Harvard Astronomy 201b

ARTICLE: Spitzer Survey of the Large Magellanic Cloud: Surveying the Agents of a Galaxy’s Evolution (SAGE). I. Overview and Initial Results

In Journal Club, Journal Club 2011 on April 26, 2011 at 3:54 pm

ADS Article Link

Paper Summary by Sukrit Ranjan and Gregory Green

Abstract

We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; \sim 7^{\circ} \times 7^{\circ}) using the IRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS (24, 70, and 160 μm) 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 \times 10^{21} \, \mathrm{H} / \mathrm{cm^2} 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 Msolar that will determine the current star formation rate in the LMC. SAGE’s detection of evolved stars with mass-loss rates >1×10-8 Msolar yr-1 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 μm 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 μm band and decrease dramatically toward longer wavelengths, consistent with the fact that stars dominate the point-source catalogs and the dusty objects detected at the longer wavelengths are rare in comparison. The SAGE epoch 1 point-source catalog has \sim 4 \times 10^6 sources, and more are anticipated when the epoch 1 and 2 data are combined. Using Milky Way (MW) templates as a guide, we adopt a simplified point-source classification to identify three candidate groups-stars without dust, dusty evolved stars, and young stellar objects-that offer a starting point for this work. We outline a strategy for identifying foreground MW stars, which may comprise as much as 18% of the source list, and background galaxies, which may comprise ~12% of the source list.

Overview

Whew! What a mouthful of text! What does that abstract boil down to?

This article presents an overview and preliminary results from the SAGE survey. The SAGE survey was a survey of the Large Magellanic Cloud (LMC) in the infra-red from 3.6-160 microns using the Spitzer Space Telescope. The goal of the survey was to obtain a dataset that could be used to study star formation, the ISM, and mass loss from evolved stars.

This paper does not directly answer science questions. Rather, the purpose of this paper is to report the status of the survey to the community so the community can plan follow up work. What is the nature and quality of the data? What sources have been identified, and how many of them? What are the limits of the survey? Using this information, other scientists can then pursue science questions, likely related to the broad science goals of the survey.

Science Objectives

The SAGE survey was built around three broad science goals:

  • Studying the properties of diffuse ISM, in particular the properties of dust. SAGE aims to detect emission corresponding to column densities of 1.2 \times 10^{21} cm^{-2} of H or greater. This will allow SAGE to measure the dust-to gas ratio across the ISM and look for variations. This traces the degree of dust-gas mixing in the LMC. One can imagine using this as a proxy for processes like turbulence. Further, by using color ratios and fitting the SED for different grain size distributions, SAGE will be able to study the spatial variation in grain size distribution in the LMC. In particular, SAGE aims to get high-resolution data on Polycyclic Aromatic Hydrocarbon (PAH) emission. This emission traces the smallest dust particles, which are important because of their role in heating. Thanks to their small size relative to the photon reabsorption length and processes like secondary and Auger electron emission, these small dust grains are particularly efficient at heating their surrounding gas via the photoelectric effect. Finally, SAGE will have the resolution and spectral range sufficient to distinguish stars from ISM, and identify individual regions of the ISM such as HII regions, molecular clouds, and photodissociation regions. This census will greatly improve the sample size of objects available to researchers studying these topics.
  • Obtaining a complete census of young stellar objects (YSOs). SAGE is sensitive to newly-formed stars with mass exceeding 3 \mathrm{M_{\odot}}. The survey should be able to obtain an unbiased, complete census of all such objects throughout the LMC. This should include both massive star formation regions (traced by HII regions) and lower mass star formation regions (traced by Taurus complexes). The two epochs of SAGE data should also allow variability studies for YSOs, looking for objects like FU Orionis stars, a class of star that exhibit abrupt increases in luminosity on timescales of a year. Current models to explain these stars revolve around accretion from disks on to pre-main-sequence objects.
  • Study mass loss in evolved stars. SAGE can detect mass-loss rates of 10^{-8} \mathrm{M_{\odot}} / \mathrm{yr}. This should allow a complete census of evolved stars with appreciable mass loss rates throughout the LMC. The two epochs of SAGE data should also inform studies of variability in mass loss rates, since mass loss processes in evolved stars are also thought to occur on timescales of around a year.

Figure 2 from the paper, showing the discovery space of SAGE. The black line is SAGE

Observational Approach

The SAGE survey aims to understand and characterize the ISM, star formation, and mass outflows. These phenomena are permeated with dust, which impede observations in the visible due to extinction. However, dust extinction in the near-IR is minimal, while in the far-IR the presence of dust becomes an asset, as irradiated dust reradiates in that wavelength regime. This motivates the use of IR observations to achieve SAGE’s science goals.

SAGE builds on a number of IR surveys, both ground-based (2MASS) and space-based (IRAS). Thanks to improvements in telescope design and detector sensitivity, Spitzer is able to combine a greater wavelength range than IRAS with being above the atmospheric extinction 2MASS had to deal with. These advances allow SAGE to push beyond previous surveys and target fainter – and more numerous – dusty sources in the LMC out of the reach of previous surveys. Specific goals of SAGE include detecting IR point sources down to the spatial resolution limit of Spitzer, and mapping dust emission from HII regions, molecular clouds, and other features from the ISM at high SNR.

SAGE observed the LMC using Spitzer’s IRAC and MIPS cameras. A 7^{\circ} \times 7^{\circ} region corresponding to the LMC was imaged by the telescope in 7 different bands, from the near to far IR. IRAC imaged in the near-IR in the 3.5, 4.5, 5.8 and 8.0 \mathrm{\mu m} bands (291 hours) and MIPS in the mid and far IR in the 24, 70 and 160 \mathrm{\mu m} bands (217 hours). Individual tiles underwent absolute photometric calibration using a network of pre-selected standard stars and were then mosaiced together to form the final survey. Data was taken in two epochs separated by 3 months to allow variability studies. The combined epoch 1 and 2 data has data gaps smaller than a pixel for MIPS. The survey achieved an angular resolution of 2” for IRAC, corresponding to a 0.5 pc spatial resolution at the distance of the LMC. Similarly, MIPS achieved a resolution of 6”, which corresponds to 1.5 pc in the LMC.

The LMC has a number of properties that make it useful for exploring the SAGE science goals. First, it is an observationally opportune target. Its close proximity (50 kpc) relative to other galaxies allows for high photon flux and high SNR. A viewing angle of 35^{\circ} means the LMC is nowhere near edge-on, making it much easier to distinguish features. The combination of these features allow resolving features in much greater detail than permitted by imaging more distant galaxies. Further, lines of sight to the LMC are clear of the Small Magellanic Cloud (SMC) and Milky Way (MW). The expectation on the number of substantial clouds per line of sight is small, so if a cloud is detected along a line of sight, it is the only one. Not having to try to separate clouds along the line of sight greatly simplifies the analysis. Further, all clouds are located at approximately the same distance, making comparisons along the line of sight much easier.

Scientifically, previous surveys such as IRAS and 2MASS have shown that the LMC possesses features at all spatial scales, offering a rich diversity of structures to study and promising the opportunity for a wide variety of science. The low metallicity of the LMC (Z=0.3-0.5) corresponds to the metallicity of the universe at Z~1.5, the era of peak star formation. Studying the LMC offers insight into what star formation and galaxy evolution might have been like during that key epoch.

Preliminary Results

The authors chose two HII regions within the LMC, N79 and N83, to investigate qualitatively in this paper in order to demonstrate the utility of the SAGE survey. N79 and N83 were chosen because they had not been studied at Spitzer wavelengths, contain both young and old stellar populations, and happened to be some of the first regions processed by the SAGE pipeline. Some preliminary results are sketched out below.

The authors compared dust emission in several bands to various tracers. They found that diffuse HI emission (from 21-cm maps) is well traced by the 70- and 160-μm bands, while the Hα and CO J=1-0 lines are traced by the 24 μm band (the origins of the HI, Hα and CO maps are given in the paper). The latter two lines have strong peaks in the HII regions of N79 and N83. The corresponding 24 μm emission is most likely from warm (~120 K) dust heated by young, massive stars. By contrast, 8 μm emission, which traces PAHs, is largely absent from bright star-forming regions, indicating that PAHs are destroyed by the intense UV radiation. See Fig. 9 from the paper, which is reproduced below:

Point-source classification was carried out by dividing up regions of color-color space based on Milky-Way templates. Objects were divided into three groups:

  1. Young Stellar Objects (YSOs)
  2. Stars without dust – main-sequence stars and red giants
  3. Dusty evolved stars – O-rich and C-rich AGB stars, OH/IR stars and carbon stars
The 3.6, 8.0 and 24 μm bands were chosen for the color-color diagram, as they provide the widest range of objects. Longer wavelengths contain too few point sources, while use of just the shorter bands would provide a smaller range of colors. The classification is shown in Fig. 10 from the paper:

Figure 12 of the paper plots these classified objects on various choices of color-magnitude diagrams. The authors comment that many of the point sources classified as young stellar objects are actually background galaxies. As noted in class by Alyssa, a more sophisticated (and accurate) method of classifying point sources is to define regions of the space defined by point-source flux in several observed bands. In this larger space, different populations of objects separate out more clearly, while in color-magnitude space, one is effectively taking a projection of the higher-dimensional magnitude space. In this projection, different populations – e.g. YSOs and background galaxies – may be projected onto one another. Figure 12 in the paper shows several choices of color-magnitude space (the last two panels are reproduced below):

In these two panels, the point sources classified as YSOs separate out nicely from the stars without dust and dusty evolved stars, which in the right panel cluster along the main-sequence.

Follow-up Work

The SAGE paper that we read in for Journal Club serves to introduce the survey and present some of its capabilities through the preliminary analysis sketched out above. More detailed results from two subsequent papers are outlined in what follows.

The first paper by Meixner et al. (2009), and titled “Measuring the lifecycle of baryonic matter in the Large Magellanic Cloud with the Spitzer SAGE-LMC survey” (ADS Link), summarizes certain SAGE results from a 2008 IAU symposium on the Magellanic system. The paper summarizes efforts to put lower bounds on the total rate of star formation and mass loss from AGB stars within the LMC.  The authors first present a map of dust column density derived from 160 μm emission, which is optically thin throughout the LMC, and thus a good tracer of the total dust mass of the galaxy. This map is compared to the column density of HI (from 21 cm emission) and CO. An “infrared excess map” is obtained by subtracting the total gas column density from HI and CO from that determined by dust emission. The infrared excess seems to follow the HI density, and the authors propose three possible origins for the excess: Hgas not traced by CO emission, HI not detected by 21 cm emission, or variations in the gas-to-dust ratio. The total ISM mass is estimated at 10^9 \, \mathrm{M_{\odot}}, as compared to a total stellar mass of 3 \times 10^9 \, \mathrm{M_{\odot}}. The authors summarize the work of Whitney et al. (2008) (ADS Link), who fit the most massive of ~1000 YSOs detected by the survey to a Kroupa (2001) (ADS Link) initial mass function and infer a total star formation rate of 0.1 \mathrm{M_{\odot}} \, / \mathrm{yr}. Whitney et al. note, however, that the UV flux from the LMC gives a higher star formation rate, and that their catalogue of YSOs in the LMC is by no means complete. Meixner et al. note that more recent work presented at the proceedings finds five times as many YSOs using the same color cuts as Whitney et al. Finally, the mass-loss rate of AGB stars is estimated by comparing the 8 μm emission to scaled atmospheric models. The 8 μm excess has been previously correlated to AGB mass-loss rates, and the resulting relation is applied to the entire sample of AGB stars, giving a total mass-loss rate of > 8.7 \times 10^{-3} \, \mathrm{M_{\odot}} / \mathrm{yr}.

The second paper, by Bernard et al. (2008) (ADS Link), which is cited by the above Meixner et al. (2009) paper with regards to excess dust emission in the 160 μm band, deals with dust properties in the LMC. The authors find that dust temperature in the LMC ranges from 12.1 to 37.4 K, with the warmest dust residing in the stellar bar. The authors find two types of IR-excess. The first type, dealing with disparities in column-density measures, was described above. The second “excess” occurs in the MIR, where the spectral energy distribution of the LMC departs significantly from that of the Milky Way, as shown in Fig. 5 of the paper:

The departure is not limited to the LMC, but it also present in SMC. The authors consider a flatter distribution of very small grains (VSGs) in the LMC to be the most likely explanation of the infrared excess. In all, however, they find the relative abundances of dust species in the LMC to be very similar to those in the MW.

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