Hubble Finds Source of Magellanic Stream

The Magellanic Stream  

Credit for the radio/visible-light image: David L. Nidever et al., NRAO/AUI/NSF and A. Mellinger, LAB Survey, Parkes Observatory, Westerbork Observatory, and Arecibo Observatory. Credit for the radio image: LAB Surve. More Images

Astronomers using NASA's Hubble Space Telescope have solved a 40-year mystery on the origin of the Magellanic Stream, a long ribbon of gas stretching nearly halfway around our Milky Way galaxy.

The Large and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way, are at the head of the gaseous stream. Since the stream's discovery by radio telescopes in the early 1970s, astronomers have wondered whether the gas comes from one or both of the satellite galaxies. Now, new Hubble observations reveal that most of the gas was stripped from the Small Magellanic Cloud about 2 billion years ago, and a second region of the stream originated more recently from the Large Magellanic Cloud.

A team of astronomers, led by Andrew J. Fox of the Space Telescope Science Institute in Baltimore, Md., and the European Space Agency, determined the source of the gas filament by using Hubble's Cosmic Origins Spectrograph (COS) to measure the amount of heavy elements, such as oxygen and sulfur, at six locations along the Magellanic Stream. COS observed faraway quasars whose emitted light passes through the stream and detected these elements from the way they absorb ultraviolet light. Quasars are the brilliant cores of active galaxies.

Fox's team found a low amount of oxygen and sulfur along most of the stream, matching the levels in the Small Magellanic Cloud about 2 billion years ago, when the gaseous ribbon was thought to have been formed.

In a surprising twist, the team discovered a much higher level of sulfur in a region closer to the Magellanic Clouds. "We're finding a consistent amount of heavy elements in the stream until we get very close to the Magellanic Clouds, and then the heavy element levels go up," said Fox. "This inner region is very similar in composition to the Large Magellanic Cloud, suggesting it was ripped out of that galaxy more recently."

This discovery was a wrinkle Fox's team didn't expect, because computer models of the stream predicted that the gas came entirely out of the Small Magellanic Cloud, which has less gravity than its more massive cousin.

"Only Hubble can measure these abundances," Fox explained. "You have to go to space because the absorption lines we need to measure these abundances are all in the ultraviolet, and Earth's atmosphere absorbs ultraviolet light."

Astronomers have debated whether the two Magellanic Clouds are on their first pass near our Milky Way or are bound to it.

"What's interesting is that all the other nearby satellite galaxies of the Milky Way have lost their gas," Fox said. "The Magellanic Clouds have been able to retain their gas and are still forming stars because they're more massive than the other satellites. However, as they're now approaching the Milky Way, they're feeling its gravity more and also encountering its halo of hot gas, which puts pressure on them. That process, together with the gravitational tug-of-war between the Magellanic Clouds, leads to the production of the stream. You're seeing material stripped out of the Clouds as they come in toward the Milky Way."

Ultimately, the gaseous stream may rain down onto the Milky Way's disk, fueling the birth of new stars. This infusion of fresh gas is part of one process that triggers star formation in a galaxy. Astronomers want to know the origin of that wayward gas in order to more fully understand how galaxies make new stars.

"We want to understand how galaxies like the Milky Way strip the gas from small galaxies that fall into them and use that to form new stars," Fox explained. "This seems like it's an episodic process. It's not a smooth process where a slow stream of gas comes in continuously. Instead, once in a while a large gas cloud falls in. We've got a way of testing that here, where two galaxies are coming in. We have shown which of them is producing the gas that ultimately will fall into the Milky Way."

The team reported its results in two papers that appeared in the Aug. 1 issue of The Astrophysical Journal. Fox is the lead author of one paper; the other paper's lead author is Philipp Richter of the University of Potsdam in Germany.

CONTACT

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493/4514
dweaver@stsci.edu / villard@stsci.edu

Andrew Fox
Space Telescope Science Institute, Baltimore, Md.
410-338-5083
afox@stsci.edu

 

NGC 602: Taken Under the "Wing" of the Small Magellanic Cloud

NGC 602

Credit : X-ray: NASA/CXC/Univ.Potsdam/L.Oskinova et al; 

Optical: NASA/STScI; 

Infrared: NASA/JPL-Caltech

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Tour of NGC 602

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The Small Magellanic Cloud (SMC) is one of the Milky Way's closest galactic neighbors. Even though it is a small, or so-called dwarf galaxy, the SMC is so bright that it is visible to the unaided eye from the Southern Hemisphere and near the equator. Many navigators, including Ferdinand Magellan who lends his name to the SMC, used it to help find their way across the oceans.

Modern astronomers are also interested in studying the SMC (and its cousin, the Large Magellanic Cloud), but for very different reasons. Because the SMC is so close and bright, it offers an opportunity to study phenomena that are difficult to examine in more distant galaxies.

New Chandra data of the SMC have provided one such discovery: the first detection of X-ray emission from young stars with masses similar to our Sun outside our Milky Way galaxy. The new Chandra observations of these low-mass stars were made of the region known as the "Wing" of the SMC. In this composite image of the Wing the Chandra data is shown in purple, optical data from the Hubble Space Telescope is shown in red, green and blue and infrared data from the Spitzer Space Telescope is shown in red.

Astronomers call all elements heavier than hydrogen and helium - that is, with more than two protons in the atom's nucleus - "metals." The Wing is a region known to have fewer metals compared to most areas within the Milky Way. There are also relatively lower amounts of gas, dust, and stars in the Wing compared to the Milky Way.

Taken together, these properties make the Wing an excellent location to study the life cycle of stars and the gas lying in between them. Not only are these conditions typical for dwarf irregular galaxies like the SMC, they also mimic ones that would have existed in the early Universe.

Most star formation near the tip of the Wing is occurring in a small region known as NGC 602, which contains a collection of at least three star clusters. One of them, NGC 602a, is similar in age, mass, and size to the famous Orion Nebula Cluster. Researchers have studied NGC 602a to see if young stars - that is, those only a few million years old - have different properties when they have low levels of metals, like the ones found in NGC 602a.

Using Chandra, astronomers discovered extended X-ray emission, from the two most densely populated regions in NGC 602a. The extended X-ray cloud likely comes from the population of young, low-mass stars in the cluster, which have previously been picked out by infrared and optical surveys, using Spitzer and Hubble respectively. This emission is not likely to be hot gas blown away by massive stars, because the low metal content of stars in NGC 602a implies that these stars should have weak winds. The failure to detect X-ray emission from the most massive star in NGC 602a supports this conclusion, because X-ray emission is an indicator of the strength of winds from massive stars. No individual low-mass stars are detected, but the overlapping emission from several thousand stars is bright enough to be observed.

The Chandra results imply that the young, metal-poor stars in NGC 602a produce X-rays in a manner similar to stars with much higher metal content found in the Orion cluster in our galaxy. The authors speculate that if the X-ray properties of young stars are similar in different environments, then other related properties -- including the formation and evolution of disks where planets form -- are also likely to be similar.

X-ray emission traces the magnetic activity of young stars and is related to how efficiently their magnetic dynamo operates. Magnetic dynamos generate magnetic fields in stars through a process involving the star's speed of rotation, and convection, the rising and falling of hot gas in the star's interior.

The combined X-ray, optical and infrared data also revealed, for the first time outside our Galaxy, objects representative of an even younger stage of evolution of a star. These so-called “young stellar objects” have ages of a few thousand years and are still embedded in the pillar of dust and gas from which stars form, as in the famous "Pillars of Creation" of the Eagle Nebula. A labeled version shows the location of these young stellar objects (roll your mouse over the image above).

A paper describing these results was published online and in the March 1, 2013 issue of The Astrophysical Journal. The first author is Lidia Oskinova from the University of Potsdam in Germany and the co-authors are Wei Sun from Nanjing University, China; Chris Evans from the Royal Observatory Edinburgh, UK; Vincent Hénault-Brunet from University of Edinburgh, UK; You-Hua Chu from the University of Illinois, Urbana, IL; John Gallagher III from the University of Wisconsin-Madison, Madison, WI; Martin Guerrero from the Instituto de Astrofísica de Andalucía, Spain; Robert Gruendl from the University of Illinois, Urbana, IL; Manuel Güdel from the University of Vienna, Austria; Sergey Silich from the Instituto Nacional de Astrofísica Optica y Electrónica, Puebla, Mexico; Yang Chen from Nanjing University, China; Yael Nazé from Université de Liège, Liège, Belgium; Rainer Hainich from the University of Potsdam, Germany, and Jorge Reyes-Iturbide from the Universidade Estadual de Santa Cruz, Ilhéus, Brazil.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.

Fast Facts for NGC 602: 

Scale: Image is 3 arcmin across (160 light years)
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 01h 29m 12.40s | Dec -73° 32' 01.70"
Constellation: Hydrus
Observation Date: 11 pointings between 31 March and 29 April, 2010
Observation Time: 80 hours 45 min (3 days 8 hours 45 min)
Obs. ID: 10985-10986, 11978-11979, 11988-11989, 12130-12131, 12134, 12136, 12207
Instrument: ACIS
References: Oskinova, L. et al, 2013, ApJ, 765 73; arXiv:1301.3500
Color Code: X-ray (Purple); Optical (Red, Green, Blue); Infrared (Red)

Herschel and Spitzer See Nearby Galaxies' Stardust

This new image shows the Large Magellanic Cloud galaxy in infrared light as seen by the Herschel Space Observatory, a European Space Agency-led mission with important NASA contributions, and NASA's Spitzer Space Telescope. Image credit: ESA/NASA/JPL-Caltech/STScI. Full image and caption

This new image shows the Small Magellanic Cloud galaxy in infrared light from the Herschel Space Observatory a European Space Agency-led mission with important NASA contributions, and NASA's Spitzer Space Telescope. Image credit: ESA/NASA/JPL-Caltech/STScI. Full image and caption - enlarge image

PASADENA, Calif. - The cold dust that builds blazing stars is revealed in new images that combine observations from the Herschel Space Observatory, a European Space Agency-led mission with important NASA contributions; and NASA's Spitzer Space Telescope. The new images map the dust in the galaxies known as the Large and Small Magellanic Clouds, two of the closest neighbors to our own Milky Way galaxy.

The new images are available at the following links: http://www.nasa.gov/mission_pages/herschel/multimedia/pia15254.html and http://www.nasa.gov/mission_pages/herschel/multimedia/pia15255.html

The Large Magellanic Cloud looks like a fiery, circular explosion in the combined Herschel-Spitzer infrared data. Ribbons of dust ripple through the galaxy, with significant fields of star formation noticeable in the center, center-left and top right (the brightest center-left region is called 30 Doradus, or the Tarantula Nebula, for its appearance in visible light). The Small Magellanic Cloud has a much more irregular shape. A stream of dust extends to the left in this image, known as the galaxy's "wing," and a bar of star formation appears on the right.

The colors in these images indicate temperatures in the dust that permeate the Magellanic Clouds. Colder regions show where star formation is at its earliest stages or is shut off, while warm expanses point to new stars heating dust surrounding them. The coolest areas and objects appear in red, corresponding to infrared light taken up by Herschel's Spectral and Photometric Imaging Receiver at 250 microns, or millionths of a meter. Herschel's Photodetector Array Camera and Spectrometer fills out the mid-temperature bands, shown in green, at 100 and 160 microns. The warmest spots appear in blue, courtesy of 24- and 70-micron data from Spitzer.

"Studying these galaxies offers us the best opportunity to study star formation outside of the Milky Way," said Margaret Meixner, an astronomer at the Space Telescope Science Institute, Baltimore, Md., and principal investigator for the mapping project. "Star formation affects the evolution of galaxies, so we hope understanding the story of these stars will answer questions about galactic life cycles."

The Large and Small Magellanic Clouds are the two biggest satellite galaxies of our home galaxy, the Milky Way, though they are still considered dwarf galaxies compared to the big spiral of the Milky Way. Dwarf galaxies also contain fewer metals, or elements heavier than hydrogen and helium. Such an environment is thought to slow the growth of stars. Star formation in the universe peaked around 10 billion years ago, even though galaxies contained lesser abundances of metallic dust. Previously, astronomers only had a general sense of the rate of star formation in the Magellanic Clouds, but the new images enable them to study the process in more detail.

The results were presented today at the 219th meeting of the American Astronomical Society in Austin, Texas.

Herschel is a European Space Agency cornerstone mission, with science instruments provided by consortia of European institutes and with important participation by NASA. NASA's Herschel Project Office is based at NASA's Jet Propulsion Laboratory, Pasadena, Calif. JPL contributed mission-enabling technology for two of Herschel's three science instruments. The NASA Herschel Science Center, part of the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena, supports the United States' astronomical community.

JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Caltech manages JPL for NASA.

For more information about Herschel, visit http://www.herschel.caltech.edu, http://www.nasa.gov/herschel and http://www.esa.int/SPECIALS/Herschel/index.html.

For more information about Spitzer, visit http://spitzer.caltech.edu/ and http://www.nasa.gov/spitzer .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

Trent J. Perrotto 202-358-0321
NASA Headquarters, Washington
trent.j.perrotto@nasa.gov

Neighbor Galaxy Caught Stealing Stars

The Milky Way’s near neighbor, the Large Magellanic Cloud (LMC), has accreted a smattering of stars from its smaller neighbor, the Small Magellanic Cloud (SMC). In this image, the LMC is shown as it appears in observations by the Spitzer Space Telescope at 3.6, 8.0, and 24 microns. Overlaid in red and blue, with colors representing the light of sight velocities (red = away, blue = towards) are the locations of stars whose origin has been traced to the SMC. These stars were discovered by a team led by NOAO astronomer Knut Olsen, through analysis of spectra obtained at the CTIO 4-m Blanco telescope. Spitzer image credit: Karl Gordon and Margaret Meixner (Space Telescope Science Institute/AURA/NASA). Compilation by K. Olsen (NOAO/AURA/NSF). Download: Web resolution - Print resolution

Astronomers from the National Optical Astronomy Observatory (NOAO) and their collaborators have found that hundreds of the stars found in the Large Magellanic Cloud (LMC) were stolen from another nearby galaxy – the Small Magellanic Cloud (SMC). The Large and Small Magellanic Clouds are both neighbor galaxies to our Milky Way Galaxy and easily visible to the unaided eye from the southern hemisphere.

By analyzing the spectra of 5900 giant and supergiant stars in the Large Magellanic Cloud galaxy, NOAO astronomers Knut Olsen and Bob Blum, and their collaborators Dennis Zaritsky (University of Arizona), and Martha Boyer and Karl Gordon (Space Telescope Science Institute) found that over 5% of the stars they observed in the LMC are rotating counter to the direction of the majority of LMC stars, or perhaps in a plane that is greatly inclined to the rotation of the LMC. An ambiguity remains in the result, because the astronomers were only able to measure the projection of the stellar velocities into the line of sight, and not their full velocity vectors. In either case, these peculiar orbits indicate that these stars probably did not form from the rotating and collapsing cloud of gas that formed the LMC, a galaxy located about 160,000 light years away.

Further examination of these counter-rotating stars revealed another anomaly. The chemical composition of these stars is different. They have fewer heavy elements such as iron and calcium than typical stars in the Large Magellanic Cloud. However, their composition closely matches that of stars in another nearby galaxy, the Small Magellanic Cloud, whose stars are also depleted in these “metals”.

These two lines of evidence – motion and composition – indicated to the research team that these stars were stolen from the smaller galaxy by the gravitational pull of the larger galaxy. The astronomers used a multi-object spectrometer on the Cerro Tololo Inter-American Observatory 4-meter Blanco Telescope in Chile to observe 4600 stars. The spectrometer allowed the spectral characteristics of a large number of stars to be observed simultaneously. These observations were then combined with data on 1300 other stars to look for patterns. According to Olsen “It is not always easy to tell whether the stars in a galaxy formed in the galaxy or formed somewhere else and then were captured. Since the LMC is so close to us, we were able to observe a large number of individual stars. And to our surprise, the LMC contained a significant number of stars that must have formed elsewhere.”

The same team is also using infrared observations made with the Spitzer Space Telescope to study how stars form and evolve in the Large Magellanic Cloud. NOAO Deputy Director Bob Blum indicated the importance of this approach: “Using observations with the Spitzer Space Telescope, we were able to get a complete census of the stellar populations in the LMC. With the ground-based observations we could determine the properties and motions of a large sample of stars throughout that galaxy. By combining both, we were able to tell that some of the stars must have come from the neighboring SMC. This led us to a deeper understanding of how galaxies can and do interact, and change over time.”

This result might also help explain the unusually large amount of star formation in the LMC nebula called 30 Doradus, also known as the Tarantula Nebula due to its appearance in a small telescope. This area is on the southwestern rim of the Large Magellanic Cloud and is a current hotbed of star formation.

If the 30 Doradus region was in our galaxy and as close to us as the Orion Nebula (the nearest stellar nursery to us), it would have the area of 60 full moons in our sky and its glow would cast shadows on the ground. The 30 Doradus region is located at the position where gas from the Small Magellanic Cloud that is being pulled into the Large Magellanic Cloud along with the captured stars collides with the LMC’s own gas at high velocity. The resultant shock wave from this collision of gas pressurizes and concentrates the gas, making star formation much more likely and leading to the formation of large, unstable stars which can explode, like the famous supernova that appeared in 1987. Remnants from an older supernova have also been found in this region by x-ray telescopes.

***

This research appears in a paper “A Population Of Accreted Small Magellanic Cloud Stars In The Large Magellanic Cloud” accepted for publication by The Astrophysical Journal. The paper authors are: Knut A.G. Olsen, National Optical Astronomy Observatory, Tucson, Arizona Dennis Zaritsky, Steward Observatory, University of Arizona Robert D. Blum, National Optical Astronomy Observatory, Tucson, Arizona Martha L. Boyer, Space Telescope Science Institute, Baltimore, Maryland Karl D. Gordon, Space Telescope Science Institute, Baltimore, Maryland

NOAO is operated by Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.

Science Contacts

Dr. Knut Olsen
National Optical Astronomy Observatory
950 N Cherry Ave
Tucson AZ 85719 USA
+1 520-318-8555
Email: kolsen@noao.edu

Dr. Robert Blum
National Optical Astronomy Observatory
950 N Cherry Ave
Tucson AZ 85719 USA
+1 520-318-8233
Email: rblum@noao.edu

Milky Way Sidelined in Galactic Tug of War

This plot shows the simulated gas distribution of the Magellanic System resulting from the tidal encounter between the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) as they orbit our home Milky Way Galaxy. The entire sky is plotted in galactocentric coordinates of longitude and latitude. The Magellanic Stream is the pronounced tail of material that stretches 150 degrees across the southern sky. The solid line shows the calculated path of the LMC and the dotted line is the path of the SMC. The color range from dark to light shows the density (lower to higher) of the hydrogen gas making up the Magellanic Stream and the Bridge that connects the two dwarf galaxies. Credit: Plot by G. Besla, Milky Way background image by Axel Mellinger (used with permission). Low Resolution Image (jpg)

Cambridge, MA - The Magellanic Stream is an arc of hydrogen gas spanning more than 100 degrees of the sky as it trails behind the Milky Way's neighbor galaxies, the Large and Small Magellanic Clouds. Our home galaxy, the Milky Way, has long been thought to be the dominant gravitational force in forming the Stream by pulling gas from the Clouds. A new computer simulation by Gurtina Besla (Harvard-Smithsonian Center for Astrophysics) and her colleagues now shows, however, that the Magellanic Stream resulted from a past close encounter between these dwarf galaxies rather than effects of the Milky Way.

"The traditional models required the Magellanic Clouds to complete an orbit about the Milky Way in less than 2 billion years in order for the Stream to form," says Besla. Other work by Besla and her colleagues, and measurements from the Hubble Space Telescope by colleague Nitya Kallivaylil, rule out such an orbit, however, suggesting the Magellanic Clouds are new arrivals and not long-time satellites of the Milky Way.

This creates a problem: How can the Stream have formed without a complete orbit about the Milky Way?

To address this, Besla and her team set up a simulation assuming the Clouds were a stable binary system on their first passage about the Milky Way in order to show how the Stream could form without relying on a close encounter with the Milky Way.

The team postulated that the Magellanic Stream and Bridge are similar to bridge and tail structures seen in other interacting galaxies and, importantly, formed before the Clouds were captured by the Milky Way.

"While the Clouds didn't actually collide," says Besla, "they came close enough that the Large Cloud pulled large amounts of hydrogen gas away from the Small Cloud. This tidal interaction gave rise to the Bridge we see between the Clouds, as well as the Stream."

"We believe our model illustrates that dwarf-dwarf galaxy tidal interactions are a powerful mechanism to change the shape of dwarf galaxies without the need for repeated interactions with a massive host galaxy like the Milky Way."

While the Milky Way may not have drawn the Stream material out of the Clouds, the Milky Way's gravity now shapes the orbit of the Clouds and thereby controls the appearance of the tail.

"We can tell this from the line-of-sight velocities and spatial location of the tail observed in the Stream today," says team member Lars Hernquist of the Center.

The paper describing this work has been accepted for publication in the October 1 issue of the Astrophysical Journal Letters and is available online at http://arxiv.org/abs/1008.2210v1.

Besla's co-authors were Nitya Kallivayalil (MIT Kavli Institute for Astrophysics & Space Research), Lars Hernquist, R. P. van der Marel (STScI), T.J. Cox (Carnegie Observatories) and D. Keres (Harvard-Smithsonian Center for Astrophysics). Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463,
cpulliam@cfa.harvard.edu

Dusty, Little Galaxy

Little Galaxy with a Tail

Image credit: NASA/JPL-Caltech/STScI

The infrared portrait of the Small Magellanic Cloud, taken by NASA's Spitzer Space Telescope, reveals the stars and dust in this galaxy as never seen before. The Small Magellanic Cloud is a nearby satellite galaxy to our Milky Way galaxy, approximately 200,000 light-years away.

The image shows the main body of the Small Magellanic Cloud, which is comprised of the "bar" and "wing" on the left and the "tail" extending to the right. The bar contains both old stars (in blue) and young stars lighting up their natal dust (green/red). The wing mainly contains young stars. The tail contains only gas, dust and newly formed stars. Spitzer data has confirmed that the tail region was recently torn off the main body of the galaxy. Two of the tail clusters, which are still embedded in their birth clouds, can be seen as red dots.

In addition, the image contains a galactic globular cluster in the lower left (blue cluster of stars) and emission from dust in our own galaxy (green in the upper right and lower right corners).

The data in this image are being used by astronomers to study the lifecycle of dust in the entire galaxy: from the formation in stellar atmospheres, to the reservoir containing the present day interstellar medium, and the dust consumed in forming new stars. The dust being formed in old, evolved stars (blue stars with a red tinge) is measured using mid-infrared wavelengths. The present day interstellar dust is weighed by measuring the intensity and color of emission at longer infrared wavelengths. The rate at which the raw material is being consumed is determined by studying ionized gas regions and the younger stars (yellow/red extended regions). The Small Magellanic Cloud, and its companion galaxy the Large Magellanic Cloud, are the two galaxies where this type of study is possible, and the research could not be done without Spitzer.

This image was captured by Spitzer's multiband imaging photometer, with 24-micron light colored blue; 70-micron light colored green and 160-micron light colored red. The blue, green, and red colors trace hot, warm and cool dust emission, respectively.

The image was taken as part of the Spitzer Legacy program known as SAGE-SMC: Surveying the Agents of Galaxy Evolution in the Tidally-Stripped, Low Metallicity Small Magellanic Cloud.