Posted on by Matt Williams

Thanks to a Massive Release from Gaia, we now Know Where 1.7 BILLION Stars are in the Milky Way

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

On December 19th, 2013, the European Space Agency’s (ESA) Gaia spacecraft took to space with one of the most ambitious missions ever. 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.

The first release of Gaia data, which took place in September 2016, contained the distances and motions of over two million stars. But the second data release, which took place on April 25th, 2018, is even more impressive. Included in the release are the positions, distance indicators and motions of more than one billion stars, asteroids within our Solar System, and even stars beyond the Milky Way.

Whereas the first data release was based on just over a year’s worth of observations, the new data release covers a period of about 22 months – which ran from July 25th, 2014, to May 23rd, 2016. Preliminary analysis of this data has revealed fine details about 1.7 billion stars in the Milky Way and how they move, which is essential to understanding how our galaxy evolved over time.

ESA’s Gaia is currently on a five-year mission to map the stars of the Milky Way. Image credit: ESA/ATG medialab; background: ESO/S. Brunier.

As Günther Hasinger, the ESA Director of Science, explained in a recent ESA press release:

“The observations collected by Gaia are redefining the foundations of astronomy. Gaia is an ambitious mission that relies on a huge human collaboration to make sense of a large volume of highly complex data. It demonstrates the need for long-term projects to guarantee progress in space science and technology and to implement even more daring scientific missions of the coming decades.“

The precision of Gaia‘s instruments has allowed for measurements that are so accurate that it was possible to separate the parallax of stars – the apparent shift caused by the Earth’s orbit around the Sun – from their movements through the galaxy. Of the 1.7 billion stars cataloged, the parallax and velocity (aka. proper motion) of more than 1.3 billion were measured and listed.

For about 10% of these, the parallax measurements were so accurate that astronomers can directly estimate distances to the individual stars. As Anthony Brown of Leiden University, who is also the chair of the Gaia Data Processing and Analysis Consortium Executive Board, explained:

“The second Gaia data release represents a huge leap forward with respect to ESA‘s Hipparcos satellite, Gaia‘s predecessor and the first space mission for astrometry, which surveyed some 118 000 stars almost thirty years ago… The sheer number of stars alone, with their positions and motions, would make Gaia‘s new catalogue already quite astonishing. But there is more: this unique scientific catalogue includes many other data types, with information about the properties of the stars and other celestial objects, making this release truly exceptional.“

In addition to the proper motions of stars, the catalog provides information on a wide range of topics that will be of interest to astronomers and astrophysicists. These include brightness and color measurements of nearly all of the 1.7 billion stars cataloged, as well as information on how the brightness and color change for half a million variable stars over time.

It also contains the velocities along the line of sight of seven million stars, the surface temperatures of about 100 million, and the effect interstellar dust has on 87 million. The Gaia data also contains information on objects in our Solar System, which includes the positions of 14,000 known asteroids (which will allow for the precise determination of their orbits).

Beyond the Milky Way, Gaia obtained more accurate measurements of the positions of half a million distant quasars – bright galaxies that emit massive amounts of energy due to the presence of a supermassive black hole at their centers. In the past, quasars have been used as a reference frame for the celestial coordinates of all objects in the Gaia catalogue based on radio waves.

However, this information will now be available at optical wavelengths for the first time. This, and other developments made possible by Gaia, could revolutionize how we study our galaxy and the Universe. As Antonella Vallenari, from the Istituto Nazionale di Astrofisica (INAF), the Astronomical Observatory of Padua, Italy, and the deputy chair of the Data Processing Consortium Executive Board, indicated:

“The new Gaia data are so powerful that exciting results are just jumping at us. For example, we have built the most detailed Hertzsprung-Russell diagram of stars ever made on the full sky and we can already spot some interesting trends. It feels like we are inaugurating a new era of Galactic archaeology.“

The Hertzsprung-Russell diagram, which is named after the two astronomers who devised it in the early 20th century, is fundamental to the study of stellar populations and their evolution. Based on four million stars that were selected from the catalog (all of which are withing five thousand light-years from the Sun), scientist were able to reveal many fine details about stars beyond our Solar System for the first time.

Along with measurements of their velocities, the Gaia Hertzsprung-Russell diagram enables astronomers to distinguish between populations of stars that are of different ages, are located in different regions of the Milky Way (i.e. the disk and the halo), and that formed in different ways. These include fast moving stars that were previously thought to belong to the halo, but are actually part of two stellar populations.

“Gaia will greatly advance our understanding of the Universe on all cosmic scales,” said Timo Prusti, a Gaia project scientist at ESA. “Even in the neighborhood of the Sun, which is the region we thought we understood best, Gaia is revealing new and exciting features.”

For instance, for a subset of stars within a few thousand light-years of the Sun, Gaia measured their velocity in all three dimensions. From this, it has been determined that they follow a similar pattern to stars that are orbiting the galaxy at similar speeds. The cause of these patterns will be the subject of future research, as it is unclear whether its caused by our galaxy itself or are the result of interactions with smaller galaxies that merged with us in the past.

Last, but not least, Gaia data will be used to learn more about the orbits of 75 globular clusters and 12 dwarf galaxies that revolve around the Milky Way. This information will shed further light on the evolution of our galaxy, the gravitational forces affecting it, and the role played by dark matter. As Fred Jansen, the Gaia mission manager at ESA, put it:

“Gaia is astronomy at its finest. Scientists will be busy with this data for many years, and we are ready to be surprised by the avalanche of discoveries that will unlock the secrets of our Galaxy.“

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). A series of scientific papers describing what has been learned from this latest release will also appear in a special issue of Astronomy & Astrophysics.

From the evolution of stars to the evolution of our galaxy, the second Gaia data release is already proving to be a boon for astronomers and astrophysicists. Even after the mission concludes, we can expect scientists will still be analyzing the data and learning a great deal more about the structure and evolution of our Universe.

Further Reading: ESA

Posted on by Matt Williams

Did the Milky Way Steal These Stars or Kick Them Out of the Galaxy?

The Milky Way galaxy, perturbed by the tidal interaction with a dwarf galaxy, as predicted by N-body simulations. The locations of the observed stars above and below the disk, which are used to test the perturbation scenario, are indicated. Credit: T. Mueller/C. Laporte/NASA/JPL-Caletch

Despite thousands of years of research and observation, there is much that astronomers still don’t know about the Milky Way Galaxy. At present, astronomers estimate that it spans 100,000 to 180,000 light-years and consists of 100 to 400 billion stars. In addition, for decades, there have been unresolved questions about how the structure of our galaxy evolved over the course of billions of years.

For example, astronomers have long suspected that galactic halo came from – giant structures of stars that orbit above and below the flat disk of the Milky Way – were formed from debris left behind by smaller galaxies that merged with the Milky Way. But according to a new study by an international team of astronomers, it appears that these stars may have originated within the Milky Way but were then kicked out.

The study recently appeared in the journal Nature under the title “Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk“. The study was led by Margia Bergmann, a researcher from the Max Planck Institute for Astronomy, and included members from the Australian National University, the California Institute of Technology, and multiple universities.

Artist’s impression of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

For the sake of their study, the team relied on data from the W.M. Keck Observatory to determine the chemical abundance patterns from 14 stars located in the galactic halo. These stars were located in two different halo structures – the Triangulum-Andromeda (Tri-And) and the A13 stellar overdensities – which are bout 14,000 light years above and below the Milky Way disc.

As Bergemann explained in a Keck Observatory press release:

“The analysis of chemical abundances is a very powerful test, which allows, in a way similar to the DNA matching, to identify the parent population of the star. Different parent populations, such as the Milky Way disk or halo, dwarf satellite galaxies or globular clusters, are known to have radically different chemical compositions. So once we know what the stars are made of, we can immediately link them to their parent populations.”

The team also obtained spectra from one additional using the European Southern Observatory’s Very Large Telescope (VLT) in Chile. By comparing the chemical compositions of these stars with the ones found in other cosmic structures, the scientists noticed that the chemical compositions were almost identical. Not only were they similar within and between the groups being studies, they closely matched the abundance patterns of stars found within the Milky Way’s outer disk.

Computer model of the Milky Way and its smaller neighbor, the Sagittarius dwarf galaxy. Credit: Tollerud, Purcell and Bullock/UC Irvine

From this, they concluded that these stellar population in the Galactic Halo were formed in the Milky Way, but then relocated to locations above and below the Galactic Disk. This phenomena is known as “galactic eviction”, where structures are pushed off the plane of the Milky Way when a massive dwarf galaxy passes through the galactic disk. This process causes oscillations that eject stars from the disk, in whichever the dwarf galaxy is moving.

“The oscillations can be compared to sound waves in a musical instrument,” added Bergemann. “We call this ‘ringing’ in the Milky Way galaxy ‘galactoseismology,’ which has been predicted theoretically decades ago. We now have the clearest evidence for these oscillations in our galaxy’s disk obtained so far!”

These observations were made possible thanks to the High-Resolution Echelle Spectrometer (HiRES) on the Keck Telescope. As Judy Cohen, the Kate Van Nuys Page Professor of Astronomy at Caltech and a co-author on the study, explained:

“The high throughput and high spectral resolution of HIRES were crucial to the success of the observations of the stars in the outer part of the Milky Way. Another key factor was the smooth operation of Keck Observatory; good pointing and smooth operation allows one to get spectra of more stars in only a few nights of observation. The spectra in this study were obtained in only one night of Keck time, which shows how valuable even a single night can be.”

360-degree panorama view of the Milky Way (an assembled mosaic of photographs) by ESO. Credit: ESO/S. Brunier

These findings are very exciting for two reasons. On the one hand, it demonstrates that halo stars likely originated in the Galactic think disk – a younger part of the Milky Way. On the other hand, it demonstrates that the Milky Way’s disk and its dynamics are much more complex than previously thought. As Allyson Sheffield of LaGuardia Community College/CUNY, and a co-author on the paper, said:

“We showed that it may be fairly common for groups of stars in the disk to be relocated to more distant realms within the Milky Way – having been ‘kicked out’ by an invading satellite galaxy. Similar chemical patterns may also be found in other galaxies, indicating a potential galactic universality of this dynamic process.”

As a next step, the astronomers plan to analyze the spectra of additional stars in the Tri-And and A13 overdensities, as well as stars in other stellar structures further away from the disk. They also plan to determine masses and ages of these stars so they can constrain the time limits of when this galactic eviction took place.

In the end, it appears that another long-held assumption on galactic evolution has been updated. Combined with ongoing efforts to probe the nuclei of galaxies – to see how their Supermassive Black Holes and star formation are related – we appear to be getting closer to understanding just how our Universe evolved over time.

Further Reading: W.M. Keck Observatory, Nature

Posted on by Matt Williams

Amazing High Resolution Image of the Core of the Milky Way, a Region with Surprisingly Low Star Formation Compared to Other Galaxies

NASA's Spitzer Space Telescope captured this stunning infrared image of the center of the Milky Way Galaxy, where the black hole Sagitarrius A resides. Credit: NASA/JPL-Caltech

Compared to some other galaxies in our Universe, the Milky Way is a rather subtle character. In fact, there are galaxies that are a thousands times as luminous as the Milky Way, owing to the presence of warm gas in the galaxy’s Central Molecular Zone (CMZ). This gas is heated by massive bursts of star formation that surround the Supermassive Black Hole (SMBH) at the nucleus of the galaxy.

The core of the Milky Way also has a SMBH (Sagittarius A*) and all the gas it needs to form new stars. But for some reason, star formation in our galaxy’s CMZ is less than the average. To address this ongoing mystery, an international team of astronomers conducted a large and comprehensive study of the CMZ to search for answers as to why this might be.

The study, titled “Star formation in a high-pressure environment: an SMA view of the Galactic Centre dust ridge” recently appeared in the Monthly Notices of the Royal Astronomical Society. The study was led by Daniel Walker of the Joint ALMA Observatory and the National Astronomical Observatory of Japan, and included members from multiple observatories, universities and research institutes.

A false color Spitzer infrared image of the Milky Way’s Central Molecular Zone (CMZ). Credit: Spitzer/NASA/CfA

For the sake of their study, the team relied on the Submillimeter Array (SMA) radio interferometer, which is located atop Maunakea in Hawaii. What they found was a sample of thirteen high-mass cores in the CMZ’s “dust ridge” that could be young stars in the initial phase of development. These cores ranged in mass from 50 to 2150 Solar Masses and have radii of 0.1 – 0.25 parsecs (0.326 – 0.815 light-years).

They also noted the presence of two objects that appeared to be previously unknown young, high-mass protostars. As they state in their study, all of this indicated that stars in CMZ had about the same rate of formation as those in the galactic disc, despite their being vast pressure differences:

“All appear to be young (pre-UCHII), meaning that they are prime candidates for representing the initial conditions of high-mass stars and sub-clusters. We compare all of the detected cores with high-mass cores and clouds in the Galactic disc and find that they are broadly similar in terms of their masses and sizes, despite being subjected to external pressures that are several orders of magnitude greater.”

To determine that the external pressure in the CMZ was greater, the team observed spectral lines of the molecules formaldehyde and methyl cyanide to measure the temperature of the gas and its kinetics. These indicated that the gas environment was highly turbulent, which led them to the conclusion that the turbulent environment of the CMZ is responsible for inhibiting star formation there.

A radio image from the NSF’s Karl G. Jansky Very Large Array showing the center of our  galaxy. Credit: NSF/VLA/UCLA/M. Morris et al.

As they state in their study, these results were consistent with their previous hypothesis:

“The fact that >80 percent of these cores do not show any signs of star-forming activity in such a high-pressure environment leads us to conclude that this is further evidence for an increased critical density threshold for star formation in the CMZ due to turbulence.”

So in the end, the rate of star formation in a CMZ is not only dependent on their being a lot of gas and dust, but on the nature of the gas environment itself. These results could inform future studies of not only the Milky Way, but of other galaxies as well – particularly when it comes to the relationship that exists between Supermassive Black Holes (SMBHs), star formation, and the evolution of galaxies.

For decades, astronomers have studied the central regions of galaxies in the hopes of determining how this relationship works. And in recent years, astronomers have come up with conflicting results, some of which indicate that star formation is arrested by the presence of SMBHs while others show no correlation.

In addition, further examinations of SMBHs and Active Galactic Nuclei (AGNs) have shown that there may be no correlation between the mass of a galaxy and the mass of its central black hole – another theory that astronomers previously subscribed to.

As such, understanding how and why star formation appears to be different in galaxies like the Milky Way could help us to unravel these other mysteries. From that, a better understanding of how stars and galaxies evolved over the course of cosmic history is sure to emerge.

Further Reading: CfA, MNRAS

Posted on by Matt Williams

It Turns Out, Andromeda is Younger Than Earth… Sort Of

Andromeda Galaxy. Credit: Wikipedia Commons/Adam Evans

Since ancient times, astronomers have looked up at the night sky and seen the Andromeda galaxy. As the closest galaxy to our own, scientists have been able to observe and scrutinize this giant spiral galaxy for millennia. By the 20th century, astronomers realized that Andromeda was the Milky Way’s sister galaxy and was moving towards us. In 4.5 billion years, it will even merge with our own to form a supergalaxy.

However, it seems astronomers were wrong about the Andromeda galaxy in one major respect. According to recent study led by a team of French and Chinese astronomers, this giant spiral galaxy formed from a major merger that occurred less than 3 billion years ago. This means that Andromeda, as we know it today, is effectively younger than our very own Solar System, which has it beat by about 1.5 billion years!

The study, titled “A 2-3 billion year old major merger paradigm for the Andromeda galaxy and its outskirts“, recently appeared in the Monthly Notices of the Royal Astronomical Society. Led by Francois Hammer, the Principle Investigator of the Galaxies, Etoiles, Physique et Instrumentation (GEPI) department at the Paris Observatory, the team included members from the Chinese Academy of Sciences and the University of Strasbourg.

For the sake of their study, the relied on data gathered by recent surveys that noted considerable differences between the Andromeda and Milky Way galaxies. The first of these studies, which took place between 2006 and 2014, demonstrated all Andromeda has a wealth of young blue stars in its disk (less than 2 billion years old) that undergo random motions over large scales. This is contrast to the stars in the Milky Way’s disk, which are subject only to simple rotation.

In addition, deep observations conducted between 2008 and 2014 with the French-Canadian telescope in the Hawaiian Islands (CFHT) indicated some interesting things about Andromeda’s halo. This vast region, which is 10 times the size of the galaxy itself, is populated by gigantic currents of stars. The most prominent of which is called the “Giant Stream”, a warped disk that has shells and clumps at its very edges.

Using this data, the French-Chinese collaboration then created a detailed numerical model of Andromeda using the two most powerful computers available in France – the Paris Observatory’s MesoPSL and the National Center for Scientific Research’s (CNRS) IDRIS-GENCI supercomputer. With the resulting numerical model, the team was able to demonstrate that these recent observations could be explained only by a recent collision.

Basically, they concluded that between 7 and 10 billion years ago, Andromeda consisted of  two galaxies that had slowly achieved a encountering orbit. After optimizing the trajectories of both galaxies, they determined that they would have collided 1.8 to 3 billion years ago. This collision is what gave birth to Andromeda as we know it today, which effectively makes it younger than our Solar System – which formed almost 4.6 billion years ago.

What’s more, they were able to calculate mass distributions for both parent galaxies that merged to formed Andromeda, which indicated that the larger galaxy was four times the size of the smaller. But most importantly, the team was able to reproduce in detail all the structures that compose Andromeda today – including the bulge, the bar, the huge disk, and the presence of young stars.

The presence of young blue stars in its disk, which has remained unexplained until now, is attributable to a period of intense star formation that took place after the collision. In addition, structures like the “Giant Stream” and the shells of the halo belonged to the smaller parent galaxy, whereas the diffuse clumps and the warped nature of the halo were derived from the larger one.

Their study also explains why the features attributed to the smaller galaxy have an under-abundance in heavy elements compared to the others – i.e. it was less massive so it formed fewer heavy elements and stars. This study is immensely significant when it comes to galactic formation and evolution, mainly because it is the first numerical simulation that has succeeded in reproducing a galaxy in such detail.

It is also of significance given that such a recent impact it could have left materials in the Local Group. In other words, this study could have implications that range far beyond our galactic neighborhood. It is also a good example of how increasingly sophisticated instruments are leading to more detailed observations which, when combined with increasingly sophisticated computers and algorithms, are leading to more detailed models.

One can only wonder if future extra-terrestrial intelligence (ETI) will draw similar conclusions about our own galaxy once it merges with Andromeda, billions of years from now. The collision and resulting features are sure to be of interest to anyone advanced species that’s around to study it!

Further Reading: Paris Observatory, Monthly Notices of the Royal Astronomical Society search and more info website

Posted on by Matt Williams

Astronomers Find One of the Oldest Stars in the Milky Way

A recent survey has discovered the first stars of the Milky Way. Credit: Gabriel Pérez, SMM (IAC)

According to modern cosmological models, the Universe began in a cataclysm event known as the Big Bang. This took place roughly 13.8 billion years ago, and was followed by a period of expansion and cooling. During that time, the first hydrogen atoms formed as protons and electrons combined and the fundamental forces of physics were born. Then, about 100 million years after the Big Bang, that the first stars and galaxies began to form.

The formation of the first stars was also what allowed for the creation of heavier elements, and therefore the formation of planets and all life as we know it. However, until now, how and when this process took place has been largely theoretical since astronomers did not know where the oldest stars in our galaxy were to be found. But thanks to a new study by a team of Spanish astronomers, we may have just found the oldest star in the Milky Way!

The study, titled “J0815+4729: A chemically primitive dwarf star in the Galactic Halo observed with Gran Telescopio Canarias“, recently appeared in The Astrophysical Journal Letters. Led by David S. Aguado of the Instituto de Astrofisica de Canarias (IAC), the team included members from the University of La Laguna and the Spanish National Research Council (CSIC).

Artist’s impression of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

This star is located roughly 7,500 light years from the Sun, and was found in the halo of the Milky Way along the line of sight to the Lynx constellation. Known as J0815+4729, this star is still in its main sequence and has a low mass, (around 0.7 Solar Masses), though the research team estimates that it has a surface temperature that is about 400 degrees hotter – 6,215 K (5942 °C; 10,727 °F) compared to 5778 K (5505 °C; 9940 °F).

For the sake of their study, the team was looking for a star that showed signs of being metal-poor, which would indicate that it has been in its main sequence for a very long time. The team first selected J0815+4729 from the Sloan Digital Sky Survey-III Baryon Oscillation Spectroscopic Survey (SDSS-III/BOSS) and then conducted follow-up spectroscopic investigations to determine its composition (and hence its age).

This was done using the Intermediate dispersion Spectrograph and Imaging System (ISIS) at the William Herschel Telescope (WHT) and the Optical System for Imaging and low-intermediate-Resolution Integrated Spectroscopy (OSIRIS) at Gran Telescopio de Canarias (GTC), both of which are located at the Observatorio del Roque de los Muchachos on the island of La Palma.

Consistent with what modern theory predicts, the star was found in the Galactic halo – the extended component of our galaxy that reaches beyond the galactic disk (the visible portion). It is in this region that the oldest and most metal-poor stars are believed to be found in galaxies, hence why the team was confident that a star dating back to the early Universe would be found here.

The William Herschel Telescope, part of the Isaac Newton group of telescopes, located on Canary Island. Credit: ing.iac.es

As Jonay González Hernández – a professor from the University of La Laguna, a member of the IAC and a co-author on the paper – explained in an IAC press release:

“Theory predicts that these stars could use material from the first supernovae, whose progenitors were the first massive stars in the galaxy, around 300 million years after the Big Bang. In spite of its age, and its distance away from us, we can still observe it.”

Spectra obtained by both the ISIS and OSIRIS instruments confirmed that the star was poor in metals, indicating that J0815+4729 has only one-millionth of the calcium and iron that the Sun contains. In addition, the team also noticed that the star has a higher carbon content than our Sun, accounting for almost 15% percent of its solar abundance (i.e. the relative abundance of its elements).

In short, J0815+4729 may be the most iron-poor and carbon-rich star currently known to astronomers. Moreover, finding it was rather difficult since the star is both weak in luminosity and was buried within a massive amount of SDSS/BOSS archival data. As Carlos Allende Prieto, another IAC researcher and a co-author on the paper, indicated:

“This star was tucked away in the database of the BOSS project, among a million stellar spectra which we have analysed, requiring a considerable observational and computational effort. It requires high-resolution spectroscopy on large telescopes to detect the in the star, which can help us to understand the first supernovae and their progenitors.”

In the near future, the team predicts that next-generation spectrographs could allow for further research that would reveal more about the star’s chemical abundances. Such instruments include the HORS high-resolution spectrograph, which is presently in a trial phase on the Gran Telescopio Canarias (GTC).

“Detecting lithium gives us crucial information related to Big Bang nucleosynthesis,” said Rafael Rebolo, the director of the IAC and a coauthor of the paper. “We are working on a spectrograph of high-resolution and wide spectral range in order to measure the detailed chemical composition of stars with unique properties such as J0815+4719.”

These future studies are sure to be a boon for astronomers and cosmologists. In addition to being a chance to study stars that formed when the Universe was still in its infancy, they could provide new insight into the early stages of the universe, the formation of the first stars, and the properties of the first supernovae. In other words, they would put us a step closer to know how the Universe as we know it formed and evolved.

Further Reading: IAC, The Astrophysical Journal Letters

Posted on by Matt Williams

New Map Shows the Motion of all the Galaxies in Our Supercluster

A mosaic of telescopic images showing the galaxies of the Virgo Supercluster. It's part of the cosmic web in which a galaxy can exist during part of its evolution. Credit: NASA/Rogelio Bernal Andreo
A mosaic of telescopic images showing the galaxies of the Virgo Supercluster. It's part of the cosmic web in which a galaxy can exist during part of its evolution. Credit: NASA/Rogelio Bernal Andreo

For almost a century, astronomers have understood that the Universe is in a state of expansion. This is a consequence of General Relativity, and the rate at which it is expanding is known as the Hubble Constant – named after the man who first noticed the phenomena. However, astronomers have also learned that withing the large-scale structures of the Universe, galaxies and clusters have also been moving closer and relative to one other.

For decades, astronomers have sought to track how these movements have taken place over the course of cosmic history. And thanks to the efforts of international team of astronomers, the most detailed map to date of the orbits of galaxies that lie within the Virgo Supercluster has been created. This map encompasses the past motions of almost 1,400 galaxies within 100 million light years of space, showing how our cosmic neighborhood has changed.

The study which details their research recently appeared in The Astrophysical Journal under the title “Action Dynamics of the Local Supercluster“. Led by Edward J. Shaya of the University of Maryland, the team included members from the UH Institute of Astronomy, the Racah Institute of Physics in Jerusalem, and the Institute for Research of the Fundamental Laws of the Universe (IRFU) in Paris.

Orbits of galaxies in the Local Supercluster. Credit: Brent Tully.

For the sake of their study, the team used data from the CosmicFlows surveys, a series of three studies that calculated the distance and speed of neighboring galaxies between 2011 and 2016. Several members of the study team were involved in these surveys, which they then paired with other distance and gravity field estimates to construct a massive flow study of the Virgo Supercluster.

From this, they were able to create computer models that charted the motions of almost 1,400 galaxies within 100 million light years, and over the course of 13 billion years (just 800 million years after the Big Bang). As Brent Tully, an astronomer with the UH Institute of Astronomy and a co-author on the study, explained in a UH press release:

“For the first time, we are not only visualizing the detailed structure of our Local Supercluster of galaxies but we are seeing how the structure developed over the history of the universe. An analogy is the study of the current geography of the Earth from the movement of plate tectonics.”

What they found was that their models fit the present day velocity flow well, meaning that the structures and speeds they observed in their models fit with what has been observed from galaxies in the present day. They also determined that within the area of space they mapped, the main gravitational attractor is the Virgo Cluster – which is located about 50 million light years away and contains between 1300 and 2000 galaxies.

Moreover, their study indicated that more than a thousand galaxies have fallen into the Virgo Cluster in the past 13 billion years, while all galaxies within 40 million light-years of the cluster will eventually be captured. At present, the Milky Way lies just outside this capture zone, but both the Milky Way and the Andromeda Galaxy are destined to merge in the next 4 billion years.

Once they do, the fate of the resulting massive galaxy will be similar to the rest of the galaxies in the area of study. This was another takeaway from the study, where the team determined that these merger events are merely part of a larger pattern. Basically, within the region of space they observed, there are two overarching flow patterns. Within one hemisphere of this region, all galaxies – including the Milky Way – are streaming towards a single flat sheet.

At the same time, every galaxy over the entire volume of space is moving towards gravitational attractors that are located far beyond the area of study. They determined that these outside forces are none other than the Centaurus Supercluster – a cluster of hundreds of galaxies, located approximately 170 million light years away in the Centaurus constellation – and the Great Attractor.

The Great Attractor is located 150 million light years away, and is a mysterious region that cannot be seen because of its location (on the opposite side of the Milky Way). However, for decades, scientists have known that our galaxy and other nearby galaxies are moving towards it. The region is also the core of the Laniakea Supercluster, a region that spans more than 500 million light-years and contains about 100,000 large galaxies.

In short, while the Universe is in a state of expansion, the dynamics of galaxies and galaxy clusters indicate that they still gravitate into tighter structures.  Within our cosmic neighborhood, the main attractor is clearly the Virgo Cluster, which is affecting all galaxies within a 40 million light-year radius. Beyond this, it is the Centaurus Supercluster and the Great Attractor (as part of the larger Laniakea Supercluster) that is tugging at our strings.

By charting this process of attraction that has been taking place over the past 13 billion years, astronomers and cosmologists are able to see just how our Universe has evolved over the course of the majority of its history. With time, and improved instruments that are capable of looking even deeper into the cosmos (such as the James Webb Space Telescope) we are expected to be able to probe even further back towards the beginning of the cosmos.

Charting how our Universe has changed over time not only confirms our cosmological models and verifies predominant theories about how matter behaves on the largest of scales (i.e. General Relativity). It also allows scientists to predict the future of our Universe with a fair degree of certainty, modelling how galaxies and superclusters will eventually come together to form even larger structures.

The team also created a video showing the results of their study, as well as an interactive model that let’s users examine the frame of reference from multiple vantage points. Be sure to check out the video below, and head on over to the UH page to access their interactive model.

Further Reading: University of Hawaii, The Astrophysical Journal

Posted on by Matt Williams

Astronomers Start Mapping the Structure of the Far Side of the Milky Way

Artist's impression of the spiral structure of the Milky Way with two major stellar arms and a bar. Credit: NASA/JPL-Caltech/ESO/R. Hurt

Since the 18th century, astronomers have been aware that our Solar System is embedded in a vast disk of stars and gas known as the Milky Way Galaxy. Since that time, the greatest scientific minds have been attempting to obtain accurate distance measurements in order to determine just how large the Milky Way is. This has been no easy task, since the fact that we are embedded in our galaxy’s disk means that we cannot view it head-on.

But thanks to a time-tested technique called trigonometric parallax, a team of astronomers from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, and the Harvard-Smithsonian Center for Astrophysics (CfA) were recently able to directly measure the distance to the opposite side of the Milky Way Galaxy. Aside from being an historic first, this feat has nearly doubled the previous record for distance measurements within our galaxy.

The study which described this accomplishment, titled “Mapping Spiral Structure on the far side of the Milky Way“, recently appeared in the journal Science. Led by Alberto Sanna, a researcher from the Max Planck Institute for Radio Astronomy, the team consulted data from the National Radio Astronomy Observatory’s Very Long Baseline Array (VLBA) to determine the distance to a star-forming region on the other side of our galaxy.

Artist’s view of the Milky Way with the location of the Sun and the star forming region at the opposite side in the Scutum-Centaurus spiral arm. Credit: Bill Saxton, NRAO/AUI/NSF; Robert Hurt, NASA.

To do this, the team relied on a technique first applied by Freidrich Wilhelm Bessel in 1838 to measure the distance to the star 61 Cygni. Known as trigonometric parallax, this technique involves viewing an object from opposite sides of the Earth’s orbit around the Sun, and then measuring the angle of the object’s apparent shift in position. In this way, astronomers are able to use simple trigonometry to calculate the distance to that object.

In short, the smaller the measured angle, the greater the distance to the object. These measurements were performed using data from the Bar and Spiral Structure Legacy (BeSSeL) Survey, which was named in honor of Freidrich Wilhelm Bessel. But whereas Bessel and his contemporaries were forced to measure parallax using basic instruments, the VLBA has ten dish antennas distributed across North America, Hawaii, and the Caribbean.

With such an array at its disposal, the VLBA is capable of measuring parallaxes with one thousand times the accuracy of those performed by astronomers in Bessel’s time. And rather than being confined to nearby star systems, the VLBA is capable of measuring the minuscule angles associated with vast cosmological distances. As Sanna explained in a recent MPIfR press release:

“Using the VLBA, we now can accurately map the whole extent of our Galaxy. Most of the stars and gas in our Galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the Galaxy’s spiral arms and learn their true shapes.”

With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.

The VLBA observations, which were conducted in 2014 and 2015, measured the distance to the star-forming region known as G007.47+00.05. Like all star-forming regions, this one contains molecules of water and methanol, which act as natural amplifiers of radio signals. This results in masers (the radio-wave equivalent of lasers), an effect that makes the radio signals appear bright and readily observable with radio telescopes.

This particular region is located over 66,000 light years from Earth and at on opposite side of the Milky Way, relative to our Solar System. The previous record for a parallax measurement was about 36,000 light-years, roughly 11,000 light years farther than the distance between our Solar System and the center of our galaxy. As Sanna explained, this accomplishment in radio astronomy will enable surveys that reach much farther than previous ones:

“Most of the stars and gas in our Galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the Galaxy’s spiral arms and learn their true shapes.”

Hundreds of star-forming regions exist within the Milky Way. But as Karl Menten – a member of the MPIfR and a co-author on the study – explained, this study was significant because of where this one is located. “So we have plenty of ‘mileposts’ to use for our mapping project,” he said. “But this one is special: Looking all the way through the Milky Way, past its center, way out into the other side.”

The band of light (the Milky Way) that is visible in the night sky, showing the stellar disk of our galaxy. Credit: Bob King

In the coming years, Sanna and his colleagues hope to conduct additional observations of G007.47+00.05 and other distant star-forming regions of the Milky Way. Ultimately, the goal is to gain a complete understanding of our galaxy, one that is so accurate that scientists will be able to finally place precise constraints on its size, mass, and its total number of stars.

With the necessary tools now in hand, Sanna and his team even estimate that a complete picture of the Milky Way could be available in about ten years time. Imagine that! Future generations will be able to study the Milky Way with the same ease as one that is located nearby, and which they can view edge-on. At long last, all those artist’s impression of our Milky Way will be to scale!

Further Reading: MPIfR, Science

Posted on by Matt Williams

Determining the Mass of the Milky Way Using Hypervelocity Stars

An artist's conception of a hypervelocity star that has escaped the Milky Way. Credit: NASA

For centuries, astronomers have been looking beyond our Solar System to learn more about the Milky Way Galaxy. And yet, there are still many things about it that elude us, such as knowing its precise mass. Determining this is important to understanding the history of galaxy formation and the evolution of our Universe. As such, astronomers have attempted various techniques for measuring the true mass of the Milky Way.

So far, none of these methods have been particularly successful. However, a new study by a team of researchers from the Harvard-Smithsonian Center for Astrophysics proposed a new and interesting way to determine how much mass is in the Milky Way. By using hypervelocity stars (HVSs) that have been ejected from the center of the galaxy as a reference point, they claim that we can constrain the mass of our galaxy.

Their study, titled “Constraining Milky Way Mass with Hypervelocity Stars“, was recently published in the journal Astronomy and Astrophysics. The study was produced by Dr. Giacomo Fragione, an astrophysicist at the University of Rome, and Professor Abraham Loeb – the Frank B. Baird, Jr. Professor of Science, the Chair of the Astronomy Department, and the Director of the Institute for Theory and Computation at Harvard University.

Stars speeding through the Galaxy. Credit: ESA

To be clear, determining the mass of the Milky Way Galaxy is no simple task. On the one hand, observations are difficult because the Solar System lies deep within the disk of the galaxy itself. But at the same time, there’s also the mass of our galaxy’s dark matter halo, which is difficult to measure since it is not “luminous”, and therefore invisible to conventional methods of detection.

Current estimates of the galaxy’s total mass are based on the motions of tidal streamers of gas and globular clusters, which are both influenced by the gravitational mass of the galaxy. But so far, these measurements have produced mass estimates that range from one to several trillion solar-masses. As Professor Loeb explained to Universe Today via email, precisely measuring the mass of the Milky Way is of great importance to astronomers:

“The Milky Way provides a laboratory for testing the standard cosmological model. This model predicts that the number of satellite galaxies of the Milky Way depends sensitively on its mass. When comparing the predictions to the census of known satellite galaxies, it is essential to know the Milky Way mass. Moreover, the total mass calibrates the amount of invisible (dark) matter and sets the depth of the gravitational potential well and implies how fast should stars move for them to escape to intergalactic space.”

For the sake of their study, Prof. Loeb and Dr. Fragione therefore chose to take a novel approach, which involved modeling the motions of HVSs to determine the mass of our galaxy. More than 20 HVSs have been discovered within our galaxy so far, which travel at speeds of up to 700 km/s (435 mi/s) and are located at distances of about 100 to 50,000 light-years from the galactic center.

Artist’s conception of a hyperveloctiy star heading out from a spiral galaxy (similar to the Milky Way) and moving into dark matter nearby. Credit: Ben Bromley, University of Utah

These stars are thought to have been ejected from the center of our galaxy thanks to the interactions of binary stars with the supermassive black hole (SMBH) at the center of our galaxy – aka. Sagittarius A*. While their exact cause is still the subject of debate, the orbits of HVSs can be calculated since they are completely determined by the gravitational field of the galaxy.

As they explain in their study, the researchers used the asymmetry in the radial velocity distribution of stars in the galactic halo to determine the galaxy’s gravitational potential. The velocity of these halo stars is dependent on the potential escape speed of HVSs, provided that the time it takes for the HVSs to complete a single orbit is shorter than the lifetime of the halo stars.

From this, they were able to discriminate between different models for the Milky Way and the gravitational force it exerts. By adopting the nominal travel time of these observed HVSs – which they calculated to about 330 million years, about the same as the average lifetime of halo stars – they were able to derive gravitational estimates for the Milky Way which allowed for estimates on its overall mass.

“By calibrating the minimum speed of unbound stars, we find that the Milky Way mass is in the range of 1.2-1.9 trillions solar masses,” said Loeb. While still subject to a range, this latest estimate is a significant improvement over previous estimates. What’s more, these estimates are consistent our current cosmological models that attempt to account for all visible matter in the Universe, as well as dark matter and dark energy – the Lambda-CDM model.

Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. Credit: VIRGO Consortium/Alexandre Amblard/ESA

“The inferred Milky Way mass is in the range expected within the standard cosmological model,” said Leob, “where the amount of dark matter is about five times larger than that of ordinary (luminous) matter.”

Based on this breakdown, it can be said that normal matter in our galaxy – i.e. stars, planets, dust and gas – accounts for between 240 and 380 billion Solar Masses. So not only does this latest study provide more precise mass constraints for our galaxy, it could also help us to determine exactly how many star systems are out there – current estimates say that the Milky Way has between 200 to 400 billion stars and 100 billion planets.

Beyond that, this study is also significant to the study of cosmic formation and evolution. By placing more precise estimates on our galaxy’s mass, ones which are consistent with the current breakdown of normal matter and dark matter, cosmologists will be able to construct more accurate accounts of how our Universe came to be. One step clsoer to understanding the Universe on the grandest of scales!

Further Reading: Harvard Smithsonian CfA, Astronomy and Astrophysics

Posted on by Matt Williams

Chinese Astronomers Spot Two New Hypervelocity Stars

An artist's conception of a hypervelocity star that has escaped the Milky Way. Credit: NASA

Most stars in our galaxy behave predictably, orbiting around the center of the Milky Way at speeds of about 100 km/s (62 mi/s). But some stars achieve velocities that are significantly greater, to the point that they are even able to escape the gravitational pull of the galaxy. These are known as hypervelocity stars (HVS), a rare type of star that is believed to be the result of interactions with a supermassive black hole (SMBH).

The existence of HVS is something that astronomers first theorized in the late 1980s, and only 20 have been identified so far. But thanks to a new study by a team of Chinese astronomers, two new hypervelocity stars have been added to that list. These stars, which have been designated LAMOST-HVS2 and LAMOST-HVS3, travel at speeds of up to 1,000 km/s (620 mi/s) and are thought to have originated in the center of our galaxy.

The study which describes the team’s findings, titled “Discovery of Two New Hypervelocity Stars From the LAMOST Spectroscopic Surveys“, recently appeared online. Led by Yang Huang of the South-Western Institute for Astronomy Research at Yunnan University in Kunming, China, the team relied on data from Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) to detect these two new hypervelocity stars.

Footprint of the LAMOST pilot survey and the first three years’ general survey. Credit: LAMOST

Astronomers estimates that only 1000 HVS exist within the Milky Way. Given that there are as many as 200 billion stars in our galaxy, that’s just 0.0000005 % of the galactic population. While these stars are thought to originate in the center of our galaxy – supposedly as a result of interaction with our SMBH, Sagittarius A* – they manage to travel pretty far, sometimes even escaping our galaxy altogether.

It is for this very reason that astronomers are so interested in HVS. Given their speed, and the vast distances they can cover, tracking them and creating a database of their movements could provide constraints on the shape of the dark matter halo of our galaxy. Hence why Dr. Huang and his colleagues began sifting through LAMOST data to find evidence of new HVS.

Located in Hebei Province, northwestern China, the LAMOST observatory is operated by the Chinese Academy of Sciences. Over the course of five years, this observatory conducted a spectroscopic survey of 10 million stars in the Milky Way, as well as millions of galaxies. In June of 2017, LAMOST released its third Data Release (DR3), which included spectra obtained during the pilot survey and its first three years’ of regular surveys.

Containing high-quality spectra of 4.66 million stars and the stellar parameters of an additional 3.17 million, DR3 is currently the largest public spectral set and stellar parameter catalogue in the world. Already, LAMOST data had been used to identify one hypervelocity star, a B1IV/V-type (main sequence blue subgiant/subdwarf) star that was 11 Solar Masses, 13490 times as bright as our Sun, and had an effective temperature of 26,000 K (25,727 °C; 46,340 °F).

Artist’s impression of hypervelocity stars (HVSs) speeding through the Galaxy. Credit: ESA

This HVS was designated LAMOST-HSV1, in honor of the observatory. After detecting two new HVSs in the LAMOST data, these stars were designated as LAMOST-HSV2 and LAMOST-HSV3. Interestingly enough, these newly-discovered HVSs are also main sequence blue subdwarfs – or a B2V-type and B7V-type star, respectively.

Whereas HSV2 is 7.3 Solar Masses, is 2399 times as luminous as our Sun, and has an effective temperature of 20,600 K (20,327 °C; 36,620 °F), HSV3 is 3.9 Solar Masses, is 309 times as luminous as the Sun, and has an effective temperature of 14,000 K (24,740 °C; 44,564 °F). The researchers also considered the possible origins of all three HVSs based on their spatial positions and flight times.

In addition to considering that they originated in the center of the Milky Way, they also consider alternate possibilities. As they state in their study:

“The three HVSs are all spatially associated with known young stellar structures near the GC, which supports a GC origin for them. However, two of them, i.e. LAMOST-HVS1 and 2, have life times smaller than their flight times, indicating that they do not have enough time to travel from the GC to the current positions unless they are blue stragglers (as in the case of HVS HE 0437-5439). The third one (LAMOST-HVS3) has a life time larger than its flight time and thus does not have this problem.

In other words, the origins of these stars is still something of a mystery. Beyond the idea that they were sped up by interacting with the SMBH at the center of our galaxy, the team also considered other possibilities that have suggested over the years.

Artist’s impression of the ESA’s Gaia spacecraft, looking into the heart of the Milky Way  Galaxy. Credit: ESA/ATG medialab/ESO/S. Brunier

As they state in these study, these “include the tidal debris of an accreted and disrupted dwarf galaxy (Abadi et al. 2009), the surviving companion stars of Type Ia supernova (SNe Ia) explosions (Wang & Han 2009), the result of dynamical interaction between multiple stars (e.g, Gvaramadze et al. 2009), and the runaways ejected from the Large Magellanic Cloud (LMC), assuming that the latter hosts a MBH (Boubert et al. 2016).”

In the future, Huang and his colleagues indicate that their study will benefit from additional information that will be provided by the ESA’s Gaia mission, which they claim will shed additional light on how HVS behave and where they come from. As they state in their conclusions:

“The upcoming accurate proper motion measurements by Gaia should provide a direct constraint on their origins. Finally, we expect more HVSs to be discovered by the ongoing LAMOST spectroscopic surveys and thus to provide further constraint on the nature and ejection mechanisms of HVSs.”

Further Reading: arXiv

Posted on by Matt Williams

Ancient Impacts Shaped the Structure of the Milky Way

Accroding to new research, the Milky Way may still bear the marks of "ancient impacts". Credit: NASA/Serge Brunier

Understanding how the Universe came to be is one of the greater challenges of being an astrophysicist. Given the observable Universe’s sheer size (46.6 billion light years) and staggering age (13.8 billion years), this is no easy task. Nevertheless, ongoing observations, calculations and computer simulations have allowed astrophysicists to learn a great deal about how galaxies and larger structures have changed over time.

For example, a recent study by a team from the University of Kentucky (UK) has challenged previously-held notions about how our galaxy has evolved to become what we see today. Based on observations made of the Milky Way’s stellar disk, which was previously thought to be smooth, the team found evidence of asymmetric ripples. This indicates that in the past, our galaxy may have been shaped by ancient impacts.

The study, titled “Milky Way Tomography with K and M Dwarf Stars: The Vertical Structure of the Galactic Disk“, recently appeared in the The Astrophysical Journal. Led by Deborah Ferguson, a 2016 UK graduate, the team consisted of Professor Susan Gardner – from the UK College of Arts and Sciences – and Brian Yanny, an astrophysicist from the Fermilab Center for Particle Astrophysics (FCPA).

This study evolved from Ferguson’s senior thesis, which was overseen by Prof. Gardner. At the time, Ferguson sought to expand on previous research by Gardner and Yanny, which also sought to understand the presence of ripples in our galaxy’s stellar disk. For the sake of this new study, the team relied on data obtained by the Sloan Digital Sky Survey‘s (SDSS) 2.5m Telescope, located at the Apache Point Observatory in New Mexico.

This allowed the team to examine the spatial distribution of 3.6 million stars in the Milky Way Galaxy, from which they confirmed the presence of asymmetric ripples. These, they claim, can be interpreted as evidence of the Milky Way’s ancient impacts – in other words, that these ripples resulted from our galaxy coming into contact with other galaxies in the past.

These could include a merger between the Milky Way and the Sagittarius dwarf galaxy roughly 0.85 billion years ago, as well as our galaxy’s current merger with the Canis Major dwarf galaxy. As Prof. Gardner explained in a recent UK press release:

“These impacts are thought to have been the ‘architects’ of the Milky Way’s central bar and spiral arms. Just as the ripples on the surface of a smooth lake suggest the passing of a distant speed boat, we search for departures from the symmetries we would expect in the distributions of the stars to find evidence of ancient impacts. We have found extensive evidence for the breaking of all these symmetries and thus build the case for the role of ancient impacts in forming the structure of our Milky Way.”

Illustration showing a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

As noted, Gardner’s previous work also indicated that when it came to north/south symmetry of stars in the Milky Way’s disk, there was a vertical “ripple”. In other words, the number of stars that lay above or below the stellar disk would increase from one sampling to the next the farther they looked from the center of the galactic disk. But thanks to the most recent data obtained by the SDSS, the team had a much larger sample to base their conclusions on.

And ultimately, these findings confirmed the observations made by Ferguson and Lally, and also turned up evidence of an asymmetry in the plane of the galactic disk as well. As Ferguson explained:

“Having access to millions of stars from the SDSS allowed us to study galactic structure in an entirely new way by breaking the sky up into smaller regions without loss of statistics. It has been incredible watching this project evolve and the results emerge as we plotted the stellar densities and saw intriguing patterns across the footprint. As more studies are being done in this field, I am excited to see what we can learn about the structure of our galaxy and the forces that helped to shape it.”

Understanding how our galaxy evolved and what role ancient impact played is essential to understanding the history and evolution of the Universe as a whole. And in addition to helping us confirm (or update) our current cosmological models, studies like this one can also tell us much about what lies in store for our galaxy billions of years from now.

For decades, astronomers have been of the opinion that in roughly 4 billion years, the Milky Way will collide with Andromeda. This event is likely to have tremendous repercussions, leading to the merger of both galaxy’s supermassive black holes, stellar collisions, and stars being ejected. While it’s doubtful humanity will be around for this event, it would still be worthwhile to know how this process will shape our galaxy and the local Universe.

Further Reading: University of Kentucky, The Astrophysical Journal