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. 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