, as well as the differences in the spectral responses of the instruments. We selected only sources where the IRAC 8.0 μm and MSX 8.28 μm fluxes differ by more than a factor of 2, in order to minimize contamination from sources where the difference in fluxes at 8 μm is due to a strong 10 μm silicate feature. We present a subset of 40 sources for which additional evidence suggests variability, using 2MASS and MIPSGAL data. Based on a comparison with the variability flags in the IRAS and MSX point-source catalogs we estimate that at least a quarter of the 552 sources and at least half of the 40 sources in the subset are truly variable. In addition, we tentatively confirm the variability of one source using multiepoch IRAS LRS spectra. We suggest that most of the sources in our list are likely to be asymptotic giant branch stars.

Infrared point source variability between the Spitzer and MSX surveys of the Galactic mid-plane

Abstract: We present a list of 552 sources with suspected variability, based on a comparison of mid-infrared photometry from the GLIMPSE I and MSX surveys, which were carried out nearly a decade apart. We were careful to address issues such as the difference in resolution and sensitivity between the two surveys, as well as the differences in the spectral responses of the instruments. We selected only sources where the IRAC 8.0 and MSX 8.28 micron fluxes differ by more than a factor of two, in order to minimize contamination from sources where the difference in fluxes at 8 micron is due to a strong 10 micron silicate feature. We present a subset of 40 sources for which additional evidence suggests variability, using 2MASS and MIPSGAL data. Based on a comparison with the variability flags in the IRAS and MSX Point-Source Catalogs we estimate that at least a quarter of the 552 sources, and at least half of the 40 sources are truly variable. In addition, we tentatively confirm the variability of one source using multi-epoch IRAS LRS spectra. We suggest that most of the sources in our list are likely to be Asymptotic Giant Branch stars.

Structure generation by irradiation: what can glimpse teach us about the ism structure?

Abstract: Diffuse emission in the mid-infrared shows a wealth of structure, which lends itself to high-resolution structure analysis of the interstellar gas. A large part of the emission comes from polycyclic aromatic hydrocarbons (PAHs), excited by nearby ultraviolet sources. Can the observed diffuse emission structure be interpreted as column density structure?Wediscussthisquestionwiththehelpofasetof modelmolecularcloudsbathedintheradiationfieldofa nearby O star. The correlation strength between column density and ‘‘observed’’ flux density strongly depends on theabsolutevolumedensityrangeintheregion.Shadowingandirradiationeffectsmaycompletelyalter theappearance of an object. Irradiation introduces additional small-scale structure, and it can generate structures resembling shells around H ii regions in objects that do not possess any shell-like structures whatsoever. Nevertheless, with some effort, structural information about the underlying interstellar medium can be retrieved. In the more diffuse regime [n(Hi)P100cm � 3 ],fluxdensitymapsmaybeusedtotracethe3Ddensitystructureof thecloudviadensitygradients. Thus, while caution definitely is in order, mid-infrared surveys such as GLIMPSE will provide quantitative insight into the turbulent structure of the interstellar medium. Subject headingg dust, extinction — infrared: ISM — ISM: structure — methods: numerical — radiative transfer — turbulence 1. THE PROBLEM

Discovery of two galaxies deeply embedded in the great attractor wall

Abstract: We report on the discovery of two spiral galaxies located behind the southern Milky Way, within the leastexplored region of the Great Attractor. They lie at l;b ðÞ �317 � ; � 0:5 � ðÞ , where obscuration from Milky Way stars and dust exceeds 13Y15 mag of visual extinction. The galaxies were the most prominent of a set identified using mid-infrared images of the low-latitude (jbj < 1 � ) Spitzer Legacy program Galactic Legacy Infrared Mid-Plane Survey Extraordinaire. Follow-up H i radio observations reveal that both galaxies have redshifts that place them squarely in the Norma Wall of galaxies, which appears to extend diagonally across the Galactic plane from Norma in the south to Centaurus/Vela in the north. We report on the near-infrared, mid-infrared, and radio properties of these newly discovered galaxies and discuss their context in the larger view of the Great Attractor. The work presented here demonstrates that mid-infrared surveys open up a new window to study galaxies in the zone of avoidance.

Galactic Structure and Star Formation in Vela-Carina

Abstract: 3 Quality Checks and Source List Validation 6 3.1 Astrometric Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 Photometric Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.3 Color-Color and Color-Magnitude Plots . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.4 Other checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

A glimpse of the Southern Jellyfish Nebula and its massive YSO

Abstract: In Spitzer/IRAC images obtained under the GLIMPSE Legacy Survey, we have identified a unique and provocative nebular object we call the "Southern Jellyfish Nebula." The Southern Jellyfish Nebula is characterized by a fan of narrow tendrils with extreme length-to-width ratios that emanate from the vicinity of a bright infrared point source embedded in a smaller resolved nebula. From CO observations of the Nebula's morphologically associated molecular cloud, we have derived a kinematic distance of 5.7 ± 0.8 kpc and a cloud mass of 3.2 ± 0.9 × 10^3 M_⊙. The tendril-like ropes of the Nebula have widths of ~0.1 pc and lengths of up to ~2 pc. We have integrated the infrared spectral energy distribution (SED) of the point source to establish it as a massive young stellar object (MYSO), most likely forming alone, but possibly masking fainter cluster members. The shape of the SED is consistent with the shape of a late Class 0 SED model. Based on its far-IR luminosity of 3.3 ± 0.9 × 10^4 L_⊙, the Southern Jellyfish's MYSO has a zero-age main sequence (ZAMS) spectral type of B0. Given the curious nature of this nebula, we suspect its peculiar IR-bright structure is directly related to its current state of star formation.

Molecular Outflows and a Mid-Infrared Census of the Massive Star-Formation Region Associated with IRAS 18507+0121

Abstract: We have observed the central region of the IR-dark cloud filament associated with IRAS 18507+0121 at millimeter wavelengths in CO(1-0), 13CO(1-0), and C18O(1-0) line emission and with the Spitzer Space Telescope at mid-IR wavelengths. Five massive outflows from two cloud cores were discovered. Three outflows are centered on or near an Ultracompact HII region (G34.4+0.23) while the remaining two outflows originate from the millimeter core G34.4+0.23 MM. Modeling of the SEDs of the mid-IR sources identified 31 young stellar objects in the filament with a combined stellar mass of ~127 +/- 27 Msun. An additional 22 sources were identified as probable cluster members based on the presence of strong 24 micron emission. The total star formation efficiency in the G34.4 cloud filament is estimated to be ~7% while the massive and intermediate mass star formation efficiency in the entire cloud filament is estimated to be roughly 2%. A comparison of the gravitational binding energy with the outflow kinetic energy suggests that the compact core containing G34.4+0.23 MM is being destroyed by its molecular outflows whereas the outflows associated with more massive core surrounding the G34.4 UC HII region are not likely to totally disrupt the cloud. Additionally, a qualitative evaluation of the region appears to suggest that stars in this region may have formed in two stages: first lower mass stars formed and then, a few Myrs later, the more massive stars began to form.

SPITZER SAGE Observations of Large Magellanic Cloud Planetary Nebulae

Abstract: We present IRAC and MIPS images and photometry of a sample of previously known planetary nebulae (PNe) from the SAGE survey of the Large Magellanic Cloud (LMC) performed with the Spitzer Space Telescope. Of the 233 known PNe in the survey field, 185 objects were detected in at least two of the IRAC bands, and 161 detected in the MIPS 24 micron images. Color-color and color-magnitude diagrams are presented using several combinations of IRAC, MIPS, and 2MASS magnitudes. The location of an individual PN in the color-color diagrams is seen to depend on the relative contributions of the spectral components which include molecular hydrogen, polycyclic aromatic hydrocarbons (PAHs), infrared forbidden line emission from the ionized gas, warm dust continuum, and emission directly from the central star. The sample of LMC PNe is compared to a number of Galactic PNe and found to not significantly differ in their position in color-color space. We also explore the potential value of IR PNe luminosity functions (LFs) in the LMC. IRAC LFs appear to follow the same functional form as the well-established [O III] LFs although there are several PNe with observed IR magnitudes brighter than the cut-offs in these LFs.

SAGE-SMC: Surveying the Agents of Galaxy Evolution in the Tidally-Disrupted, Low-Metallicity Small Magellanic Cloud

Abstract: The observable properties of galaxy evolution are largely driven by the life−cycle of baryonic matter: stars precipitate out of a complex, multi−phase interstellar medium; and eventually, evolved stellar populations return enriched material back to the ISM via stellar winds or supernova explosions. As demonstrated by the SAGE−LMC survey, comprehensive Spitzer imaging of a nearby galaxy provides an incredibly rich view of this baryonic lifecycle, allowing for an unprecedented understanding of the physical processes which drive galaxy evolution. This proposal will extend the SAGE analysis to the whole SMC (Bar, Wing, and high−density portion of the Magellanic Bridge), a galaxy whose properties are uniquely similar to those of star−forming galaxies at high redshift. Specifically, the SMC’s metallicity is below the critical threshold (1/3− 1/4 Z_sun) where interstellar medium properties are observed to change dramatically (sharp reduction in the PAH dust mass fraction, reduced dust−to−gas ratio, and extreme ultraviolet extinction curve variations). In addition, the SMC has been profoundly influenced by past interactions with the LMC and Milky Way, allowing us to study the impact of periodic interactions on the structure of the ISM and the physical processes of star formation. We will gain crucial insight into the ISM and star formation in a known tidal debris structure (Bridge portion of SMC), which has a metallicity 4 times lower than the rest of the SMC. When combined with observations of the Milky Way (GLIMPSE, MIPSGAL) and the LMC (SAGE−LMC), our survey of the SMC (SAGE−SMC) will provide a complete and detailed picture of the life−cycle of baryons in galactic environments spanning orders of magnitude in metallicity, and wide ranges in star formation history. This understanding will equip us to properly interpret the infrared properties of more distant galaxies, both in the local (e.g., SINGS) and high−redshift (e.g., GOODS and SWIRE) universe. SAGE-SMC, K. D. Gordon et al. 1 1 Scientific Justification The interstellar medium (ISM) plays a central role in the galaxy evolution as the birthsite of new stars and repository of old stellar ejecta. The formation of new stars slowly consumes the ISM, locking it up for millions to billions of years. As these stars age, the winds from low mass, asymptotic giant branch (AGB) stars and high mass, red supergiants (RSGs), and supernova explosions inject nucleosynthetic products of stellar interiors into the ISM, slowly increasing its metallicity. This constant recycling and associated enrichment drives the evolution of a galaxy’s visible matter and changes its emission characteristics. To understand this recycling, we have to study the physical processes of the ISM, the formation of new stars, and the injection of mass by evolved stars, and their relationships on a galaxy-wide scale. Among the nearby galaxies, the Small Magellanic Cloud (SMC) represents a unique astrophysical laboratory for studies of the lifecycle of the ISM, because of its proximity (∼60 kpc, Hilditch et al. 2005), low ISM metallicity (1/5-1/20 Z⊙; Russell & Dopita 1992; Rolleston et al. 1999) and tidally-disrupted interaction status (Zaritsky & Harris 2004). The SMC offers a rare glimpse into the physical processes in an environment with a metallicity which is below the threshold of 1/4–1/3 Z⊙ where the properties of the ISM in galaxies changes significantly as traced by the rapid reduction in the PAH dust mass fractions and dust-to-gas ratios (Engelbracht et al. 2005; Draine et al. 2007). In addition, the SMC is the only local galaxy which has the ultraviolet dust characteristics (lack of 2175 Å extinction bump; Gordon et al. 2003) of starburst galaxies in the local (Calzetti et al. 1994; Gordon et al. 1997) and high-redshift (2 < z < 4; Vijh et al. 2003) universe. The evolution of stars in the SMC is also clearly affected by the low metallicities (Cioni et al. 2006) with the corresponding expected differences in stellar mass loss. The Large and Small Magellanic clouds represent the nearest example of tidally interacting galaxies and the Magellanic Bridge is a clear manifestation of a close encounter of these two galaxies some 200 Myr ago (Zartisky & Harris 2004). Over cosmological timescales, galaxy interactions are one of the key drivers of galaxy evolution and, thus, tidally interacting galaxies allow us to examine star formation in an unusual and disturbed environment, which resembles the conditions in the early universe when galaxies were forming. The Magellanic Bridge is a filament of neutral hydrogen, which joins the SMC and LMC over some 15 kpc (Staveley-Smith et al. 1998; Muller et al. 2004). Recent studies have revealed the presence of locally formed, young (<200 Myrs) massive stars associated with the highest-density portion of the Bridge which is adjacent to the main SMC body (Harris 2007). Finally, the Magellanic bridge is characterized by a much lower metallicity than the main SMC body (1/20 instead of 1/5 Z⊙) which provides an even more extreme star formation environment than the main SMC body. We propose to survey the full SMC (33 ⊓⊔) and the star forming portion of the Magellanic bridge using IRAC and MIPS. The multiwavelength appearance of the SMC (Fig. 1) clearly shows this galaxy is made of three main components: the Bar, Wing, and high-density portion of the Magellanic Bridge. The proposed observations will allow us to trace the life cycle of dust (and thereby gas) on a galaxy wide scale from their injection by late-type stars, through their sojourn in the violent ISM, until their demise during the process of star formation. In addition, the IR emission will trace the global structure of the ISM on a galaxy-wide scale and allow us to trace the interrelationship of the various phases of the ISM. This survey will provide a complete census of the star formation population in this low and spatially varying metallicity environment. Full and uniform coverage of the SMC is necessary to understand the galaxy as a complete system, to develop a template SAGE-SMC, K. D. Gordon et al. 2 for more distant galaxies, and to create an archival data set that promises a lasting legacy to match SMC surveys at other wavelengths. With much improved wavelength coverage, up to ∼1000 times better point source sensitivity and ∼11 times better angular resolution than the MSX and IRAS surveys (Fig. 2) and >10X spatial coverage than the SMC Spitzer mini-survey (Bolatto et al. 2007), SAGE-SMC will reveal over 3 million sources including 8,000 mass-losing evolved stars and 3,000 young stellar objects (YSOs). The SMC minisurvey was mainly concerned with the characterizing SMC low metallicity star formation and was limited to cover only the Bar and a portion of the Wing which represent ∼10% of the whole SMC. As a result, the SMC cannot address the science goals which are at the core of this proposal: the lifecycle of interstellar dust, the global structure of the ISM, and the characteristics of tidally driven star formation. Combining the results from this proposed SMC survey with the existing LMC (SAGELMC, Meixner et al. 2006) and Milky Way (GLIMPSE, Benjamin et al. 2003; MIPSGAL, Casey et al. 2005) surveys will provide a foundation for understanding the physics of the ISM, current star formation, and evolved stellar mass loss as a function of metallicity. This foundation is crucial for interpreting the observations of more distant galaxies like those in the SINGS (Kennicutt et al. 2003), SWIRE (Lonsdale et al. 2003), and GOODS (Dickinson et al. 2003) Spitzer Legacy programs. Without the SAGE-SMC survey, there would be a missing link in our understanding of galaxies at the low metallicity, less chemically evolved stage. Our imaging survey is a base for future work in the SMC with SOFIA, Herschel, the James Webb Space Telescope (JWST), and the Atacama Large Millimeter Array. Interstellar Medium The SMC presents a distinct mix of ISM components different from that found in the MW and LMC. For example, the molecular phase in the MW dominates the inner disk and atomic gas dominates elsewhere, while the diffuse ISM only has ∼15% of the gas mass. In contrast, in the SMC, the ionized ISM dominates, then the atomic gas and, finally, the molecular ISM which is relatively confined and lower mass (Leroy et al. 2007). The differences seen in the SMC are likely related to its low metallicity which varies from ∼1/5 Z⊙ (Bar/Wing, Russell & Dopita 1992) to ∼1/20 Z⊙ (Bridge, Rolleston et al. 1999). Observations with ISO (Madden et al. 2006) and Spitzer (Engelbracht et al. 2005) have revealed that the ISM in low-metallicity environments has weak/absent PAH emission. The absence of PAHs has a profound influence on the gas heating and the existence of cold/warm phases in the ISM (Wolfire et al. 1995). In particular, variations in the small grain properties, as traced by PAH emission, are of fundamental importance to the ISM thermodynamics since these grains are efficient in heating the gas through the photoelectric effect (Bakes & Tielens 1994). The basic question that will be answered by the SAGE-SMC survey is: How do the dust properties vary across the SMC and what do they tell us about the physics of ISM processing? Previous observations in the SMC have shown large variations in dust properties: dust in the Bar has very weak aromatic features and has UV extinction with a steep UV rise and no 2175 Å bump, while the dust in the Wing shows MW-like UV extinction and aromatic features (Gordon et al. 2003; Li & Draine 2002; Bolatto et al. 2006). Additionally, the gasto-dust ratio has been seen to vary spatially across the SMC by a factor of a few (Bot et al. 2004, Leroy et al. 2007). Using the average SMC Bar extinction curve, Galliano et al. (2007) found that the proportion of carbonaceous:silicaceous grains is 0.02:0.98 in the SMC, whereas it is 0.14:0.86 in the LMC and 0.36:0.64 in the MW, quantifying how silicate rich the dust in t