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Spitzer Survey of the Large Magellanic Cloud: Surveying the Agents of a Galaxy's Evolution (SAGE-LMC)

The recycling of matter between the interstellar medium (ISM) and stars drives the evolution of a galaxy's visible matter. To understand this recycling, we propose 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 the galaxy-wide scale of the Large Magellanic Cloud (LMC). Due to its proximity, favorable viewing angle, multi-wavelength information, and measured tidal interactions with the Milky Way (MW) and Small Magellanic Cloud (SMC), the LMC is uniquely suited for surveying the agents of a galaxy's evolution (SAGE), the ISM and stars. Our uniform and unbiased survey of the LMC (7x7 degrees) in all IRAC and MIPS bands will have much better wavelength coverage, up to ~1000 times better point source sensitivity and ~11 times better angular resolution than previous IR surveys. Full and uniform coverage of the LMC is necessary to study the galaxy as a system, to develop a template for more distant galaxies and to create an archival data set (rights waived) that promises a lasting legacy to match current LMC surveys at other wavelengths. SAGE will reveal over 6 million sources including ~150,000 evolved stars, ~50,000 young stellar objects and the diffuse ISM with column densities >1.2e21 H/cm2. In contrast to the MW and SMC, the diffuse IR emission in the LMC can be unambiguously associated with individual gas/dust clouds, thereby permitting unique studies of dust processes in the ISM. SAGE's complete census of newly formed stars with masses >1-3 Msun will reveal whether tidally-triggered star formation events in the LMC are sustained or short-lived. SAGE's complete census of evolved stars with mass loss rates >1e-8 Msun/yr will quantitatively measure the rate at which evolved stars inject mass into the ISM. SAGE will be the crucial link between Spitzer's survey of individual IR sources in the MW (GLIMPSE) and its surveys of galaxies (e.g., SINGS) and a stepping stone to the deep surveys (e.g., GOODS & SWIRE).


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

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. The SAGE-SMC program 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-Spectroscopy: the life cycle of dust and gas in the Large Magellanic Cloud

SAGE-Spec leverages the SAGE-LMC program by conducting a comprehensive IRS and MIPS - SED spectroscopy program of dust with the goal to determine the composition, origin, evolution, and observational characteristics of interstellar dust and its role in the LMC. Analysis of the spectra will yield composition and abundance of the dust compounds in different LMC objects, including AGB stars, post - AGB, young stellar objects, HII regions and the general diffuse ISM and provide a quantitative picture of the dust lifecyle. Besides dust features, the spectra will also contain molecular and atomic emission and absorption lines, providing the diagnostics to determine physical parameters such as temperature, density and radiation field - all important to the formation and processing of dust, and understanding the life cycle of matter.
The SAGE-Spec spectroscopic survey will provide critical underpinning for the SAGE-LMC survey by linking observed IRAC and MIPS colors of LMC objects to the infrared spectral type of the object. We will to the maximum extent utilize the LMC spectroscopy available in the Spitzer archive. A subset of the IRS point sources from this proposal will also be surveyed in MIPS SED. Legacy data products that will be made available to the public include all reduced single point spectra and data cubes, feature maps, a spectral catalog, and a fully classified SAGE point source catalog.



Stellar feedback on circumcluster gas and dust in 30 Doradus, the nearest super-star cluster

Massive stars dominate the evolution of their host galaxies by energetic feedback into the interstellar medium. Therefore, if we wish to understand galaxy evolution, we must understand how massive star clusters process local gas and dust (radiatively and mechanically), and how strong stellar winds interact with the HII region. In particular, the most energetic form of star formation in the universe occurs in super star clusters, which are an increasingly dominant mode of star formation as one looks further back in time. The only super star cluster near enough to be studied in detail with Spitzer (or any other existing telescope) is 30 Doradus in the Large Magellanic Cloud. 30 Doradus is also the ideal massive cluster to study because it has a very well characterized stellar population, is extremely massive, and has subsolar metallicity. We propose a complete spectral map of the 30 Doradus region with the IRS low-esolution modules (5-8um) and MIPS/SED mode (55-5um). Analysis of the fine structure lines and aromatic features, using sophisticated modeling tools already developed by our team, will allow a complete self-onsistent understanding of how this super-tar cluster is affecting its circumcluster gas and dust. Understanding the infrared emitting species (dust, PAHs, ionized gas) with this unprecedented level of detail is a necessary step to quantitavely connect the spectra of distant unresolved galaxies to the star formation in those galaxies. This detailed analysis relies on simultaneous mapping of multiple diagnostic line ratios and dust features, and thus can only be accomplished with Spitzer/IRS (many of the diagnostic lines are not observable through the atmosphere). Furthermore, conditions in 30 Dor are known to vary dramatically on small scales, so a spatially and spectrally complete data cube will be the only way to link the physical conditions of the gas, radiation, and dust.


HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) in the Magellanic Clouds

We propose a uniform survey of the Large Magellanic Cloud (LMC, 8x8.5 degrees), Small Magellanic Cloud (SMC, 5x5 degrees), and the Magellanic bridge (4x3 degrees) in SPIRE 250, 350, 500 um and PACS 100 and 160 um bands in order to produce a HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE), the interstellar medium (ISM) and massive stars. Herschel images will provide key insights into the life cycle of galaxies because the far-infrared and submm emission from dust grains is an effective tracer of the coldest ISM dust, the most deeply embedded young stellar objects (YSOs), and the dust ejected over the lifetime of massive stars. The ISM dust map will directly measure dust on a scale size of individual regions (~10pc, ~5-20 K) with column densities >0.85x10^{21} and >6x10^{21} H-atoms cm^{-2} for the LMC and SMC, respectively. Dust emission per beam will be detected for regions with >0.1 Msun at ~25 K, >5 Msun of 10 K. HERITAGE will complete 1) the census of massive YSOs down to >4 Msun Class 0 sources and 2) the inventory of dust injected into the ISM by massive evolved stars and supernova remnants (SNRs). The variation in dust properties discerned from the dust maps and the measurements of the main cooling lines of the SNR shocks will quantify their effect on the interstellar grain size distribution. HERITAGE will create an archival data set that promises a lasting legacy to match current LMC and SMC surveys at other wavelengths. HERITAGE will bridge the gap between Herschel studies of the Milky Way and those of nearby galaxies and provide a template for high red shift galaxies.


SMC-Spec: The SMC as a probe of dust in the early Universe

We propose a comprehensive spectroscopic survey of the Small Magellanic Cloud (SMC), using the IRS and MIPS-SED. The SMC has a metallicity similar to high-redshift galaxies, and its proximity makes it a spatially resolved proxy for star-forming galaxies in the distant, early Universe. The sensitivity of the Spitzer Space Telescope allows us to to observe dust in nearly every stage of its life cycle in the SMC so that we can study how the interactions of dust and its host galaxy differ from more metal-rich systems like the Galaxy and the LMC. Our proposed observations concentrate on important classes underrepresented in the archive of SMC spectra such as young stellar objects, compact H II regions, objects in transition to and from the asymptotic giant branch, and supergiants. These observations, in combination with those already in the archive, will give us a complete picture of the dust in a metal-poor star-forming galaxy similar to those in the early Universe. This proposal received 62 hours of Spitzer observing time, allocated as part of the IRS Guaranteed Time Observations.
 
 

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