Massive galaxies like our Milky Way gain mass by absorbing smaller galaxies. The Large Magellanic Cloud and the Small Magellanic Cloud are irregular dwarf galaxies that are gravitationally bound to the Milky Way. Both the clouds are distorted by the Milky Way’s gravity, and astronomers think that the Milky Way is in the process of digesting both galaxies.
A new study says that process is already happening, and that the Milky Way is enjoying the Magellanic Clouds’ halos of gas as an appetizer, creating a feature called the Magellanic Stream as it eats. It also explains a 50 year old mystery: Why is the Magellanic Stream so massive?
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Astronomers: Gary Gonella, Andrew Dumbleton, Stuart Foreman, David Dickinson, Shahrin Ahmad and special guest Henna Khan from Bombay, India
the Moon’s surface
M44 Beehive Cluster
Neutron Star B224 from HST
Mars with ice caps and Hellas Basin visible
Comet C/2012 K1 PanSTARRS
Stuart demonstrating how to work with software to process images
M51a Whirlpool Galaxy
M53 Globular Cluster
Rosette Nebula – NGC 2237, 2238, 2239 and 2246
Horsehead Nebula (Barnard 33 in emission nebula IC 434) and Flame Nebula (NGC 2024) with a satellite trail
NGC 5139 Omega Centauri
M42 Orion Nebula
M63 Sunflower Galaxy
NGC 7635 Bubble Nebula
Large and Small Magellanic Clouds
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Most people agree that the Magellanic Clouds are in orbit around the Milky Way. What’s not clear is whether it is a bound orbit or just a temporary ‘ships passing in the night’ arrangement. Something which could clarify the relationship is the Magellanic Stream, a 600,000 light year long string of gas dragged through and beyond the Small and Large Magellanic Clouds.
For the complete picture, note that there is also a shorter trail of gas drawn out ahead of the Clouds, known as the Leading Arm – and the gas flow between the Clouds is known as the Magellanic Bridge. The Bridge is an indication that the Clouds are gravitationally bound in a binary pair – at least for now. The Large Magellanic Cloud may dragging the Small Magellanic Cloud behind it, since the Magellanic Stream ‘skid mark’ is most chemically similar to the contents of the Small Magellanic Cloud.
What remains unresolved is whether the Clouds are in a bound orbit around the Milky Way – or are they just passing by? The level of uncertainty about the dynamics of objects that are relatively close to us, and are easily visible to the naked eye, may seem surprising.
Firstly, it is tricky to gain an accurate estimation of each Cloud’s velocity relative to the Milky Way – partly because we, the observers, have our own independent movement and we need to find a reference frame that we can reliably measure the Clouds’ velocity against.
Estimates derived from Hubble Space Telescope observations by Kallivayalil and colleagues in 2006, measured the Clouds’ velocities against a background of distant quasars, which are visible through the Clouds. These data were then used by Besla and colleagues to propose that the Clouds’ velocities were too fast to be in bound orbits around the Milky Way and so must be just passing by.
But there is another area of uncertainty, where – even with the Clouds’ velocity determined – you still need to decide what escape velocity they need to avoid being caught in a bound orbit of the Milky Way. While we can estimate the Milky Way’s mass, there is the issue of dark matter – which we can’t see and hence can’t locate accurately – so there is some uncertainty about how the combined mass of the Milky Way’s visible and dark matter is distributed.
If, like the visible matter, the dark matter is centralized around the galactic hub, the Clouds won’t need so much velocity to escape. But if the dark matter is more evenly distributed with the galactic disk of visible matter being surrounded by a spherical halo of dark matter, then it’s less clear as to whether the Cloud’s could escape (a scenario that was acknowledged by Besla et al).
A spherical halo of dark matter is the generally preferred model for the Milky Way’s total mass distribution – since, without it, the outer edges of the Milky Way’s visible disk are rotating so fast that they should fly off into space.
Diaz and Bekki have run with this idea by computer-modeling a Milky Way with a circular velocity of 250 kilometres a second (a recent new estimate), which hence requires a more substantial dark matter halo than was assumed by Besla et al. Otherwise, they still use the same Cloud velocities determined from the 2006 Hubble Space Telescope observations.
Their model, when wound back in time, suggests the Clouds have been locked in bound orbits around the Milky Way for more than 5 billion years – with the Magellanic Stream and Leading Arm arising more recently, following a close encounter between the two Clouds (an idea also proposed in Besla et al’s unbound orbit model).
Diaz and Bekki suggest that the Clouds began separate orbits, but passed close to each other around 1.25 billion years ago and then became the binary pair we observe today. The Leading Arm is freed gas being drawn into the Milky Way’s halo – an indication that both Clouds may eventually be assimilated.
The Magellanic Clouds are an oddity. Their relative velocity is suspiciously close to the escape velocity of the Milky Way system making it somewhat difficult for them to have been formed as part of the system. Additionally, their direction of motion is nearly perpendicular to the disk of the galaxy and systems, especially ones as large as the Magellanic Clouds, should show more orientation to the plane if they formed along side. Their gas content is also notably different than other satellite galaxies of our galaxy. The combination of these features suggests to some, that the Magellanic Clouds aren’t native to the Milky Way and were instead intercepted.
But where did they come from? Although the suggestion is not entirely new, a recent paper, accepted to the Astrophysical Journal Letters, suggests they may have been captured after a past merger in the Andromeda Galaxy (M31).
To analyze this proposition, the researchers, Yang (from the Chinese Academy of Sciences) and Hammers (of the University of Paris, Diderot), conducted simulations backtracking the positions of the Magellanic Clouds. While this may sound straightforward, the process is anything but. Since galaxies are extended objects, their three dimensional shapes and mass profiles must be worked out extremely well to truly account for the path of motion. Additionally, the Andromeda galaxy is certainly moving and would have been in a different position that it is observed today. But exactly where was it when the Magellanic Clouds would have been expelled? This is an important question, but not easy to answer given that observing the proper motions of objects so far away is difficult.
But wait. There’s more! As always, there’s a significant amount of the mass that can’t be seen at all! The presence and distribution of dark matter would greatly have affected the trajectory of the expelled galaxies. Fortunately, our own galaxy seems to be in a fairly quiescent phase and other studies have suggested that dark matter halos would be mostly spherical unless perturbed. Furthermore, distant galaxy clusters such as the Virgo supercluster as well as the “Great Attractor” would have also played into the trajectories.
These uncertainties take what would be a fairly simple problem and turn it into a case in which the researchers were instead forced to explore the parameter space with a range of reasonable inputs to see which values worked. In doing so, the pair of astronomers concluded “it could be the case, within a reasonable range of parameters for both the Milky Way and M31.” If so, the clouds spent 4 – 8 billion years flying across intergalactic space before being caught by our own galaxy.
But could there be further evidence to support this? The authors note that if Andromeda underwent a merger event of such scale would likely have induced vast amounts of star formation. As such, we should expect to see an increase in numbers of stars with this age. The authors do not make any statements as to whether or not this is the case. Regardless, the hypothesis is interesting and reminds us how dynamic our universe can be.
Sorry – a bit of southern sky bias in this one. But it does seem that our favourite down under naked eye objects are even more unique than we might have thought. The two dwarf galaxies, the Large and Small Magellanic Clouds, orbit the Milky Way and have bright star forming regions. It would seem that most satellite galaxies, in orbit around other big galaxies, don’t. And, taking this finding a step further, our galaxy may be one of a declining minority of galaxies still dining on gas-filled dwarf galaxies to maintain a bright and youthful appearance.
We used to think that the Sun was an ordinary, unremarkable star – but these days we should acknowledge that it’s out of statistical mid-range, since the most common stars in the visible universe are red dwarfs. Also, most stars are in binary or larger groups – unlike our apparently solitary one.
The Sun is also fortunately positioned in the Milky Way’s habitable zone – not too close-in to be constantly blasted with gamma rays, but close-in enough for there to be plenty of new star formation to seed the interstellar medium with heavy elements. And the Milky Way itself is starting to look a bit out of the ordinary. It’s quite large as spiral galaxies go, bright with active star formation – and it’s got bright satellites.
The Lambda Cold Dark Matter (CDM) model of large scale structure and galaxy formation has it that galaxy formation is a bottom-up process, with the big galaxies we see today having formed from the accretion of smaller structures – including dwarf galaxies – which themselves may have first formed upon some kind of dark matter scaffolding.
Through this building-up process, spinning spiral galaxies with bright star forming regions should become common place – only dimming if they run out of new gas and dust to feast on, only losing their structure if they collide with another big galaxy – first becoming a ‘train wreck’ irregular galaxy and then probably evolving into an elliptical galaxy.
The Lambda CDM model suggests that other bright spiral galaxies should also be surrounded by lots of gas-filled satellite galaxies, being slowly draw in to feed their host. Otherwise how is it that these spiral galaxies get so big and bright? But, at least for the moment, that’s not what we are finding – and the Milky Way doesn’t seem to be a ‘typical’ example of what’s out there.
The relative lack of satellites observed around other galaxies could mean the era of rapidly accreting and growing galaxies is coming to a close – a point emphasised by the knowledge that we observe distant galaxies at various stages of their past lives anyway. So the Milky Way may already be a relic of a bygone era – one of the last of the galaxies still growing from the accretion of smaller dwarf galaxies.
On the other hand – maybe we just have some very unusual satellites. To a distant observer, the Large MC would have nearly a tenth of the luminosity of the Milky Way and the Small MC nearly a fortieth– we don’t find anything like this around most other galaxies. The Clouds may even represent a binary pair which is also fairly unprecedented in any current sky survey data.
They are thought to have passed close together around 2.5 billion years ago – and it’s possible that this event may have set off an extended period of new star formation. So maybe other galaxies do have lots of satellites – it’s just that they are dim and difficult to observe as they are not engaged in new star formation.
Either way, using our galaxy as a basis for modelling how other galaxies work might not be a good idea – apparently it’s not so ordinary.
At the end of the proverbial day, space-based missions like Spitzer produce millions of observations of astronomical objects, phenomena, and events. And those terabytes of data are used to test hypotheses in astrophysics which lead to a deeper understanding of the universe and our home in it, and perhaps some breakthrough whose here-on-the-ground implementation leads to a major, historic improvement in human welfare and planetary ecosystem health.
But such missions also leave more immediate legacies, in terms of the pleasure they bring millions of people, via the beauty of their images (not to mention posters, computer wallpaper and screen savers, and even inspiration for avatars).
Some recent results from one of Spitzer’s programs – SAGE-SMC – are no exception.
The image shows the main body of the Small Magellanic Cloud (SMC), which is comprised of the “bar” on the left and a “wing” 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. 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 SMC is one of very few galaxies where this type of study is possible, and the research could not be done without Spitzer.
This image was captured by Spitzer’s infrared array camera and multiband imaging photometer (blue is 3.6-micron light; green is 8.0 microns; and red is combination of 24-, 70- and 160-micron light). The blue color mainly traces old stars. The green color traces emission from organic dust grains (mainly polycyclic aromatic hydrocarbons). The red traces emission from larger, cooler dust grains.
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.
The Small Magellanic Cloud (SMC), and its larger sister galaxy, the Large Magellanic Cloud (LMC), are named after the seafaring explorer Ferdinand Magellan, who documented them while circling the globe nearly 500 years ago. From Earth’s southern hemisphere, they can appear as wispy clouds. The SMC is the further of the pair, at 200,000 light-years away.
Recent research has shown that the galaxies may not, as previously suspected, orbit around our galaxy, the Milky Way. Instead, they are thought to be merely sailing by, destined to go their own way. Astronomers say the two galaxies, which are both less evolved than a galaxy like ours, were triggered to create bursts of new stars by gravitational interactions with the Milky Way and with each other. In fact, the LMC may eventually consume its smaller companion.
Karl Gordon, the principal investigator of the latest Spitzer observations at the Space Telescope Science Institute in Baltimore, Maryland, and his team are interested in the SMC not only because it is so close and compact, but also because it is very similar to young galaxies thought to populate the universe billions of years ago. The SMC has only one-fifth the amount of heavier elements, such as carbon, contained in the Milky Way, which means that its stars haven’t been around long enough to pump large amounts of these elements back into their environment. Such elements were necessary for life to form in our solar system.
Studies of the SMC therefore offer a glimpse into the different types of environments in which stars form.
“It’s quite the treasure trove,” said Gordon, “because this galaxy is so close and relatively large, we can study all the various stages and facets of how stars form in one environment.” He continued: “With Spitzer, we are pinpointing how to best calculate the numbers of new stars that are forming right now. Observations in the infrared give us a view into the birthplace of stars, unveiling the dust-enshrouded locations where stars have just formed.”
This image shows the main body of the SMC, which is comprised of the “bar” and “wing” on the left and the “tail” extending to the right. 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.
This week at the AAC Conference, astronomers released a new image of the Small Magellanic Cloud (SMC, a dwarf galaxy just outside our Milky Way) from Spitzer. The purpose of the image was to study “the life cycle of dust in this galaxy.” In this life cycle, clouds of gas and dust collapse to form new stars. As those stars die, they create new dust in their atmosphere which will enrich the galaxy and, when the stars give off that dust, will be made available future generations of stars. The rate at which this process occurs determines how fast the galaxy will evolve. This research has shown that the SMC is far less evolved than our on galaxy and only has 20% of the heavy elements that our own galaxy has. Such unevolved galaxies are reminiscent of the building blocks of larger galaxies.
As with most astronomical images, this new image is taken in different filters which correspond to different wavelengths of light. The red is 24 microns and traces mainly cool dust which is part of the reservoir from which new star formation can occur. Green represents the 8 micron wavelength and traces warmer dust in which new stars are forming. The blue is even warmer at 3.6 microns and shows older stars which have already cleared out their local region of gas and dust. By combining the amount of each of these, astronomers are able to determine the current rate at which evolution is taking place in order to understand how the evolution of the SMC is progressing.
The new research shows that the tail (lower right in this image) is tidal in nature as it’s being tugged on by gravitational interactions with the Milky Way. This tidal interaction has caused new star formation in the galaxy. Surprisingly, the team of researchers also indicated that their work may indicate that the Magellanic Clouds are not gravitational bound to the Milky Way and may just be passing.
Our galaxy has a streamer, though it’s not like the ones you had on your bike as a kid: this streamer is a flow of largely hydrogen gas that originates in the Large and Small Magellanic Clouds, two of our closest galactic neighbors. New observations of the stream have helped to revise its age and extent, and show it to be longer and much older than previous estimates.
The Magellanic Stream, which was discovered over 30 years ago, flows from the two galaxies closest to the Milky Way, the Large and Small Magellanic Clouds. These clouds, which are actually two irregular dwarf galaxies, are 150,000 to 200,000 light-years away, and are visible in the southern hemisphere.
The stream connects up with the Milky Way about 70,000 light years from the Solar System, in the constellation of the Southern Cross.
Using the Green Bank Telescope (GBT), a team of astronomers took over 100 hours of observations of the streamer. These observations were combined with those from other radio telescopes, including the Aricebo telescope in Puerto Rico, to further constrain both its extent and age.
Their observations were presented at the American Astronomical Society’s meeting in Washington D.C., and a paper has been submitted to the Astrophysical Journal. The team included David Nidever and Steven Majewski of the Department of Astronomy at the University of Virginia, Butler Burton of the Leiden Observatory and the National Radio Astronomy Observatory and Lou Nigra of the University of Wisconsin.
Previous observations of the stream showed it to have gaps between the Magellanic Clouds and where it enters the Milky Way, but these revised observations show it to be one continuous stream between the three galaxies. The stream is also at least forty percent longer that previously estimated.
The Magellanic Stream was also determined by the astronomers to be much older than had been estimated before: up from 1.75 billion years old to 2.5 billion years old. Just how does this long-lived intergalactic trail of hydrogen crumbs start off in the Magellanic Clouds?
“The new age of the stream puts its beginning at about the time when the two Magellanic Clouds may have passed close to each other, triggering massive bursts of star formation. The strong stellar winds and supernova explosions from that burst of star formation could have blown out the gas and started it flowing toward the Milky Way,” said David Nidever in a NRAO press release.
By getting a better picture of how the gas flows from the Magellanic Clouds into the Milky Way, astronomers have been able to determine with better accuracy just how far away the two galaxies are, as well as their interactions with the tidal forces of the Milky Way.
This team has collaborated before on the exploration of the Magellanic Stream and its origins. You can read about their previous findings on Arxiv right here, which were also published in the Astrophysical Journal.