The outer reaches of the Milky Way galaxy are a different place. Stars are much harder to come by, with most of this “galactic halo” being made up of empty space. But scientists theorize that there is an abundance of one particular thing in this desolate area – dark matter. Now, a team from Harvard and the University of Arizona (UA) spent some time studying and modeling one of the galaxy’s nearest neighbors to try to tease out more information about that dark matter, and as a result came up with an all new way to look at the halo itself.
The Magellanic Clouds are two of our closest neighbours, in galactic terms. The pair of irregular dwarf galaxies were drawn into the Milky Way’s orbit in the distant past, and we’ve been looking up at them since the dawn of humanity. Some of our ancestors even gathered pigments and created images of them in petroglyphs and cave paintings.
Following in the footsteps of those ancient artists, astronomers recently used the Dark Energy Camera (DECam) to capture an in-depth portrait of the pair of galaxies.
The Magellanic Clouds are a pair of dwarf galaxies that are bound to the Milky Way. The Milky Way is slowly consuming them in Borg-like fashion, starting with the gas halo that surrounds both Clouds. They’re visible in the southern sky, and for centuries people have gazed up at them. They’re named after the Portuguese explorer Ferdinand Magellan, in our current times.
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?
For some time, astronomers have known that collisions or mergers between galaxies are an integral part of cosmic evolution. In addition to causing galaxies to grow, these mergers also trigger new rounds of star formation as fresh gas and dust are injected into the galaxy. In the future, astronomers estimate that the Milky Way Galaxy will merge with the Andromeda Galaxy, as well as the Small and Large Magellanic Clouds in the meantime.
According to new results obtained by researchers at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York city, the results of our eventual merger with the Magellanic Clouds is already being felt. According to results presented at the 235th meeting of the American Astronomical Society this week, stars forming in the outskirts of our galaxy could be the result of these dwarf galaxies merging with our own.
Astronomers at Cardiff University have done something nobody else has been able to do. A team, led by Dr. Phil Cigan from Cardiff University’s School of Physics and Astronomy, has found the neutron star remnant from the famous supernova SN 1987A. Their evidence ends a 30 year search for the object.
When stars reach the end of their life cycle, many will blow off their outer layers in an explosive process known as a supernova. While astronomers have learned much about this phenomena, thanks to sophisticated instruments that are able to study them in multiple wavelengths, there is still a great deal that we don’t know about supernovae and their remnants.
For example, there are still unresolved questions about the mechanisms that power the resulting shock waves from a supernova. However, an international team of researchers recently used data obtained by the Chandra X-Ray Observatory of a nearby supernova (SN1987A) and new simulations to measure the temperature of the atoms in the resulting shock wave.
On December 19th, 2013, the European Space Agency’s (ESA) Gaia spacecraft took to space with for a very ambitious mission. Over the course of its planned 5-year mission (which was recently extended), this space observatory would map over a billion stars, planets, comets, asteroids and quasars in order to create the largest and most precise 3D catalog of the Milky Way ever created.
Since that time, the ESA has made two data releases that cover the first three years of the Gaia mission. The second data release, which took on April 25th, 2018, has already proven to be a treasure trove for astronomers. In addition to the positions, distance indicators and motions of over a billion stars and celestial objects in the Milky Way Galaxy, it also contained a hidden gem – the proper motions of stars within the Large Magellanic Cloud (LMC).
Located about 200,000 light-years from Earth, the LMC has dense clouds of dust that results in it experiencing high rates of star formation. In addition, it’s central bar is warped (where the east and west ends are nearer to the Milky Way), suggesting that it was once a barred dwarf spiral galaxy who’s spiral arms were disrupted by interaction with the Small Magellanic Cloud (SMC) and the Milky Way.
The proper motions of the stars in the LMC, as captured by the Gaia spacecraft (without color). Copyright: ESA/Gaia/DPAC
For these reasons, astronomers have been hoping to derive the orbits of dwarf galaxies (and globular clusters) that revolve around the Milky Way. In so doing, they hope to learn more about how our galaxy evolved due to mergers with clusters and other galaxies. By determining the proper motions of the LMC’s stars, the Gaia mission has provided clues as to how the Milky Way and its largest satellite galaxy have interacted over time.
As you can see from the image (at top), the bar of the LMC is outlined in great detail, along with individual star-forming regions like the Tarantula Nebula (aka. 30 Doradus, which is visible just above the center of the galaxy). The image combines the total amount of radiation detected by the observatory in each pixel. The radiation measurements were then taken through different filters on the spacecraft to generate color information.
This allowed Gaia to obtain information about the total density of stars within the LMC as well as their proper motions. As you can see, the image is dominated by the brightest, most massive stars, which greatly outshine their fainter, lower-mass counterparts. The proper motions of the stars observed is represented as the texture of the image – which looks a lot like a fingerprint.
The Large Magellanic Cloud (LMC), one of the nearest galaxies to our Milky Way, as viewed by ESA’s Gaia satellite using information from the mission’s second data release. Copyright ESA/Gaia/DPAC
From this, scientists were able to see an imprint of the stars rotating clockwise around the center of the galaxy. Using this information, astronomers will be able to create new models on how the LMC, SMC, and Milky Way evolved together over time. This, in turn, could shed light on how galaxies like our own, formed and evolved over the course of billions of years.
As with other information contained in the first and second data releases, this latest discovery demonstrates that the Gaia mission is fulfilling its intended purpose. The third release of Gaia data is scheduled to take place in late 2020, with the final catalog being published in the 2020s. Meanwhile, an extension has already been approved for the Gaia mission, which will now remain in operation until the end of 2020 (to be confirmed at the end of this year).
And be sure to enjoy this animated view of the LMC’s rotation, courtesy of the ESA:
The European Space Agency’s (ESA) Gaia mission is an ambitious project. Having launched in December of 2013, the purpose of this space observatory has been to measure the position and distances of 1 billion objects – including stars, extra-solar planets, comets, asteroids and even quasars. From this, astronomers hope to create the most detailed 3D space catalog of the cosmos ever made.
Back in 2016, the first batch of Gaia data (based on its first 14 months in space) was released. Since then, scientists have been poring over the raw data to obtain clearer images of the neighboring stars and galaxies that were studied by the mission. The latest images to be released, based on Gaia data, included revealing pictures of the Large Magellanic Cloud (LMC), the Andromeda galaxy, and the Triangulum galaxy.
The first catalog of Gaia data consisted of information on 1.142 billion stars, including their precise position in the night sky and their respective brightness. Most of these stars are located in the Milky Way, but a good fraction were from galaxies beyond ours, which included about ten million belonging to the LMC. This satellite galaxy, located about 166 000 light-years away, has about 1/100th the mass of the Milky Way.
Gaia’s view of the Large Magellanic Cloud. Click here for further details, full credits, and larger versions of the image. Credit: ESA/Gaia/DPAC
The two images shown above display composite data obtained by the Gaia probe. The image on the left, which was compiled by mapping the total density of stars detected by Gaia, shows the large-scale distribution of stars in the LMC. This image also delineates the extent of the LMC’s spiral arms, and is peppered with bright dots that represent faint clusters of stars.
The image on the right, on the other hand, reveals other aspects of the LMC and its stars. This image was created by mapping radiation flux in the LMC and is dominated by the brightest and most massive stars. This allows the bar of the LMC to be more clearly defined and also shows individual regions of star-formation – like 30 Doradus, which is visible just above the center of the galaxy in the picture.
The next set of images (shown below), which were also obtained using data from the first 14 months of the Gaia mission, depict two nearby spiral galaxies – the Andromeda galaxy (M31) and its neighbor, the Triangulum galaxy (M33). The Andromeda galaxy, located 2.5 million light-years away, is the largest galaxy in our vicinity and slightly more massive than our own. It is also destined to merge with the Milky Way in roughly 4 billion years.
The Triangulum galaxy, meanwhile, is a fraction the size of the Milky Way (with an estimated fifty billion stars) and is located slightly farther from us than Andromeda – about 2.8 million light-years distant. As with the LMC images, the images on the left are based on the total density of stars and show stars of all types, while images on the right are based on the radiation flux of each galaxy and mainly show the bright end of the stellar population.
Gaia’s view of the Andromeda galaxy. Credit: ESA/Gaia/DPAC
Another benefit of the images on the right is that they indicate the regions where the most intense star formation is taking place. For many years, astronomers have known that the LMC boasts a significant amount of star-forming activity, forming stars at five times the rate of the Milky Way Galaxy. Andromeda, meanwhile, has reached a point of near-inactivity in the past 2 billion years when it comes to star formation.
In comparison, the Triangulum Galaxy still shows signs of star formation, at a rate that is about four and a half times that of Andromeda. Thanks to the Gaia images, which indicate the relative rates of star formation from elevated levels of radiation flux and brightness, these differences between Andromeda, Triangulum and the LMC is illustrated quite beautifully.
What’s more, by analyzing the motions of individual stars in external galaxies like the LMC, Andromeda, or Triangulum, it will be possible to learn more about the overall rotation of stars within these galaxies. It will also be possible to determine the orbits of the galaxies themselves, which are all part of the larger structure known as the Local Group.
This region of space, which the Milky Way is part of, measures roughly 10 million light-years across and has an estimated 1.29 billion Solar masses. This, in turn, is just one of several collections of galaxies in the even larger Virgo Supercluster. Measuring how stars and galaxies orbit about these larger structures is key to determining cosmic evolution, how the Universe came to be as it is today and where it is heading.
The Triangulum galaxy (M33), based on data compiled by the Gaia mission. Credit: ESA/Gaia/DPAC
An international team of astronomers recently attempted to do just that using the CosmicFlows surveys. These studies, which were conducted between 2011 and 2016, calculated the distance and speed of neighboring galaxies. By pairing this data with other distance estimates and data on the galaxies gravity fields, they were able to chart the motions of almost 1,400 galaxies within 100 million light years over the course of the past 13 billion years.
In the case of the LMC, another team of astronomers recently attempted to measure its orbit using a subset of data from the first Gaia release – the Tycho–Gaia Astrometric Solution (TGAS). Combined with additional parallax and proper motion data from the Hipparcos mission, the team was able to identify 29 stars in the LMC and measure their proper motion, which they then used to estimate the rotation of the galaxy.
Gaia’s observations of the LMC and the Small Magellanic Cloud (SMC) are also important when it comes to studying Cepheid and RR Lyrae variables. For years, astronomers have indicated that these stars could be used as indicators of cosmic distances for galaxies beyond our own. In addition, astronomers working at the Gaia Data Processing and Analysis Consortium (DPAC) tested this method on hundreds of LMC variable stars in order to validate data from the first release.
Astronomers are eagerly awaiting the second release of Gaia data, which is scheduled for April of 2018. This will also contain measurements on stellar distances and their motions across the sky, and is expected to reveal even more about our galaxy and its neighbors. But in the meantime, there are still plenty of revelations to be found from the first release, and scientists expect to be busy with it for many years to come.
Astronomers have finally observed something that was predicted but never seen: a stream of stars connecting the two Magellanic Clouds. In doing so, they began to unravel the mystery surrounding the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). And that required the extraordinary power of the European Space Agency’s (ESA) Gaia Observatory to do it.
The Large and Small Magellanic Clouds (LMC and SMC) are dwarf galaxies to the Milky Way. The team of astronomers, led by a group at the University of Cambridge, focused on the clouds and on one particular type of very old star: RR Lyrae. RR Lyrae stars are pulsating stars that have been around since the early days of the Clouds. The Clouds have been difficult to study because they sprawl widely, but Gaia’s unique all-sky view has made this easier.
Small and Large Magellanic Clouds over Paranal Observatory Credit: ESO/J. Colosimo
The Mystery: Mass
The Magellanic Clouds are a bit of a mystery. Astronomers want to know if our conventional theory of galaxy formation applies to them. To find out, they need to know when the Clouds first approached the Milky Way, and what their mass was at that time. The Cambridge team has uncovered some clues to help solve this mystery.
The team used Gaia to detect RR Lyrae stars, which allowed them to trace the extent of the LMC, something that has been difficult to do until Gaia came along. They found a low-luminosity halo around the LMC that stretched as far as 20 degrees. For the LMC to hold onto stars that far away means it would have to be much more massive than previously thought. In fact, the LMC might have as much as 10 percent of the mass that the Milky Way has.
The Large Magellanic Cloud. Image: Public Domain, https://commons.wikimedia.org/w/index.php?curid=57110
The Arrival of the Magellanic Clouds
That helped astronomers answer the mass question, but to really understand the LMC and SMC, they needed to know when the clouds arrived at the Milky Way. But tracking the orbit of a satellite galaxy is impossible. They move so slowly that a human lifetime is a tiny blip compared to them. This makes their orbit essentially unobservable.
But astronomers were able to find the next best thing: the often predicted but never observed stellar stream, or bridge of stars, stretching between the two clouds.
A star stream forms when a satellite galaxy feels the gravitational pull of another body. In this case, the gravitational pull of the LMC allowed individual stars to leave the SMC and be pulled toward the LMC. The stars don’t leave at once, they leave individually over time, forming a stream, or bridge, between the two bodies. This action leaves a luminous tracing of their path over time.
The astronomers behind this study think that the bridge actually has two components: stars stripped from the SMC by the LMC, and stars stripped from the LMC by the Milky Way. This bridge of RR Lyrae stars helps them understand the history of the interactions between all three bodies.
A Bridge of Stars… and Gas
The most recent interaction between the Clouds was about 200 million years ago. At that time, the Clouds passed close by each other. This action formed not one, but two bridges: one of stars and one of gas. By measuring the offset between the star bridge and the gas bridge, they hope to narrow down the density of the corona of gas surrounding the Milky Way.
Mystery #2: The Milky Way’s Corona
The density of the Milky Way’s Galactic Corona is the second mystery that astronomers hope to solve using the Gaia Observatory.
The Galactic Corona is made up of ionised gas at very low density. This makes it very difficult to observe. But astronomers have been scrutinizing it intensely because they think the corona might harbor most of the missing baryonic matter. Everybody has heard of Dark Matter, the matter that makes up 95% of the matter in the universe. Dark Matter is something other than the normal matter that makes up familiar things like stars, planets, and us.
The other 5% of matter is baryonic matter, the familiar atoms that we all learn about. But we can only account for half of the 5% of baryonic matter that we think has to exist. The rest is called the missing baryonic matter, and astronomers think it’s probably in the galactic corona, but they’ve been unable to measure it.
A part of the Small Magellanic Cloud galaxy is dazzling in this image from NASA’s Great Observatories. The Small Magellanic Cloud is about 200,000 light-years way from our own Milky Way spiral galaxy. Credit: NASA.
Understanding the density of the Galactic Corona feeds back into understanding the Magellanic Clouds and their history. That’s because the bridges of stars and gas that formed between the Small and Large Magellanic Clouds initially moved at the same speed. But as they approached the Milky Way’s corona, the corona exerted drag on the stars and the gas. Because the stars are small and dense relative to the gas, they travelled through the corona with no change in their velocity.
But the gas behaved differently. The gas was largely neutral hydrogen, and very diffuse, and its encounter with the Milky Way’s corona slowed it down considerably. This created the offset between the two streams.
Eureka?
The team compared the current locations of the streams of gas and stars. By taking into account the density of the gas, and also how long both Clouds have been in the corona, they could then estimate the density of the corona itself.
When they did so, their results showed that the missing baryonic matter could be accounted for in the corona. Or at least a significant fraction of it could. So what’s the end result of all this work?
It looks like all this work confirms that both the Large and Small Magellanic Clouds conform to our conventional theory of galaxy formation.
