Our home galaxy as seen by the European Space Agency’s Gaia satellite. Image Credit: ESA/Gaia/DPAC
The Milky Way is only as massive as it is because of collisions and mergers with other galaxies. This is a messy process, and we see the same thing happening with other galaxies throughout the Universe. Currently, we see the Milky Way nibbling at its two satellite galaxies, the Large and Small Magellanic Clouds. Their fate is likely sealed, and they’ll be absorbed into our galaxy.
Researchers thought the last major merger occurred in the Milky Way’s distant past, between 8 and 11 billion years ago. But new research amplifies the idea that it was much more recent: less than 3 billion years ago.
ESA’s Gaia space telescope discovered two star streams that helped form the infant Milky Way. Both are so ancient that they likely formed before even the oldest parts of our present-day galaxy’s spiral arms and disc. This image shows the location and distribution of Shakti (yellow) and Shiva (blue) stars throughout the Milky Way. Credit:
ESA/Gaia/DPAC/K. Malhan.
Using ESA’s Gaia spacecraft, astronomers have tracked down two streams of stars that likely formed the foundation of the Milky Way. Named “Shakti and Shiva,” the two streams contain about 10 million stars, all of which are 12 to 13 billion years old and likely came together even before the spiral arms and disk were formed. These star streams are all moving in roughly similar orbits and have similar compositions. Astronomers think they were probably separate galaxies that merged into the Milky Way shortly after the Big Bang.,
A wave structure of stellar nurseries in the Milky Way. Credit: Alyssa Goodman / Harvard University
Stars are formed within large clouds of gas and dust known as stellar nurseries. While star formation was once seen as a simple gravitational process, we now know it is a complex dance of interactions. When one star forms it can send shock waves through the interstellar medium that trigger other stars to form. Supernovae and galactic collisions can trigger the creation of stars as well. One way to study stellar formation is to look at where stars form within a galaxy.
The Milky Way 10 billion years ago and the Milky Way today. Image Credit: Gabriel Pérez Díaz, SMM (IAC)
In the very early days of our Universe, just over 13 billion years ago, there was very little structure. There were stars, and they were forming at a rapid rate, kicking off what’s known as the Stelliferous Era. But the enormous, majestic galaxies that we see today, including our Milky Way galaxy, hadn’t formed yet.
Using information from Gaia's second data release, a team of scientists have made refined estimates of the Milky Way's mass. Credit: ESA/Gaia/DPAC
Since the birth of modern astronomy, scientists have sought to determine the full extent of the Milky Way galaxy and learn more about its structure, formation and evolution. At present, astronomers estimate that it is 100,000 to 180,000 light-years in diameter and consists of 100 to 400 billion stars – though some estimates say there could be as many as 1 trillion.
And yet, even after decades of research and observations, there is still much about our galaxy astronomers do not know. For example, they are still trying to determine how massive the Milky Way is, and estimates vary widely. In a new study, a team of international scientists presents a new method for weighing the galaxy based the dynamics of the Milky Way’s satellites galaxies.
Obscured Sirius reveals Gaia 1 cluster.
Credit and Copyright: H. Kaiser/ESA
In December of 2013, the European Space Agency’s Gaia mission took to space. Since that time, this space observatory has been studying a billion astronomical objects – including stars, planets, comets, asteroids and galaxies – for the sake of creating the most precise 3D space catalog ever made. By the time the mission wraps up (later this year, barring extensions), it is expected to reveal some truly amazing things about our Universe.
In fact, with the first release of its data, the Gaia probe revealed something that has gone completely unnoticed until now. While viewing Sirius, the brightest star in the night sky, Gaia revealed a stellar cluster that had previously been obscured by Sirius’ bright light. This cluster – now known as the Gaia 1 Cluster – is now available to the public thanks to a picture that was taken by an amateur astronomer from Germany.
Given its brightness and the fact that it is visible from just about anywhere on the planet, Sirius has been known since antiquity, and was featured prominently in the astrological and astronomical traditions of many cultures. To the ancient Egyptians, the star was used to keep track of time and agriculture, since its return to the sky was linked to the annual flooding of the Nile.
Artist’s impression of a white dwarf star in orbit around Sirius (a main sequence white dwarf). Credit: NASA, ESA and G. Bacon (STScI)
In Ancient Greek mythology, Sirius represented the eye of the Canis Major constellation. Along with Canis Minor, it formed the Great Dog that diligently followed Orion, the Hunter. In Chinese astronomy, the star is known as the star of the “celestial wolf” and lies in the Mansion of Jing. And when Ptolemy created his influential astronomical tract in the 3rd century CE (the Almagest), he used Sirius as the location for the globe’s central meridian.
By the mid-19th century, astronomers determined that Sirius is actually a binary star system. Essentially, the star system consists of a main sequence white dwarf that is roughly two Solar masses and a white dwarf that is slightly more massive than our Sun. Sirius’ bright appearance means that astronomers have had plenty of light to study the star’s properties, but also causes it to outshine other celestial objects in its vicinity.
However, in the course of counting the stars around Sirius, Gaia’s sophisticated instruments managed to detect the Gaia 1 Cluster for the first time. News of both this cluster and another newly-discovered one (the Gaia 2 Cluster) became public after the first release of Gaia data, which took place in September 2016. News of this discovery sent ripples through the astronomical community and has led to much research into this cluster and its companion.
News of the discovery also prompted attempts to visually capture the cluster. Roughly a year ago, Harald Kaiser – an amateur astronomer from Karlsruhe, Germany – attended a public talk about the Gaia mission, where he learned about the Gaia 1 Cluster being spotted near Sirius. Kaiser then eagerly waited for the next clear night so he could find the cluster himself using his 30 cm telescope.
The ESA’s Gaia mission is currently on a five-year mission to map the stars of the Milky Way. Credit: ESA/ATG medialab; background: ESO/S. Brunier.
After snapping a picture of Sirius and correcting for its bright glare, he was able to capture some of the brightest stars in the cluster. As you can see from the image he took (at top), the cluster lies slightly to the left of Sirius and shows a smattering of some of its largest and brightest stars. In addition to revealing the location of this cluster, Kaiser’s efforts are also part of a larger effort to capitalize on the Gaia mission’s progress.
According to a study released in February of last year – led by Sergey Kopsov of Carnegie Melon University – Gaia 1 is a particularly massive cluster. In essence, it weighs in at an impressive 22,000 Solar Masses, is about 29 light-years (9 parsecs) in diameter, and is located 15,000 light years (4.6 kiloparsecs) from Earth. In addition to its size and the fact that it was previously undiscovered, it’s proximity also makes it an opportune target for future research.
The announcement of this cluster has also caused a fair degree of excitement in the scientific community since it validates the capabilities of Gaia and serves as an example of the kinds of things it is expected to reveal. Astronomers are now looking forward to Gaia’s second data release (planned for April 25th) which is expected to provide even more possibilities for new and exciting discoveries.
And be sure to check out this video about the Gaia mission, courtesy of the ESA:
The ESA's Gaia mission is currently on a five-year mission to map the stars of the Milky Way. Gaia has found evidence for a galactic collision that occurred between 300 million and 900 million years ago. Image credit: ESA/ATG medialab; background: ESO/S. Brunier.
In February of 2016, scientists working for the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever detection of gravitational waves. Since that time, multiple detections have taken place, thanks in large to part to improvements in instruments and greater levels of collaboration between observatories. Looking ahead, its possible that missions not designed for this purpose could also “moonlight” as gravitational wave detectors.
For example, the Gaia spacecraft – which is busy creating the most detailed 3D map of the Milky Way – could also be instrumental when it comes to gravitational wave research. That’s what a team of astronomers from the University of Cambridge recently claimed. According to their study, the Gaia satellite has the necessary sensitivity to study ultra-low frequency gravitational waves that are produced by supermassive black hole mergers.
Artist’s illustration of two merging neutron stars, which are a source of gravitational waves. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
To recap, gravitational waves (GWs) are ripples in space-time that are created by violent events, such as black hole mergers, collisions between neutron stars, and even the Big Bang. Originally predicted by Einstein’s Theory of General Relativity, observatories like LIGO and Advanced Virgo detect these waves by measuring the way space-time flexes and squeezes in response to GWs passing through Earth.
However, passing GWs would also cause the Earth to oscillate in its location with respect to the stars. As a result, an orbiting space telescope (such as Gaia), would be able to pick up on this by noting a temporary shift in the position of distant stars. Launched in 2013, the Gaia observatory has spent the past few years conducting high-precision observations of the positions of stars in our Galaxy (aka. astrometry).
In this respect, Gaia would look for small displacements in the massive field of stars it is monitoring to determine if gravitational waves have passed through the Earth’s neighborhood. To investigate whether or not Gaia was up to the task, Moore and his colleagues performed calculations to determine if the Gaia space telescope had the necessary sensitivity to detect ultra-low frequency GWs.
To this end, Moore and his colleagues simulated gravitational waves produced by a binary supermassive black hole – i.e. two SMBHs orbiting one another. What they found was that by compressing the data sets by a factor of more than 106 (measuring 100,000 stars instead of a billion at a time), GWs could be recovered from Gaia data with an only 1% loss of sensitivity.
Figure showing a Gaia star field, with red and black lines indicating induced apparent motions of the stars within a hemisphere. Credit: Kavli Institute for Cosmology, Cambridge
This method would be similar to that used in Pulsar Timing Arrays, where a set of millisecond pulsars are examined to determine if gravitational waves modify the frequency of their pulses. However, in this case, stars are being monitored to see if they are oscillating with a characteristic pattern, rather than pulsing. By looking at a field of 100,000 stars at a time, researchers would be able to detect induced apparent motions (see figure above).
Because of this, the full release of Gaia data (scheduled for the early 2020s) is likely to be a major opportunity for those hunting for GW signals. As Moore explained in a APS Physicspress release:
“Gaia will make measuring this effect a realistic prospect for the first time. Many factors contribute to the feasibility of the approach, including the precision and long duration of the astrometric measurements. Gaia will observe about a billion stars over 5–10 years, locating each one of them at least 80 times during that period. Observing so many stars is the major advance provided by Gaia.”
It is also interesting to note that the potential for GW detection was something that researchers recognized when Gaia was still being designed. One such individual was Sergei A. Klioner, a researcher from the Lorhrmann Observatory and the leader of the Gaia group at TU Dresden. As he indicated in his 2017 study, “Gaia-like astrometry and gravitational waves“, Gaia could detect GWs caused by merging SMBHs years after the event:
“It is clear that the most promising sources of gravitational waves for astrometric detection are supermassive binary black holes in the centers of galaxies… It is believed that binary supermassive black holes are a relatively common product of interaction and merging of galaxies in the typical course of their evolution. This sort of objects can give gravitational waves with both frequencies and amplitudes potentially within the reach of space astrometry. Moreover, the gravitational waves from those objects can often be considered to have virtually constant frequency and amplitude during the whole period of observations of several years.”
Artist’s impression of two merging black holes, which has been theorized to be a source of gravitational waves. Credit: Bohn, Throwe, Hébert, Henriksson, Bunandar, Taylor, Scheel/SXS
But of course, there’s no guarantees that sifting through the Gaia data will reveal additional GW signals. For one thing, Moore and his colleagues acknowledge that waves at these ultra-low frequencies could be too weak for even Gaia to detect. In addition, researchers will have to be able to distinguish between GWs and conflicting signals that result from changes in the spacecraft’s orientation – which is no easy challenge!
Still, there is hope that missions like Gaia will be able to reveal GWs that are not easily visible to ground-based interferometric detectors like LIGO and Advanced Virgo. Such detectors are subject to atmospheric effects (like refraction) which prevent them from seeing extremely low frequency waves – for instance, the primordial waves produced during the inflationary epoch of the Big Bang.
In this sense, gravitational wave research is not unlike exoplanet research and many other branches of astronomy. In order to find the hidden gems, observatories may need to take to space to eliminate atmospheric interference and increase their sensitivity. It is possible then that other space telescopes will be retooled for GW research, and that next-generation GW detectors will be mounted aboard spacecraft.
In the past few years, scientists have gone from making the first detection of gravitational waves to developing new and better ways to detecting them. At this rate, it won’t be long before astronomers and cosmologists are able to include gravitational waves into our cosmological models. In other words, they will be able to show what influence these waves played in the history and evolution of the Universe.
Color view of M31 (The Andromeda Galaxy), with M32 (a satellite galaxy) shown to the lower left. Credit and copyright: Terry Hancock.
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.
