Scientist Find Treasure Trove of Giant Black Hole Pairs

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.

For decades, astronomers have known that Supermassive Black Holes (SMBHs) reside at the center of most massive galaxies. These black holes, which range from being hundreds of thousands to billions of Solar masses, exert a powerful influence on surrounding matter and are believed to be the cause of Active Galactic Nuclei (AGN). For as long as astronomers have known about them, they have sought to understand how SMBHs form and evolve.

In two recently published studies, two international teams of researchers report on the discovery of five newly-discovered black hole pairs at the centers of distant galaxies. This discovery could help astronomers shed new light on how SMBHs form and grow over time, not to mention how black hole mergers produce the strongest gravitational waves in the Universe.

The first four dual black hole candidates were reported in a study titled “Buried AGNs in Advanced Mergers: Mid-Infrared Color Selection as a Dual AGN Finder“, which was led by Shobita Satyapal, a professor of astrophysics at George Mason University. This study was accepted for publication in The Astrophysical Journal and recently appeared online.

Optical and x-ray data on two of the new black hole pairs discovered. Credit: NASA/CXC/Univ. of Victoria/S.Ellison et al./George Mason Univ./S.Satyapal et al./SDSS

The second study, which reported the fifth dual black hole candidate, was led by Sarah Ellison – an astrophysics professor at the University of Victoria. It was recently published in the Monthly Notices of the Royal Astronomical Society under the title “Discovery of a Dual Active Galactic Nucleus with ~8 kpc Separation. The discovery of these five black hole pairs was very fortuitous, given that pairs are a very rare find.

As Shobita Satyapal explained in a Chandra press statement:

“Astronomers find single supermassive black holes all over the universe. But even though we’ve predicted they grow rapidly when they are interacting, growing dual supermassive black holes have been difficult to find.

The black hole pairs were discovered by combining data from a number of different ground-based and space-based instruments. This included optical data from the Sloan Digital Sky Survey (SDSS) and the ground-based Large Binocular Telescope (LBT) in Arizona with near-infrared data from the Wide-Field Infrared Survey Explorer (WISE) and x-ray data from NASA’s Chandra X-ray Observatory.

For the sake of their studies, Satyapal, Ellison, and their respective teams sought to detect dual AGNs, which are believed to be a consequence of galactic mergers. They began by consulting optical data from the SDSS to identify galaxies that appeared to be in the process of merging. Data from the all-sky WISE survey was then used to identify those galaxies that displayed the most powerful AGNs.

Illustration of a pair of black holes. Credit: NASA/CXC/A.Hobart

They then consulted data from the Chandra’s Advanced CCD Imaging Spectrometer (ACIS) and the LBT to identify seven galaxies that appeared to be in an advanced stage of merger. The study led by Ellison also relied on optical data provided by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to pinpoint one of the new black hole pairs.

From the combined data, they found that five out of the seven merging galaxies hosted possible dual AGNs, which were separated by less than 10 kiloparsecs (over 30,000 light years). This was evidenced by the infrared data provided by WISE, which was consistent with what is predicated of rapidly growing supermassive black holes.

In addition, the Chandra data showed closely-separated pairs of x-ray sources, which is also consistent with black holes that have matter slowly being accreted onto them. This infrared and x-ray data also suggested that the supermassive black holes are buried in large amounts of dust and gas. As Ellison indicated, these findings were the result of painstaking work that consisted of sorting through multiple wavelengths of data:

“Our work shows that combining the infrared selection with X-ray follow-up is a very effective way to find these black hole pairs. X-rays and infrared radiation are able to penetrate the obscuring clouds of gas and dust surrounding these black hole pairs, and Chandra’s sharp vision is needed to separate them”.

Artist’s impression of binary black hole system in the process of merging. Credit: Bohn et al.

Before this study, less than ten pairs of growing black holes had been confirmed based on X-ray studies, and these were mostly by chance. This latest work, which detected five black hole pairs using combined data, was therefore both fortunate and significant. Aside from bolstering the hypothesis that supermassive black holes form from the merger of smaller black holes, these studies also have serious implications for gravitational wave research.

“It is important to understand how common supermassive black hole pairs are, to help in predicting the signals for gravitational wave observatories,” said Satyapa. “With experiments already in place and future ones coming online, this is an exciting time to be researching merging black holes. We are in the early stages of a new era in exploring the universe.”

Since 2016, a total of four instances of gravitational waves have been detected by instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the VIRGO Observatory. However, these detections were the result of black hole mergers where the black holes were all smaller and less massive  – between eight and 36 Solar masses.

Supermassive Black Holes, on the other hand, are much more massive and will likely produce a much larger gravitational wave signature as they continue to draw closer together. And in a few hundred million years, when these pairs eventually do merge, the resulting energy produced by mass being converted into gravitational waves will be incredible.

Artist’s conception of two merging black holes, similar to those detected by LIGO on January 4th, 2017. Credit: LIGO/Caltech

At present, detectors like LIGO and Virgo are not able to detect the gravitational waves created by Supermassive Black Hole pairs. This work is being done by arrays like the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which relies on high-precision millisecond pulsars to measure the influence of gravitational waves on space-time.

The proposed Laser Interferometer Space Antenna (LISA), which will be the first dedicated space-based gravitational wave detector, is also expected to help in the search. In the meantime, gravitational wave research has already benefited immensely from collaborative efforts like the one that exists between Advanced LIGO and Advanced Virgo.

In the future, scientists also anticipate that they will be able to study the interiors of supernovae through gravitational wave research. This is likely to reveal a great deal about the mechanisms behind black hole formation. Between all of these ongoing efforts and future developments, we can expect to “hear” a great deal more of the Universe and the most powerful forces at work within it.

Be sure to check out this animation that shows what the eventual merger of two of these black hole pairs will look like, courtesy of the Chandra X-ray Observatory:

Further Reading: Chandra HarvardarXiv, MNRAS

Supermassive Black Holes or Their Galaxies? Which Came First?

Which Came First, Supermassive Black Holes of their Galaxies?

There’s a supermassive black hole at the center of almost every galaxy in the Universe. How did they get there? What’s the relationship between these monster black holes and the galaxies that surround them?

Every time astronomers look farther out in the Universe, they discover new mysteries. These mysteries require all new tools and techniques to understand. These mysteries lead to more mysteries. What I’m saying is that it’s mystery turtles all the way down.

One of the most fascinating is the discovery of quasars, understanding what they are, and the unveiling of an even deeper mystery, where do they come from?

As always, I’m getting ahead of myself, so first, let’s go back and talk about the discovery of quasars.

Molecular clouds scattered by an intermediate black hole show very wide velocity dispersion in this artist’s impression. This scenario well explains the observational features of a peculiar molecular cloud CO-0.40-0.22. Credit: Keio University

Back in the 1950s, astronomers scanned the skies using radio telescopes, and found a class of bizarre objects in the distant Universe. They were very bright, and incredibly far away; hundreds of millions or even billion of light-years away. The first ones were discovered in the radio spectrum, but over time, astronomers found even more blazing in the visible spectrum.

The astronomer Hong-Yee Chiu coined the term “quasar”, which stood for quasi-stellar object. They were like stars, shining from a single point source, but they clearly weren’t stars, blazing with more radiation than an entire galaxy.

Over the decades, astronomers puzzled out the nature of quasars, learning that they were actually black holes, actively feeding and blasting out radiation, visible billions of light-years away.

But they weren’t the stellar mass black holes, which were known to be from the death of giant stars. These were supermassive black holes, with millions or even billions of times the mass of the Sun.

As far back as the 1970s, astronomers considered the possibility that there might be these supermassive black holes at the heart of many other galaxies, even the Milky Way.

The Whirlpool Galaxy (Spiral Galaxy M51, NGC 5194), a classic spiral galaxy located in the Canes Venatici constellation, and its companion NGC 5195. Credit: NASA/ESA

In 1974, astronomers discovered a radio source at the center of the Milky Way emitting radiation. It was titled Sagittarius A*, with an asterisk that stands for “exciting”, well, in the “excited atoms” perspective.

This would match the emissions of a supermassive black hole that wasn’t actively feeding on material. Our own galaxy could have been a quasar in the past, or in the future, but right now, the black hole was mostly silent, apart from this subtle radiation.

Astronomers needed to be certain, so they performed a detailed survey of the very center of the Milky Way in the infrared spectrum, which allowed them to see through the gas and dust that obscures the core in visible light.

They discovered a group of stars orbiting Sagittarius A-star, like comets orbiting the Sun. Only a black hole with millions of times the mass of the Sun could provide the kind of gravitational anchor to whip these stars around in such bizarre orbits.

Further surveys found a supermassive black hole at the heart of the Andromeda Galaxy, in fact, it appears as if these monsters are at the center of almost every galaxy in the Universe.

But how did they form? Where did they come from? Did the galaxy form first, and cause the black hole to form at the middle, or did the black hole form, and build up a galaxy around them?

Until recently, this was actually still one of the big unsolved mysteries in astronomy. That said, astronomers have done plenty of research, using more and more sensitive observatories, worked out their theories, and now they’re gathering evidence to help get to the bottom of this mystery.

Astronomers have developed two models for how the large scale structure of the Universe came together: top down and bottom up.

In the top down model, an entire galactic supercluster formed all at once out of a huge cloud of primordial hydrogen left over from the Big Bang. A supercluster’s worth of stars.

As the cloud came together it, it spun up, kicking out smaller spirals and dwarf galaxies. These could have combined later on to form the more complex structure we see today. The supermassive black holes would have formed as the dense cores of these galaxies as they came together.

Hubble image of Messier 54, a globular cluster located in the Sagittarius Dwarf Galaxy. Credit: ESA/Hubble & NASA

If you want to wrap your mind around this, think of the stellar nursery that formed our Sun and a bunch of other stars. Imagine a single cloud of gas and dust forming multiple stars systems within it. Over time, the stars matured and drifted away from each other.

That’s top down. One big event that leads to the structure we see today.

In the bottom up model, pockets of gas and dust collected together into larger and larger masses, eventually forming dwarf galaxies, and even the clusters and superclusters we see today. The supermassive black holes at the heart of galaxies were grown from collisions and mergers between black holes over eons.

In fact, this is actually how astronomers think the planets in the Solar System formed. By pieces of dust attracting one another into larger and larger grains until the planet-sized objects formed over millions of years.

Bottom up, small parts coming together.

Shortly after the Big Bang, the entire Universe was incredibly dense. But it wasn’t the same density everywhere. Tiny quantum fluctuations in density at the beginning evolved over billions of years of expansion into the galactic superclusters we see today.

Colliding galaxies can force the supermassive black holes in their cores together (NCSA)

I want to stop and let this sink into your brain for a second. There were microscopic variations in density in the early Universe. And these variations became the structures hundreds of millions of light-years across we see today.

Imagine the two forces at play as the expansion of the Universe happened. On the one hand, you’ve got the mutual gravity of the particles pulling one another together. And on the other hand, you’ve got the expansion of the Universe separating the particles from one another. The size of the galaxies, clusters and superclusters were decided by the balance point of those opposing forces.

If small pieces came together, then you’d get that bottom up formation. If large pieces came together, you’d get that top down formation.

When astronomers look out into the Universe at the largest scales, they observe clusters and superclusters as far as they can see – which supports the top down model.

On the other hand, observations show that the first stars formed just a few hundred million years after the Big Bang, which supports bottom up.

So the answer is both?

No, the most modern observations give the edge to the bottom up processes.

The key is that gravity moves at the speed of light, which means that the gravitational interactions between particles spreading away from each other needed to catch up, going the speed of light.

In other words, you wouldn’t get a supercluster’s worth of material coming together, only a star’s worth of material. But these first stars were made of pure hydrogen and helium, and could grow much more massive than the stars we have today. They would live fast and die in supernova explosions, creating much more massive black holes than we get today.

This illustration shows the final stages in the life of a supermassive star that fails to explode as a supernova, but instead implodes to form a black hole. Credit: NASA/ESA/P. Jeffries (STScI)

The first protogalaxies came together, collecting together these first monster black holes and the massive stars surrounding them. And then, over millions and billions of years, these black holes merged again and again, accumulating millions and even billions of times the mass of the Sun. This was how we got the modern galaxies we see today.

There was a recent observation that supports this conclusion. Earlier this year, astronomers announced the discovery of supermassive black holes at the center of relatively tiny galaxies. In our own Milky Way, the supermassive black hole is 4.1 million times the mass of the Sun, but accounts for only .01% of the galaxy’s total mass.

But astronomers from the University of Utah found two ultra compact galaxies with black holes of 4.4 million and 5.8 million times the mass of the Sun respectively. And yet, the black holes account for 13 and 18 percent of the mass of their host galaxies.

The thinking is that these galaxies were once normal, but collided with other galaxies earlier on in the history of the Universe, were stripped of their stars and then were spat out to roam the cosmos.

They’re the victims of those early merging events, evidence of the carnage that happened in the early Universe when the mergers were happening.

We always talk about the unsolved mysteries in the Universe, but this is one that astronomers are starting to puzzle out.

It seems most likely that the structure of the Universe we see today formed bottom up. The first stars came together into protogalaxies, dying as supernova to form the first black holes. The structure of the Universe we see today is the end result of billions of years of formation and destruction. With the supermassive black holes coming together over time.

Once telescopes like James Webb get to work, we should be able to see these pieces coming together, at the very edge of the observable Universe.

Another Monster Black Hole Found in the Milky Way

At the center of the Milky Way Galaxy resides the Supermassive Black Hole (SMBH) known as Sagittarius A*. This tremendous black hole measures an estimated 44 million km in diameter, and has the mass of over 4 million Suns. For decades, astronomers have understood that most larger galaxies have an SMBH at their core, and that these range from hundreds of thousands to billions of Solar Masses.

However, new research performed by a team of researchers from Keio University, Japan, has made a startling find. According to their study, the team found evidence of a mid-sized black hole in a gas cluster near the center of the Milky Way Galaxy. This unexpected find could offer clues as to how SMBHs form, which is something that astronomers have been puzzling over for some time.

The study, titled “Millimetre-wave Emission from an Intermediate-mass Black Hole Candidate in the Milky Way“, recently appeared in the journal Nature Astronomy. Led by Tomoharu Oka, a researcher from the Department of Physics and the School of Fundamental Science and Technology at Keio University, the team studied CO–0.40–0.22, a high-velocity compact gas cloud near the center of our galaxy.

This artist’s concept shows a galaxy with a supermassive black hole at its core. The black hole is shooting out jets of radio waves.Image credit: NASA/JPL-Caltech

This compact dust cloud, which has been a source of fascination to astronomers for years, measures over 1000 AU in diameter and is located about 200 light-years from the center of our galaxy. The reason for this interest has to do with the fact that gases in this cloud – which include hydrogen cyanide and carbon monoxide – move at vastly different speeds, which is something unusual for a cloud of interstellar gases.

In the hopes of better understanding this strange behavior, the team originally observed CO–0.40–0.22 using the 45-meter radio telescope at the Nobeyama Radio Observatory in Japan. This began in January of 2016, when the team noticed that the cloud had an elliptical shape that consisted of two components. These included a compact but low density component with varying velocities, and a dense component (10 light years long) with little variation.

After conducting their initial observations, the team then followed up with observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. These confirmed the structure of the cloud and the variations in speed that seemed to accord with density. In addition, they observed the presence of radio waves (similar to those generated by Sagittarius A*) next to the dense region. As they state in their study:

“Recently, we discovered a peculiar molecular cloud, CO–0.40–0.22, with an extremely broad velocity width, near the center of our Milky Way galaxy. Based on the careful analysis of gas kinematics, we concluded that a compact object with a mass of about 105 [Solar Masses] is lurking in this cloud.”

Change image showing the area around Sgr A*, where low, medium, and high-energy X-rays are red, green, and blue, respectively. The inset box shows X-ray flares from the region close to Sgr A*. NASA: NASA/SAO/CXC

The team also ran a series of computer models to account for these strange behaviors, which indicated that the most likely cause was a black hole. Given its mass – 100,000 Solar Masses, or roughly 500 times smaller than that of Sagittarius A* – this meant that the black hole was intermediate in size. If confirmed, this discovery will constitute the second-largest black hole to be discovered within the Milky Way.

This represents something of a first for astronomers, since the vast majority of black holes discovered to date have been either small or massive. Studies that have sought to locate Intermediate Black Holes (IMBHs), on the other hand, have found very little evidence of them. Moreover, these findings could account for how SMBHs form at the center of larger galaxies.

In the past, astronomers have conjectured that SMBHs are formed by the merger of smaller black holes, which implied the existence of intermediate ones. As such, the discovery of an IMBH would constitute the first piece of evidence for this hypothesis. As Brooke Simmons, a professor at the University of California in San Diego, explained in an interview with The Guardian:

“We know that smaller black holes form when some stars die, which makes them fairly common. We think some of those black holes are the seeds from which the much larger supermassive black holes grow to at least a million times more massive. That growth should happen in part by mergers with other black holes and in part by accretion of material from the part of the galaxy that surrounds the black hole.

“Astrophysicists have been collecting observational evidence for both stellar mass black holes and supermassive black holes for decades, but even though we think the largest ones grow from the smallest ones, we’ve never really had clear evidence for a black hole with a mass in between those extremes.”

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

Further studies will be needed to confirm the presence of an IMBH at the center of CO–0.40–0.22. Assuming they succeed, we can expect that astrophyiscists will be monitoring it for some time to determine how it formed, and what it’s ultimate fate will be. For instance, it is possible that it is slowly drifting towards Sagittarius A* and will eventually merge with it, thus creating an even more massive SMBH at the center of our galaxy!

Assuming human beings are around to detect that merger, its fair to say that it won’t go unnoticed. The gravitational waves alone are sure to be impressive!

Further Reading: Nature Astronomy

Researchers Tackle Question of How the Universe Became Filled With Light

In accordance with the Big Bang model of cosmology, shortly after the Universe came into being there was a period known as the “Dark Ages”. This occurred between 380,000 and 150 million years after the Big Bang, where most of the photons in the Universe were interacting with electrons and protons. As a result, the radiation of this period is undetectable by our current instruments – hence the name.

Astrophysicists and cosmologists have therefore been pondering how the Universe could go from being in this dark, cloudy state to one where it was filled with light. According to a new study by a team of researchers from the University of Iowa and the Harvard-Smithsonian Center for Astrophysics, it may be that black holes violently ejected matter from the early Universe, thus allowing light to escape.

Their study, titled “Resolving the X-ray emission from the Lyman continuum emitting galaxy Tol 1247-232“, recently appeared in the Monthly Notices of the Royal Astronomical Society. Led by Phillip Kaaret, a professor of Physics and Astronomy at the University of Iowa – and supported by an award from the Chandra X-ray Observatory – the research team arrived at this conclusion by observing a nearby galaxy from which ultraviolet light is escaping.

Milestones in the history of the Universe, from the Big Bang to the present day. Credit: NAOJ/NOAO

This galaxy, known as Tol 1247-232, is a small (and possibly elliptical) galaxy located 652 million light-years away, in the direction of the southern Hydra constellation. This galaxy is one of just nine in the local Universe (and one of only three galaxies close to the Milky Way) that has been shown to emit Lyman continuum photons – a type of radiation in the ultraviolet band.

Back in May of 2016, the team spotted a single X-ray source coming from a star-forming region in this galaxy, using the Chandra X-ray observatory. Based on their observations, they determined that it was not caused by the formation of a new star. For one, new stars do not experience sudden changes in brightness, as this x-ray source did. In addition, the radiation emitted by new stars does not come in the form of a point-like source.

Instead, they determined that what they were seeing had to be the result of a very small object, which left only one likely explanation: a black hole. As Philip Kaaret, a professor in the UI Department of Physics and Astronomy and the lead author on the study, explained:

“The observations show the presence of very bright X-ray sources that are likely accreting black holes. It’s possible the black hole is creating winds that help the ionizing radiation from the stars escape. Thus, black holes may have helped make the universe transparent.”

Where is the Nearest Black Hole
Artist concept of matter swirling around a black hole. Credit: NASA/Dana Berry/SkyWorks Digital

However, this also raised the question of how a black hole could be emitting matter. This is something that astrophysicists have puzzled over for quite some time. Whereas all black holes have tendency to consume all that is in their path, a small number of supermassive black holes (SMBHs) have been found to have high-speed jets of charged particles streaming from their cores.

These SMBHs are what power Active Galactic Nuclei, which are compact, bright regions that has been observed at the centers of particularly massive galaxies. At present, no one is certain how these SMBHs manage to fire off jets of hot matter. But it has been theorized that they could be caused by the accelerated rotational energy of the black holes themselves.

In keeping with this, the team considered the possibility that accreting X-ray sources could explain the escape of matter from a black hole. In other words, as a black hole’s intense gravity pulls matter inward, the black hole responds by spinning faster. As the hole’s gravitational pull increases, the speed creates energy, which inevitably causes charged particles to be pushed out. As Kaaret explained:

“As matter falls into a black hole, it starts to spin and the rapid rotation pushes some fraction of the matter out. They’re producing these strong winds that could be opening an escape route for ultraviolet light. That could be what happened with the early galaxies.”

Depiction of the tidal disruption event in F01004-2237. The release of gravitational energy as the debris of the star is accreted by the black hole leads to a flare in the optical light of the galaxy. Credit and copyright: Mark Garlick

Taking this a step further, the team hypothesized that this could be what was responsible for light escaping the “Dark Ages”. Much like the jets of hot material being emitted by SMBHs today, similarly massive black holes in the early Universe could have sped up due to the accretion of matter, spewing out light from the cloudiness and allowing for the Universe to become a clear, bright place.

In the future, the UI team plans to study Tol 1247-232 in more detail and locate other nearby galaxies that are also emitting ultraviolet light. This will corroborate their theory that black holes could be responsible for the observed point source of high-energy X-rays. Combined with studies of the earliest periods of the Universe, it could also validate the theory that the “Dark Ages” ended thanks to the presence of black holes.

Further Reading: Iowa Now, Monthly Notices of the Royal Astronomical Society

Cosmic Census Says There Could be 100 Million Black Holes in our Galaxy Alone

In January of 2016, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves. Supported by the National Science Foundation (NSF) and operated by Caltech and MIT, LIGO is dedicated to studying the waves predicted by Einstein’s Theory of General Relativity and caused by black hole mergers.

According to a new study by a team of astronomers from the Center of Cosmology at the University of California Irvine, such mergers are far more common than we thought. After conducting a survey of the cosmos intended to calculate and categorize black holes, the UCI team determined that there could be as many as 100 million black holes in the galaxy, a finding which has significant implications for the study of gravitational waves.

The study which details their findings, titled “Counting Black Holes: The Cosmic Stellar Remnant Population and Implications for LIGO“, recently appeared in the Monthly Notices of the Royal Astronomical Society. Led by Oliver D. Elbert, a postdoc student with the department of Physics and Astronomy at UC Irvine, the team conducted an analysis of gravitational wave signals that have been detected by LIGO.

LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu

Their study began roughly a year and a half ago, shortly after LIGO announced the first detection of gravitational waves. These waves were created by the merger of two distant black holes, each of which was equivalent in mass to about 30 Suns. As James Bullock, a professor of physics and astronomy at UC Irvine and a co-author on the paper, explained in a UCI press release:

“Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein’s general theory of relativity. But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, ‘How common are black holes of this size, and how often do they merge?’”

Traditionally, astronomers have been of the opinion that black holes would typically be about the same mass as our Sun. As such, they sought to interpret the multiple gravitational wave detections made by LIGO in terms of what is known about galaxy formation. Beyond this, they also sought to create a framework for predicting future black hole mergers.

From this, they concluded that the Milky Way Galaxy would be home to up to 100 million black holes, 10 millions of which would have an estimated mass of about 30 Solar masses – i.e. similar to those that merged and created the first gravitational waves detected by LIGO in 2016. Meanwhile, dwarf galaxies – like the Draco Dwarf, which orbits at a distance of about 250,000 ly from the center of our galaxy – would host about 100 black holes.

They further determined that today, most low-mass black holes (~10 Solar masses) reside within galaxies of 1 trillion Solar masses (massive galaxies) while massive black holes (~50 Solar masses) reside within galaxies that have about 10 billion Solar masses (i.e. dwarf galaxies). After considering the relationship between galaxy mass and stellar metallicity, they interpreted a galaxy’s black hole count as a function of its stellar mass.

In addition, they also sought to determine how often black holes occur in pairs, how often they merge and how long this would take. Their analysis indicated that only a tiny fraction of black holes would need to be involved in mergers to accommodate what LIGO observed. It also offered predictions that showed how even larger black holes could be merging within the next decade.

As Manoj Kaplinghat, also a UCI professor of physics and astronomy and the second co-author on the study, explained:

“We show that only 0.1 to 1 percent of the black holes formed have to merge to explain what LIGO saw. Of course, the black holes have to get close enough to merge in a reasonable time, which is an open problem… If the current ideas about stellar evolution are right, then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years.”

In other words, our galaxy could be teeming with black holes, and mergers could be happening in a regular basis (relative to cosmological timescales). As such, we can expect that many more gravity wave detections will be possible in the coming years. This should come as no surprise, seeing as how LIGO has made two additional detections since the winter of 2016.

With many more expected to come, astronomers will have many opportunities to study black holes mergers, not to mention the physics that drive them!

Further Reading: UCI, MNRAS

Stars Orbiting Supermassive Black Hole Show Einstein was Right Again!

At the center of our galaxy, roughly 26,000 light years from Earth, lies the Supermassive Black Hole (SMBH) known as Sagittarius A*. Measuring 44 million km across, this object is roughly 4 million times as massive as our Sun and exerts a tremendous gravitational pull. Since astronomers cannot detect black holes directly, its existence has been determined largely from the effect it has on the small group of stars orbiting it.

In this respect, scientists have found that observing Sagittarius A* is an effective way of testing the physics of gravity. For instance, in the course of observing these stars, a team of German and Czech astronomers noted subtle effects caused by the black hole’s gravity. In so doing, they were able to yet again confirm some of the predictions made by Einstein’s famous Theory of General Relativity.

Their study, titled “Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center“, was recently published in the Astrophysical Journal. As is indicated in the course of it, the team applied new analysis techniques to existing observations that were made by European Southern Observatory’s (ESO) Very Large Telescope (VLT) and other telescopes over the course of the past 20 years.

Artist’s impression of part of S2s orbit around the supermassive black hole at the center of the Milky Way. Credit: ESO/M. Parsa/L. Calçada

From this, they measured the orbits of the stars that orbit Sagittarius A* to test predictions made by classical Newtonian physics (i.e. Universal Gravitation), as well as predictions based on general relativity. What they found was that one of the stars (S2) showed deviations in its orbit which were defied the former, but were consistent with the latter.

This star, which has 15 times the mass of our Sun, follows an elliptical orbit around the SMBH, completing a single orbit in about 15.6 years. At its closest, it gets to within 17 light hours of the black hole, which is the equivalent of 120 times the distance between the Sun and the Earth (120 AU). Essentially, the research team noted that S2 had the most elliptical orbit of any star orbiting the Supermassive Black Hole.

They also noted a slight change in its orbit – a few percent in the shape and about one-sixth of a degree in orientation. This could only be explained as being due to the relativistic effects caused by Sagittarius A* intense gravity, which cause a precession in its orbit.  What this means is, the elliptical loop of S2’s orbit rotates around the SMBH over time, with its perihelion point aimed in different directions.

Interestingly enough, this is similar to the effect that was observed in Mercury’s orbit – aka. the “perihelion precession of Mercury” – during the late 19th century. This observation challenged classical Newtonian mechanics and led scientists to conclude that Newton’s theory of gravity was incomplete. It is also what prompted Einstein to develop his theory of General Relativity, which offered a satisfactory explanation for the issue.

Should the results of their study be confirmed, this will be the first time that the effects of general relativity have been precisely calculated using the stars that orbit a Supermassive Black Hole. Marzieh Parsa – a PhD student at the University of Cologne, Germany and lead author of the paper – was understandably excited with these results. As she stated in an ESO press statement:

The Galactic Center really is the best laboratory to study the motion of stars in a relativistic environment. I was amazed how well we could apply the methods we developed with simulated stars to the high-precision data for the innermost high-velocity stars close to the supermassive black hole.

This study was made possible thanks to the high-accuracy of the VLT’s instruments; in particular, the adaptive optics on the NACO camera and the SINFONI near-infrared spectrometer. These instruments were vital in tracking the star’s close approach and retreat from the black hole, which allowed for the team to precisely determine the shape of its orbit and thusly determine the relativistic effects on the star.

In addition to the more precise information about S2’s orbit, the team’s analysis also provided new and more accurate estimates of Sagittarius A* mass, as well as its distance from Earth. This could open up new avenues of research for this and other Supermassive Black Holes, as well as additional experiments that could help scientists to learn more about the physics of gravity.

The central parts of our Galaxy, the Milky Way, as observed in the near-infrared with the NACO instrument on ESO’s Very Large Telescope. Credit: ESO/MPE/S. Gillessen et al.

The results also provided a preview of the measurements and tests that will be taking place next year. In 2018, the star S2 will be making a very close approach to Sagittarius A*. Scientists from around the world will be using this opportunity to test the GRAVITY instrument, a second-generation instrument that was recently installed on the Very Large Telescope Interferometer (VLTI).

Developed by an international consortium led by the Max Planck Institute for Extraterrestrial Physics, this instrument has been conducting observations of the Galactic Center since 2016. In 2018, it will be used to measure the orbit of S2 with even greater precision, which is expected to be most revealing.  At this time, astrophysicists will be seeking to make additional measurements of the SMBH’s general relativistic effects.

Beyond that, they also hope to detect additional deviations in the star’s orbit that could hint at the existence of new physics! With the right tools trained on the right place, and at the right time, scientists just might find that even Einstein’s theories of gravity were not entirely complete. But in the meantime, it looks like the late and great theoretical physicist was right again!

And be sure to check out this video of the recent study, courtesy of the ESO:

Further Reading: ESO, Astrophysical Journal

Third Gravitational Wave Event Detected

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.

A third gravitational wave has been detected by the Laser Interferometer Gravitational-wave Observatory (LIGO). An international team announced the detection today, while the event itself was detected on January 4th, 2017. Gravitational waves are ripples in space-time predicted by Albert Einstein over a century ago.

LIGO consists of two facilities: one in Hanford, Washington and one in Livingston, Louisiana. When LIGO announced its first gravitational wave back in February 2016 (detected in September 2015), it opened up a new window into astronomy. With this gravitational wave, the third one detected, that new window is getting larger. So far, all three waves detected have been created by the merging of black holes.

The team, including engineers and scientists from Northwestern University in Illinois, published their results in the journal Physical Review Letters.

When the first gravitational wave was finally detected, over a hundred years after Einstein predicted it, it helped confirm Einstein’s description of space-time as an integrated continuum. It’s often said that it’s not a good idea to bet against Einstein, and this third detection just strengthens Einstein’s theory.

Like the previous two detections, this one was created by the merging of two black holes. These two were different sizes from each other; one was about 31.2 solar masses, and the other was about 19.4 solar masses. The combined 50 solar mass event caused the third wave, which is named GW170104. The black holes were about 3 billion light years away.

“…an intriguing black hole population…” – Vicky Kalogera, Senior Astrophysicist, LIGO Scientific Collaboration

LIGO is showing us that their is a population of binary black holes out there. “Our handful of detections so far is revealing an intriguing black hole population we did not know existed until now,” said Northwestern’s Vicky Kalogera, a senior astrophysicist with the LIGO Scientific Collaboration (LSC), which conducts research related to the twin LIGO detectors, located in the U.S.

“Now we have three pairs of black holes, each pair ending their death spiral dance over millions or billions of years in some of the most powerful explosions in the universe. In astronomy, we say with three objects of the same type you have a class. We have a population, and we can do analysis.”

The Laser Interferometer Gravitational-Wave Observatory (LIGO)facility in Livingston, Louisiana. The other facility is located in Hanford, Washington. Image: LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO)facility in Livingston, Louisiana. The other facility is located in Hanford, Washington. Image: LIGO

When we say that gravitational waves have opened up a new window on astronomy, that window opens onto black holes themselves. Beyond confirming Einstein’s predictions, and establishing a population of binary black holes, LIGO can characterize and measure those black holes. We can learn the holes’ masses and their spin characteristics.

“Once again, the black holes are heavy,“ said Shane Larson, of Northwestern University and Adler Planetarium in Chicago. “The first black holes LIGO detected were twice as heavy as we ever would have expected. Now we’ve all been churning our cranks trying to figure out all the interesting myriad ways we can imagine the universe making big and heavy black holes. And Northwestern is strong in this research area, so we are excited.”

This third finding strengthens the case for the existence of a new class of black holes: binary black holes that are locked in relationship with each other. It also shows that these objects can be larger than thought before LIGO detected them.

“It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us.” – David Shoemaker, MIT

“We have further confirmation of the existence of black holes that are heavier than 20 solar masses, objects we didn’t know existed before LIGO detected them,” said David Shoemaker of MIT, spokesperson for the LIGO Scientific Collaboration . “It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us.”

An artist's impression of two merging black holes. Image: NASA/CXC/A. Hobart
An artist’s impression of two merging black holes. Image: NASA/CXC/A. Hobart

“With the third confirmed detection of gravitational waves from the collision of two black holes, LIGO is establishing itself as a powerful observatory for revealing the dark side of the universe,” said David Reitze of Caltech, executive director of the LIGO Laboratory and a Northwestern alumnus. “While LIGO is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars.”

A tell-tale chirping sound confirms the detection of a gravitational wave, and you can hear it described and explained here, on a Northwestern University podcast.

Sources:

What Exactly Should We See When a Star Splashes into a Black Hole Event Horizon?

This artist's impression shows a star crossing the event horizon of a supermassive black hole located in the center of a galaxy. The black hole is so large and massive that tidal effects on the star are negligible, and the star is swallowed whole. Image: Mark A. Garlick/CfA

At the center of our Milky Way galaxy dwells a behemoth. An object so massive that nothing can escape its gravitational pull, not even light. In fact, we think most galaxies have one of them. They are, of course, supermassive black holes.

Supermassive black holes are stars that have collapsed into a singularity. Einstein’s General Theory of Relativity predicted their existence. And these black holes are surrounded by what’s known as an event horizon, which is kind of like the point of no return for anything getting too close to the black hole. But nobody has actually proven the existence of the event horizon yet.

Some theorists think that something else might lie at the center of galaxies, a supermassive object event stranger than a supermassive black hole. Theorists think these objects have somehow avoided a black hole’s fate, and have not collapsed into a singularity. They would have no event horizon, and would have a solid surface instead.

“Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not,” – Pawan Kumar Professor of Astrophysics, University of Texas at Austin.

A team of researchers at the University of Texas at Austin and Harvard University have tackled the problem. Wenbin Lu, Pawan Kumar, and Ramesh Narayan wanted to shed some light onto the event horizon problem. They wondered about the solid surface object, and what would happen when an object like a star collided with it. They published their results in the Monthly Notices of the Royal Astronomical Society.

Artist's conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library
Artist’s conception of the event horizon of a black hole. Credit: Victor de Schwanberg/Science Photo Library

“Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not,” said Pawan Kumar, Professor of Astrophysics at The University of Texas at Austin, in a press release.

Since a black hole is a star collapsed into a singularity, it has no surface area, and instead has an event horizon. But if the other theory turns out to be true, and the object has a solid surface instead of an event horizon, then any object colliding with it would be destroyed. If a star was to collide with this hard surface and be destroyed, the team surmised, then the gas from the star would enshroud the object and shine brightly for months, or even years.

This is the first in a sequence of two artist's impressions that shows a huge, massive sphere in the center of a galaxy, rather than a supermassive black hole. Here a star moves towards and then smashes into the hard surface of the sphere, flinging out debris. The impact heats up the site of the collision. Image: Mark A. Garlick/CfA
This is the first in a sequence of two artist’s impressions that shows a huge, massive sphere in the center of a galaxy, rather than a supermassive black hole. Here a star moves towards and then smashes into the hard surface of the sphere, flinging out debris. The impact heats up the site of the collision. Image:
Mark A. Garlick/CfA
In this second artist's impression a huge sphere in the center of a galaxy is shown after a star has collided with it. Enormous amounts of heat and a dramatic increase in the brightness of the sphere are generated by this event. The lack of observation of such flares from the center of galaxies means that this hypothetical scenario is almost completely ruled out. Image: Mark A. Garlick/CfA
In this second artist’s impression a huge sphere in the center of a galaxy is shown after a star has collided with it. Enormous amounts of heat and a dramatic increase in the brightness of the sphere are generated by this event. The lack of observation of such flares from the center of galaxies means that this hypothetical scenario is almost completely ruled out. Image: Mark A. Garlick/CfA

If that were the case, then the team knew what to look for. They also worked out how often this would happen.

“We estimated the rate of stars falling onto supermassive black holes,” Lu said in the same press release. “Nearly every galaxy has one. We only considered the most massive ones, which weigh about 100 million solar masses or more. There are about a million of them within a few billion light-years of Earth.”

Now they needed a way to search the sky for these objects, and they found it in the archives of the Pan-STARRS telescope. Pan-STARRS is a 1.8 meter telescope in Hawaii. That telescope recently completed a survey of half of the northern hemisphere of the sky. In that survey, Pan-STAARS spent 3.5 years looking for transient objects in the sky, objects that brighten and then fade. They searched the Pan-STARR archives for transient objects that had the signature they predicted from stars colliding with these supermassive, hard-surfaced objects.

The trio predicted that in the 3.5 year time-frame captured by the Pan-STAARS survey, 10 of these collisions would occur and should be represented in the data.

“It turns out it should have detected more than 10 of them, if the hard-surface theory is true.” – Wenbin Lu, Dept. of Astronomy, University of Texas at Austin.

“Given the rate of stars falling onto black holes and the number density of black holes in the nearby universe, we calculated how many such transients Pan-STARRS should have detected over a period of operation of 3.5 years. It turns out it should have detected more than 10 of them, if the hard-surface theory is true,” Lu said.

The team found none of the flare-ups they expected to see if the hard-surface theory is true.

“Our work implies that some, and perhaps all, black holes have event horizons…” – Ramesh Narayan, Harvard-Smithsonian Center for Astrophysics.

What might seem like a failure, isn’t one of course. Not for Einstein, anyway. This represents yet another successful test of Einstein’s Theory of General Relativity, showing that the event horizon predicted in his theory does seem to exist.

As for the team, they haven’t abandoned the idea yet. In fact, according to Pawan Kumar, Professor of Astrophysics, University of Texas at Austin, “Our motive is not so much to establish that there is a hard surface, but to push the boundary of knowledge and find concrete evidence that really, there is an event horizon around black holes.”

“General Relativity has passed another critical test.” – Ramesh Narayan, Harvard-Smithsonian Center for Astrophysics.

“Our work implies that some, and perhaps all, black holes have event horizons and that material really does disappear from the observable universe when pulled into these exotic objects, as we’ve expected for decades,” Narayan said. “General Relativity has passed another critical test.”

The team plans to continue to look for the flare-ups associated with the hard-surface theory. Their look into the Pan-STARRS data was just their first crack at it.

An artist's illustration of the Large Synoptic Survey Telescope with a simulated night sky. The team hopes to use the LSST to further refine their search for hard-surface supermassive objects. Image: Todd Mason, Mason Productions Inc. / LSST Corporation
An artist’s illustration of the Large Synoptic Survey Telescope with a simulated night sky. The team hopes to use the LSST to further refine their search for hard-surface supermassive objects. Image: Todd Mason, Mason Productions Inc. / LSST Corporation

They’re hoping to improve their test with the upcoming Large Synoptic Survey Telescope (LSST) being built in Chile. The LSST is a wide field telescope that will capture images of the night sky every 20 seconds over a ten-year span. Every few nights, the LSST will give us an image of the entire available night sky. This will make the study of transient objects much easier and effective.

More reading: Rise of the Super Telescopes: The Large Synoptic Survey Telescope

Sources:

Black Hole Imaged For First Time By Event Horizon Telescope

Illustration of the supermassive black hole at the center of the Milky Way. Credit: NRAO/AUI/NSF

For decades, scientists have held that Supermassive Black Holes (SMBHs) reside at the center of larger galaxies. These reality-bending points in space exert a extremely powerful influence on all things that surround them, consuming matter and spitting out a tremendous amount of energy. But given their nature, all attempts to study them has been confined to indirect methods.

All of that changed beginning on Wednesday, April 12th, 2017, when an international team of astronomers obtained the first-ever image of a Sagittarius A*. Using a series of telescopes from around the globe – collectively known as the Event Horizon Telescope (EHT) – they were able to visualize the  mysterious region around this giant black hole from which matter and energy cannot escape – i.e. the event horizon.

Not only is this the first time that this mysterious region around a black hole has been imaged, it is also the most extreme test of Einstein’s Theory of General Relativity ever attempted. It also represents the culmination of the EHT project, which was established specifically for the purpose of studying black holes directly and improving our understanding of them.

Simulated view of a black hole. Credit: Bronzwaer/Davelaar/Moscibrodzka/Falcke/Radboud University

Since it began capturing data in 2006, the EHT has been dedicated to the study of Sagitarrius A* since it is the nearest SMBH in the known Universe – located about 25,000 light years from Earth. Specifically, scientists hoped to determine if black holes are surrounded by a circular region from which matter and energy cannot escape (which is predicted by General Relativity), and how they accrete matter onto themselves.

Rather than constituting a single facility, the EHT relies on a worldwide network of radio astronomy facilities based on four continents, all of which are dedicated to studying one of the most powerful and mysterious forces in the Universe. This process, whereby widely-space radio dishes from across the globe are connected into an Earth-sized virtual telescope, is known as Very Long Baseline Interferometry (VLBI).

As Michael Bremer – an astronomer at the International Research Institute for Radio Astronomy (IRAM) and a project manager for the Event Horizon Telescope – said in an interview with AFP:

“Instead of building a telescope so big that it would probably collapse under its own weight, we combined eight observatories like the pieces of a giant mirror. This gave us a virtual telescope as big as Earth—about 10,000 kilometers (6,200 miles) is diameter.”

Sagittarius A is the super-massive black hole at the center of our Milky Way Galaxy. It is shown in x-ray (blue) and infrared (red) in this combined image from the Chandra Observatory and the Hubble Space Telescope. Image: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI
Combined image of Sagittarius A shown in x-ray (blue) and infrared (red), provided by the Chandra Observatory and the Hubble Space Telescope. Credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

All told, the network includes instruments like the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the Arizona Radio Observatory Submillimeter Telescope, the IRAM 30-meter Telescope in Spain, the Large Millimeter Telescope Alfonso Serrano in Mexico, the South Pole Telescope in Antarctica, and the James Clerk Maxwell Telescope and Submillimeter Array at Mauna Kea, Hawaii.

With these arrays, the EHT radio-dish network is the only one powerful enough to detect the light released when an object would disappear into Sagittarius A*. And from six nights – from Wednesday, April 5th, to Tuesday, April 11th, – all of its arrays were trained on the center of our Milky Way to do just that. By the end of the run, the international team announced that they had snapped the first-ever picture of an event horizon.

In the end, some 500 terabytes of data were collected. This data is now being transferred to the MIT Haystack Observatory in Massachusetts, where it will be processed by supercomputers and turned into an image. “For the first time in our history, we have the technological capacity to observe black holes in detail,” said Bremer. “The images will emerge as we combine all the data. But we’re going to have to wait several months for the result.”

Part of the reason for the wait is the fact that the recorded data obtained by the South Pole Telescope can only be collected when spring starts in Antarctica – which won’t happen until October 2017 at the earliest. As such, it won’t be until 2018 before the public gets to feast its eyes on the shadow region that surrounds Sagittarius A*, and it is not expected that the first image will be entirely clear.

As Heino Falcke – an astronomers from Radbound University who now chairs the Scientific Council of EHT (and was the one who proposed this experiment twenty years ago) – explained in a EHT press release prior to the observation being made:

“It is the challenge of doing something, that has never been attempted before. It is the start of an adventurous journey towards a black hole… However, I think we need more observation campaigns and eventually more telescopes in the network to make a really good image.”

Despite the wait, and the fact that repeated attempts will be needed before we can get our first clear look at a black hole, there is still plenty of reason to celebrate in the meantime. Not only was this a first that was a long time in he making, but it also represents a major leap towards understanding one of the most powerful and mysterious forces of nature.

Given time, the study of black holes may allow for us to finally resolve how gravity and the other fundamental forces of the Universe interact. At long last, we will be able to comprehend all of existence as a single, unified equation!

Further Reading: Event Horizon Telescope, NRAO

Watch Stars Orbit The Milky Way’s Supermassive Black Hole

The Milky Way’s supermassive black hole, called Sagittarius A* (or Sgr A*), is arrowed in the image made of the innermost galactic center in X-ray light by NASA’s Chandra Observatory. To the left or east of Sgr A* is Sgr A East, a large cloud that may be the remnant of a supernova. Centered on Sgr A* is a spiral shaped group of gas streamers that might be falling onto the hole. Credit: NASA/CXC/MIT/Frederick K. Baganoff et al.

When your ordinary citizen learns there’s a supermassive black hole with a mass of 4 million suns sucking on its teeth in the center of the Milky Way galaxy, they might kindly ask exactly how astronomers know this. A perfectly legitimate question. You can tell them that the laws of physics guarantee their existence or that people have been thinking about black holes since 1783. That year, English clergyman John Michell proposed the idea of “dark stars” so massive and gravitationally powerful they could imprison their own light.

This time-lapse movie in infrared light shows how stars in the central light-year of the Milky Way have moved over a period of 14 years. The yellow mark at the image center represents the location of Sgr A*, site of an unseen supermassive black hole.
Credit: A. Eckart (U. Koeln) & R. Genzel (MPE-Garching), SHARP I, NTT, La Silla Obs., ESO

Michell wasn’t making wild assumptions but taking the idea of gravity to a logical conclusion. Of course, he had no way to prove his assertion. But we do. Astronomers  now routinely find bot stellar mass black holes — remnants of the collapse of gas-guzzling supergiant stars — and the supermassive variety in the cores of galaxies that result from multiple black hole mergers over grand intervals of time.

Some of the galactic variety contain hundreds of thousands to billions of solar masses, all of it so to speak “flushed down the toilet” and unavailable to fashion new planets and stars. Famed physicist Stephen Hawking has shown that black holes evaporate over time, returning their energy to the knowable universe from whence they came, though no evidence of the process has yet been found.

On September 14, 2013, astronomers caught the largest X-ray flare ever detected from Sgr A*, the supermassive black hole at the center of the Milky Way, using NASA’s Chandra X-ray Observatory.  This event was 400 times brighter than the usual X-ray output from the source and was possibly caused when Sgr A*’s strong gravity tore apart an asteroid in its neighborhood, heating the debris to X-ray-emitting temperatures before slurping down the remains.The inset shows the giant flare. Credit: NASA

So how do we really know a massive, dark object broods at the center of our sparkling Milky Way? Astronomers use radio, X-ray and infrared telescopes to peer into its starry heart and see gas clouds and stars whirling about the center at high rates of speed. Based on those speeds they can calculate the mass of what’s doing the pulling.

The Hubble Space Telescope took this photo of the  5000-light-year-long jet of radiation ejected from the active galaxy M87’s supermassive black hole, which is aboutt 1,000 times more massive than the Milky Way’s black hole. Although black holes are dark, matter whirling into their maws at high speed is heated to high temperature, creating a bright disk of material and jets of radiation. Credit: NASA/The Hubble Heritage Team (STScI/AURA)

In the case of the galaxy M87 located 53.5 million light years away in the Virgo Cluster, those speeds tell us that something with a mass of 3.6 billion suns is concentrated in a space smaller than our Solar System. Oh, and it emits no light! Nothing fits the evidence better than a black hole because nothing that massive can exist in so small a space without collapsing in upon itself to form a black hole. It’s just physics, something that Mr. Scott on Star Trek regularly reminded a panicky Captain Kirk.

So it is with the Milky Way, only our black hole amounts to a piddling 4 million-solar-mass light thief confined within a spherical volume of space some 27 million miles in diameter or just shy of Mercury’s perihelion distance from the Sun. This monster hole resides at the location of Sagittarius A* (pronounced A- star), a bright, compact radio source at galactic center about 26,000 light years away.


Video showing a 14-year-long time lapse of stars orbiting Sgr A*

The time-lapse movie, compiled over 14 years, shows the orbits of several dozen stars within the light year of space centered on Sgr A*. We can clearly see the star moving under the influence of a massive unseen body — the putative supermassive black hole. No observations of Sgr A* in visible light are possible because of multiple veils of interstellar dust that lie across our line of sight. They quench its light to the tune of 25 magnitudes.


Merging black holes (the process look oddly biological!). Credit: SXS

How do these things grow so big in the first place? There are a couple of ideas, but astronomers don’t honestly know for sure. Massive gas clouds around early in the galaxy’s history could have collapsed to form multiple supergiants that evolved into black holes which later then coalesced into one big hole. Or collisions among stars in massive, compact star clusters could have built up stellar giants that evolved into black holes. Later, the clusters sank to the center of the galaxy and merged into a single supermassive black hole.

Whichever you chose, merging of smaller holes may explain its origin.

On a clear spring morning before dawn, you can step out to face the constellation Sagittarius low in the southern sky. When you do, you’re also facing in the direction of our galaxy’s supermassive black hole. Although you cannot see it, does it not still exert a certain tug on your imagination?