A composite image of the Abell 85 galaxy cluster. The purple is multi-million degree gas detected in X-rays by NASA's Chandra X-ray Observatory and the other colors show galaxies in an optical image from the Sloan Digital Sky Survey. Image Credit: X-ray (NASA/CXC/SAO/A.Vikhlinin et al.); Optical (SDSS); Illustration (MPE/V.Springel)
If contemplating the vast size of astronomical objects makes you feel rather puny and insignificant, then this new discovery will make you feel positively infinitesimal.
It’s almost impossible to imagine an object this large: a super massive black hole that’s 40 billion times more massive than our Sun. But there it is, sitting in the center of a super-giant elliptical galaxy called Holmberg 15A. Holmberg 15A is about 700 million light years away, in the center of the Abell 85 galaxy cluster.
An illustration of a Super-Massive Black Hole. Image Credit: Scott Woods, Western University
Super-Massive Black Holes (SMBH) are hard to explain. These gargantuan singularities are thought to be at the center of every large galaxy (our Milky Way has one) but their presence there sometimes defies easy explanation. As far as we know, black holes form when giant stars collapse. But that explanation doesn’t fit all the evidence.
Researchers from the University of Michigan confirm that a runaway star was ejected from the Milky Way's disk rather than the galactic core. Image Credit: Kohei Hattori
Every once in a while, the Milky Way ejects a star. The evicted star is typically ejected from the chaotic area at the center of the galaxy, where our Super Massive Black Hole (SMBH) lives. But at least one of them was ejected from the comparatively calm galactic disk, a discovery that has astronomers rethinking this whole star ejection phenomenon.
Researchers using the Event Horizon Telescope hope to generate more and better images like this of supermassive black hole Sag. A's event horizon. Image Credit: EHT.
The largest object in our night sky—by far!—is invisible to us. The object is the Super-Massive Black Hole (SMBH) at the center of our Milky Way galaxy, called Sagittarius A. But soon we may have an image of Sagittarius A’s event horizon. And that image may pose a challenge to Einstein’s Theory of General Relativity.
On September 14th, 2013, astronomers caught the largest X-ray flare ever detected from the supermassive black hole at the center of the Milky Way, known as Sagittarius A* (Sgr A*). Credit: NASA/CXC/Stanford/I. Zhuravleva et al.
Since the 1970s, astronomers have understood that a Supermassive Black Hole (SMBH) resides at the center of the Milky Way Galaxy. Located about 26,000 light-years from Earth between the Sagittarius and Scorpius constellations, this black hole has come to be known as Sagittarius A* (Sgr 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 that time, astronomers have discovered that most massive galaxies have SMBHs at their core, which is what separates those that have an Active Galactic Nuclei (AGN) from those that don’t. But thanks to a recent survey conducted using NASA’s Chandra X-ray Observatory, astronomers have discovered evidence for hundreds or even thousands of black holes located near the center of the Milky Way Galaxy.
The center of the Milky Way Galaxy, with X-ray binaries circled in red, other X-ray sources circled in yellow, and Sagittarius A* circled in blue at the center. Credit: NASA/CXC/Columbia University/C. Hailey et al.
Using Chandra data, the team searched for X-ray binaries containing black holes that were in the vicinity of Sgr A*. To recap, black holes are not detectable in visible light. However, black holes (or neutron stars) that are locked in close orbits with a star will pull material from their companions, which will then be accreted onto the black holes’ disks and heated up to millions of degrees.
This will result in the release of X-rays which can then be detected, hence why these systems are called “X-ray binaries”. Using Chandra data, the team sought out X-ray of sources that were located within roughly 12 light years of Sgr A*. They then selected sources with X-ray spectra similar to those of known X-ray binaries, which emit relatively large amounts of low-energy X-rays.
Using this method, they detected fourteen X-ray binaries within about three light years of Sgr A*, all of which contained stellar-mass black holes (between 5 and 30 times the mass of our Sun). Two of these sources had been identified by previous studies and were eliminated from the analysis, while the remaining twelve (circled in red in the image above) were newly-discovered.
Other sources which relatively large amounts of high energy X-rays (labeled in yellow) were believed to be binaries containing white dwarfs. Hailey and his colleagues concluded that the majority of the dozen X-ray binaries were likely to contain black holes, based on their variability and the fact that their X-ray emissions over the course of several years was different from what is expected from binaries containing neutron stars.
Artist”s impression of a black hole binary, consisting of a black hole siphoning material from its companion. Credit: ESO/L. Calçada
Given that only the brightest X-ray binaries containing black holes are likely to be detectable around Sgr A* (given its distance from Earth), Hailey and his colleagues concluded that this detection implies the existence of a much larger population. By their estimates, there could be at least 300 and as many as one thousand stellar-mass black holes present around Sgr A*.
These findings confirmed what theoretical studies on the dynamics of stars in galaxies have indicated in the past. According to these studies, a large population of stellar mass black holes (as many as 20,000) could drift inward over the course of millions of years and collect around an SMBH. However, the recent analysis conducted by Hailey and his colleagues was the first observational evidence of black holes congregating near Sgr A*.
Naturally, the authors acknowledge that there are other explanations for the X-ray emissions they detected. This includes the possibility that half of the dozen sources they observed are millisecond pulsars – very rapidly rotating neutron stars with strong magnetic fields. However, based on their observations, Hailey and his team strongly favor the black hole explanation.
In addition, a follow-up study conducted by Aleksey Generozov (et al.) of Columbia University – titled “An Overabundance of Black Hole X-Ray Binaries in the Galactic Center from Tidal Captures” – indicated that there could be as many as 10,000 to 40,000 black holes binaries at the center of our galaxy. According to this study, these binaries would be the result of companions being captured by black holes.
Artist’s impression of merging binary black holes. Credit: LIGO/A. Simonnet.
In addition to revealing much about the dynamics of stars in our galaxy, this study has implications for the emerging field of gravitational wave (GW) research. Essentially, by knowing how many black holes reside at the center of galaxies (which will periodically merge with one another), astronomers will be able to better predict how many gravitational wave events are associated with them.
From this, astronomers could create predictive models about when and how GW events are likely to happen, and well as discerning what role they may play in galactic evolution. And with next-generation instruments – like the James Webb Space Telescope (JWST) and the ESA’s Advanced Telescope for High Energy Astrophysics (ATHENA) – astronomers will be able to determine exactly how many black holes reside near the center of our galaxy.
The ESA's Gaia mission is currently on a five-year mission to map the stars of the Milky Way. Gaia has found evidence for a galactic collision that occurred between 300 million and 900 million years ago. Image credit: ESA/ATG medialab; background: ESO/S. Brunier.
In February of 2016, scientists working for the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever detection of gravitational waves. Since that time, multiple detections have taken place, thanks in large to part to improvements in instruments and greater levels of collaboration between observatories. Looking ahead, its possible that missions not designed for this purpose could also “moonlight” as gravitational wave detectors.
For example, the Gaia spacecraft – which is busy creating the most detailed 3D map of the Milky Way – could also be instrumental when it comes to gravitational wave research. That’s what a team of astronomers from the University of Cambridge recently claimed. According to their study, the Gaia satellite has the necessary sensitivity to study ultra-low frequency gravitational waves that are produced by supermassive black hole mergers.
Artist’s illustration of two merging neutron stars, which are a source of gravitational waves. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet
To recap, gravitational waves (GWs) are ripples in space-time that are created by violent events, such as black hole mergers, collisions between neutron stars, and even the Big Bang. Originally predicted by Einstein’s Theory of General Relativity, observatories like LIGO and Advanced Virgo detect these waves by measuring the way space-time flexes and squeezes in response to GWs passing through Earth.
However, passing GWs would also cause the Earth to oscillate in its location with respect to the stars. As a result, an orbiting space telescope (such as Gaia), would be able to pick up on this by noting a temporary shift in the position of distant stars. Launched in 2013, the Gaia observatory has spent the past few years conducting high-precision observations of the positions of stars in our Galaxy (aka. astrometry).
In this respect, Gaia would look for small displacements in the massive field of stars it is monitoring to determine if gravitational waves have passed through the Earth’s neighborhood. To investigate whether or not Gaia was up to the task, Moore and his colleagues performed calculations to determine if the Gaia space telescope had the necessary sensitivity to detect ultra-low frequency GWs.
To this end, Moore and his colleagues simulated gravitational waves produced by a binary supermassive black hole – i.e. two SMBHs orbiting one another. What they found was that by compressing the data sets by a factor of more than 106 (measuring 100,000 stars instead of a billion at a time), GWs could be recovered from Gaia data with an only 1% loss of sensitivity.
Figure showing a Gaia star field, with red and black lines indicating induced apparent motions of the stars within a hemisphere. Credit: Kavli Institute for Cosmology, Cambridge
This method would be similar to that used in Pulsar Timing Arrays, where a set of millisecond pulsars are examined to determine if gravitational waves modify the frequency of their pulses. However, in this case, stars are being monitored to see if they are oscillating with a characteristic pattern, rather than pulsing. By looking at a field of 100,000 stars at a time, researchers would be able to detect induced apparent motions (see figure above).
Because of this, the full release of Gaia data (scheduled for the early 2020s) is likely to be a major opportunity for those hunting for GW signals. As Moore explained in a APS Physicspress release:
“Gaia will make measuring this effect a realistic prospect for the first time. Many factors contribute to the feasibility of the approach, including the precision and long duration of the astrometric measurements. Gaia will observe about a billion stars over 5–10 years, locating each one of them at least 80 times during that period. Observing so many stars is the major advance provided by Gaia.”
It is also interesting to note that the potential for GW detection was something that researchers recognized when Gaia was still being designed. One such individual was Sergei A. Klioner, a researcher from the Lorhrmann Observatory and the leader of the Gaia group at TU Dresden. As he indicated in his 2017 study, “Gaia-like astrometry and gravitational waves“, Gaia could detect GWs caused by merging SMBHs years after the event:
“It is clear that the most promising sources of gravitational waves for astrometric detection are supermassive binary black holes in the centers of galaxies… It is believed that binary supermassive black holes are a relatively common product of interaction and merging of galaxies in the typical course of their evolution. This sort of objects can give gravitational waves with both frequencies and amplitudes potentially within the reach of space astrometry. Moreover, the gravitational waves from those objects can often be considered to have virtually constant frequency and amplitude during the whole period of observations of several years.”
Artist’s impression of two merging black holes, which has been theorized to be a source of gravitational waves. Credit: Bohn, Throwe, Hébert, Henriksson, Bunandar, Taylor, Scheel/SXS
But of course, there’s no guarantees that sifting through the Gaia data will reveal additional GW signals. For one thing, Moore and his colleagues acknowledge that waves at these ultra-low frequencies could be too weak for even Gaia to detect. In addition, researchers will have to be able to distinguish between GWs and conflicting signals that result from changes in the spacecraft’s orientation – which is no easy challenge!
Still, there is hope that missions like Gaia will be able to reveal GWs that are not easily visible to ground-based interferometric detectors like LIGO and Advanced Virgo. Such detectors are subject to atmospheric effects (like refraction) which prevent them from seeing extremely low frequency waves – for instance, the primordial waves produced during the inflationary epoch of the Big Bang.
In this sense, gravitational wave research is not unlike exoplanet research and many other branches of astronomy. In order to find the hidden gems, observatories may need to take to space to eliminate atmospheric interference and increase their sensitivity. It is possible then that other space telescopes will be retooled for GW research, and that next-generation GW detectors will be mounted aboard spacecraft.
In the past few years, scientists have gone from making the first detection of gravitational waves to developing new and better ways to detecting them. At this rate, it won’t be long before astronomers and cosmologists are able to include gravitational waves into our cosmological models. In other words, they will be able to show what influence these waves played in the history and evolution of the Universe.
Colour composite image of Centaurus A, revealing the lobes and jets emanating from the active galaxy’s central black hole. Credit: ESO/NASA/CXC/CfA/WFI/MPIfR/APEX/A.Weiss et al./R.Kraft et al.
In the 1970s, astronomers discovered that a particularly large black hole (Sagittarius A*) existed at the center of our galaxy. In time, they came to understand that similar Supermassive Black Holes (SMBHs) existed in the center of most massive galaxies. The presence of these black holes was also what differentiated galaxies that had particularly luminous cores – aka. Active Galactic Nuclei (AGN) – from those that didn’t.
Since that time, astronomers and cosmologists have pondered what role SMBHs have on galactic evolution, with some venturing that they have a profound impact on star formation. And thanks to a recent study by an international team of astronomers, there is now direct evidence for a correlation between and SMBH and a galaxy’s star formation. In fact, the team demonstrated that a black hole’s mass could determine when star formation in a galaxy will end.
The primary mirror of the Hobby-Eberly Telescope (HET) at McDonald Observatory. The mirror is made up of 91 segments, and has an effective aperture of 9.2 meters. Credit: Marty Harris/McDonald Observatory
For the sake of their study, the team relied on data gathered the Hobby-Eberle Telescope Massive Galaxy Survey in 2015. This systematic survey used the 10m Hobby-Eberly Telescope (HET) at the McDonald Observatory to conduct an optical long-slit spectroscopic survey of over 1000 galaxies. This survey not only provided spectra for these galaxies, but also produced direct mass measurements of the central black holes for 74 of these galaxies.
Using this data, Martín-Navarro and his colleagues found the first observational evidence for a direct correlation between the mass of a galaxy’s central black hole and its history of star formation. While astrophysicists have been operating under this assumption for decades, the proof was missing until now. As Jean Brodie, professor of astronomy and astrophysics at UC Santa Cruz and a coauthor of the paper, said in a UCSC press release:
“We’ve been dialing in the feedback to make the simulations work out, without really knowing how it happens. This is the first direct observational evidence where we can see the effect of the black hole on the star formation history of the galaxy.”
Roughly 15 years ago, the correlation between a SMBHs mass and the total mass of a galaxy’s stars was discovered, which led to a major unresolved question in astrophysical circles. While this correlation appeared to be a central feature of galaxies, it was unclear as to what could have caused it. How could the mass of a comparatively small and central black hole be related to the mass of billions of stars distributed throughout a galaxy?
The galaxy NGC 660 – in this and other galaxies, the rate at which new stars are formed appears to be linked to the evolution of the galaxy’s central black hole. Credit: ESA/Hubble/NASA
One possible explanation was that more massive galaxies collected larger amounts of gas, thus resulting in more stars and a more massive central black hole. However, astrophysicists also believed their was a feedback mechanism at work, where growing black holes inhibited the formation of stars in their vicinity. In short, when matter accretes on a central black hole, it sends out a tremendous amount of energy in the form of radiation and particle jets.
If this energy is transferred to gas and dust surrounding the core of the galaxy, stars will be less likely to form in this region since gas and dust need to be cold in order to undergo areas of collapse. For years, feedback of this kind has been included in cosmological simulations to explain the observed star-formation rates in galaxies. According to these same simulations, minus this mechanism, galaxies would form far more stars than have been observed.
However, no direct evidence of this phenomena had previously been available. The first step to obtaining some was to reproduce the stellar formation histories of the 74 target galaxies used for the study. Martín-Navarro and his colleagues did this by subjecting spectra obtained from each of these galaxies to computational techniques that looked for the best combination of stellar populations to fit the data.
In so doing, the team was able to reconstruct the history of star formation within the target galaxies for the past 12.5 billion years. After examining these histories, they noticed some predictable results, but also some rather significant differences. For starters, as predicted, the regions of around the galaxies’ central black holes demonstrated a clear dampening influence on the rate of star formation.
Artist’s concept of the most distant supermassive black hole ever discovered. It is part of a quasar from just 690 million years after the Big Bang. Credit: Robin Dienel/Carnegie Institution for Science
As predicted, there was also a clear correlation between the mass of the central black holes and stellar mass in these galaxies. However, the team also noted that in cases where stellar mass was slightly smaller than expected (relative to the mass of their central black holes), star formation rates were lower. In some other cases, galaxies had larger-than-expected stellar masses (again, relative to their black holes) and their star formation rates were higher.
This correlation was not only more consistent than that observed between black hole mass and stellar mass, it occurred independently of other factors (such as shape or density). As Martín-Navaro explained:
“For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. So star formation lasted longer in those galaxies with smaller central black holes.”
They also noted that this correlation extends into the deep past, where the galaxies with supermassive central black holes have been consistently producing a comparatively low rate of stars for the past 12.5 billion years. This constitutes the first strong evidence for a direct, long-term connection between star formation and the existence of a central black hole in a galaxy.
Close-up of star near a supermassive black hole (artist’s impression). Credit: ESA/Hubble, ESO, M. Kornmesser
Another interesting takeaway from the study was the way it addressed possible correlations between AGN luminosity and star formation. In the past, other researchers have sought to find evidence of a link between the two, but without success. According to Martín-Navarro and his team, this may be because the time scales are incredibly different. Whereas star formation occurs over the course of eons, outbursts from AGNs occur over shorter intervals.
What’s more, AGNs are highly variable and their properties are dependent on a number of factors relating to their black holes – i.e. size, mass, rate of accretion, etc. “We used black hole mass as a proxy for the energy put into the galaxy by the AGN, because accretion onto more massive black holes leads to more energetic feedback from active galactic nuclei, which would quench star formation faster,” said Martin-Navarro.
Looking ahead, the team hopes to conduct further research and determine exactly how central black holes arrest star formation. At present, the possibility that it could be due to radiation or jets of gas heating up surrounding matter are not definitive. As Aaron Romanowsky, an astronomer at San Jose State University and UC Observatories, indicated:
“There are different ways a black hole can put energy out into the galaxy, and theorists have all kinds of ideas about how quenching happens, but there’s more work to be done to fit these new observations into the models.”
Part of determining how the Universe came to be is knowing what mechanisms were at play and the extent of their roles. With this latest study, astrophysicists and cosmologists can take comfort in the knowledge that they’ve been getting it right – at least in this case!
A radio image from the NSF’s Karl G. Jansky Very Large Array showing the center of our galaxy. The mysterious radio filament is the curved line located near the center of the image, & the supermassive black hole Sagittarius A* (Sgr A*), is shown by the bright source near the bottom of the image. Credit: NSF/VLA/UCLA/M. Morris et al.
The core of the Milky Way Galaxy has always been a source of mystery and fascination to astronomers. This is due in part to the fact that our Solar System is embedded within the disk of the Milky Way – the flattened region that extends outwards from the core. This has made seeing into the bulge at the center of our galaxy rather difficult. Nevertheless, what we’ve been able to learn over the years has proven to be immensely interesting.
For instance, in the 1970s, astronomers became aware of the Supermassive Black Hole (SMBH) at the center of our galaxy, known as Sagittarius A* (Sgr A*). In 2016, astronomers also noticed a curved filament that appeared to be extending from Sgr A*. Using a pioneering technique, a team of astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) recently produced the highest-quality images of this structure to date.
The study which details their findings, titled “A Nonthermal Radio Filament Connected to the Galactic Black Hole?“, recently appeared in The Astrophysical Journal Letters. In it, the team describes how they used the National Radio Astronomy Observatory’s (NRAO) Very Large Array to investigate the non-thermal radio filament (NTF) near Sagittarius A* – now known as the Sgr A West Filament (SgrAWF).
Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.
As Mark Morris – a professor of astronomy at the UCLA and the lead authority the study – explained in a CfA press release:
“With our improved image, we can now follow this filament much closer to the Galaxy’s central black hole, and it is now close enough to indicate to us that it must originate there. However, we still have more work to do to find out what the true nature of this filament is.”
After examining the filament, the research team came up with three possible explanations for its existence. The first is that the filament is the result of inflowing gas, which would produce a rotating, vertical tower of magnetic field as it approaches and threads Sgr A*’s event horizon. Within this tower, particles would produce radio emissions as they are accelerated and spiral in around magnetic field lines extending from the black hole.
The second possibility is that the filament is a theoretical object known as a cosmic string. These are basically long, extremely thin cosmic structures that carry mass and electric currents that are hypothesized to migrate from the centers of galaxies. In this case, the string could have been captured by Sgr A* once it came too close and a portion crossed its event horizon.
The third and final possibility is that there is no real association between the filament and Sgr A* and the positioning and direction it has shown is merely coincidental. This would imply that there are many such filaments in the Universe and this one just happened to be found near the center of our galaxy. However, the team is confident that such a coincidence is highly unlikely.
Labelled image of the center of our galaxy, showing the mysterious radio filament & the supermassive black hole Sagittarius A* (Sgr A*). Credit: NSF/VLA/UCLA/M. Morris et al.
As Jun-Hui Zhao of the Harvard-Smithsonian Center for Astrophysics in Cambridge, and a co-author on the paper, said:
“Part of the thrill of science is stumbling across a mystery that is not easy to solve. While we don’t have the answer yet, the path to finding it is fascinating. This result is motivating astronomers to build next generation radio telescopes with cutting edge technology.”
All of these scenarios are currently being investigated, and each poses its own share of implications. If the first possibility is true – in which the filament is caused by particles being ejected by Sgr A* – then astronomers would be able to gleam vital information about how magnetic fields operate in such an environment. In short, it could show that near an SMBH, magnetic fields are orderly rather than chaotic.
This could be proven by examining particles farther away from Sgr A* to see if they are less energetic than those that are closer to it. The second possibility, the cosmic string theory, could be tested by conducting follow-up observations with the VLA to determine if the position of the filament is shifting and its particles are moving at a fraction of the speed of light.
If the latter should prove to be the case, it would constitute the first evidence that theoretical cosmic strings actually exists. It would also allow astronomers to conduct further tests of General Relativity, examining how gravity works under such conditions and how space-time is affected. The team also noted that, even if the filament is not physically connected to Sgr A*, the bend in the filament is still rather telling.
In short, the bend appears to be coincide with a shock wave, the kind that would be caused by an exploding star. This could mean that one of the massive stars which surrounds Sgr A* exploded in proximity to the filament in the past, producing the necessary shock wave that altered the course of the inflowing gas and its magnetic field. All of these mysteries will be the subject of follow-up surveys conducted with the VLA.
As co-author Miller Goss from the National Radio Astronomy Observatory in New Mexico (and a co-author on the study) said, “We will keep hunting until we have a solid explanation for this object. And we are aiming to next produce even better, more revealing images.”
Artist’s impression of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun. Credit: ESO/M. Kornmesser
Ever since the discovery of Sagittarius A* at the center of our galaxy, astronomers have come to understand that most massive galaxies have a Supermassive Black Hole (SMBH) at their core. These are evidenced by the powerful electromagnetic emissions produced at the nuclei of these galaxies – which are known as “Active Galatic Nuclei” (AGN) – that are believed to be caused by gas and dust accreting onto the SMBH.
For decades, astronomers have been studying the light coming from AGNs to determine how large and massive their black holes are. This has been difficult, since this light is subject to the Doppler effect, which causes its spectral lines to broaden. But thanks to a new model developed by researchers from China and the US, astronomers may be able to study these Broad Line Regions (BLRs) and make more accurate estimates about the mass of black holes.
An artist’s impression of the accretion disc around the supermassive black hole that powers an active galaxy. Credit: NASA/Dana Berry, SkyWorks Digital
To break it down, SMBHs are known for having a torus of gas and dust that surrounds them. The black hole’s gravity accelerates gas in this torus to velocities of thousands of kilometers per second, which causes it to heat up and emit radiation at different wavelengths. This energy eventually outshined the entire surrounding galaxy, which is what allows astronomers to determine the presence of an SMBH.
As Michael Brotherton, a UW professor in the Department of Physics and Astronomy and a co0author on the study, explained in a UW press release:
“People think, ‘It’s a black hole. Why is it so bright?’ A black hole is still dark. The discs reach such high temperatures that they put out radiation across the electromagnetic spectrum, which includes gamma rays, X-rays, UV, infrared and radio waves. The black hole and surrounding accreting gas the black hole is feeding on is fuel that turns on the quasar.”
The problem with observing these bright regions comes from the fact that the gases within them are moving so quickly in different directions. Whereas gas moving away (relative to us) is shifted towards the red end of the spectrum, gas that is moving towards us is shifted towards the blue end. This is what leads to a Broad Line Region, where the spectrum of the emitted light becomes more like a spiral, making accurate readings difficult to obtain.
Currently, the measurement of the mass of SMBHs in active galactic nuclei relies the “reverberation mapping technique”. In short, this involves using computer models to examine the symmetrical spectral lines of a BLR and measuring the time delays between them. These lines are believed to arise from gas that has been photoionized by the gravitational force of the SMBH.
Dense clouds of dust and gas, illustrated here, can obscure less energetic radiation from an active galaxy’s central black hole. High-energy X-rays, however, easily pass through. Credit: ESA/NASA/AVO/Paolo Padovani
However, since there is little understanding of broad emission lines and the different components of BLRs, this method gives rise to some uncertainties off between 200 and 300%. “We are trying to get at more detailed questions about spectral broad-line regions that help us diagnose the black hole mass,” said Brotherton. “People don’t know where these broad emission line regions come from or the nature of this gas.”
In contrast, the team led by Dr. Wang adopted a new type of computer model that considered the dynamics of the gas torus surrounding a SMBH. This torus, they assume, would be made up of discrete clumps of matter that would be tidally disrupted by the black hole, resulting in some gas flowing into it (aka. accreting on it) and some being ejected as outflow.
From this, they found that the emission lines in a BLR are subject to three characteristics – “asymmetry”, “shape” and “shift”. After examining various emissions lines – both symmetrical and asymmetrical – they found that these three characteristics were strongly dependent on how bright the gas clumps were, which they interpreted as being a result of the angle of their motion within the torus. Or as Dr. Brotherton put it:
“What we propose happens is these dusty clumps are moving. Some bang into each other and merge, and change velocity. Maybe they move into the quasar, where the black hole lives. Some of the clumps spin in from the broad-line region. Some get kicked out.”
Illustration of the supermassive black hole at the center of the Milky Way. Credit: NRAO/AUI/NSF
In the end, their new model suggests that tidally disrupted clumps of matter from a black hole torus may represent the source of the BLR gas. Compared to previous models, the one devised by Dr. Wang and his colleagues establishes a connection between different key processes and components in the vicinity of a SMBH. These include the feeding of the black hole, the source of photoionized gas, and the dusty torus itself.
While this research does not resolve all the mysteries surrounding AGNs, it is an important step towards obtaining accurate mass estimates of SMBHs based on their spectral lines. From these, astronomers could be able to more accurately determine what role these black holes played in the evolution of large galaxies.
The study was made possible thanks with support provided by the National Key Program for Science and Technology Research and Development, and the Key Research Program of Frontier Sciences, both of which are administered by the Chinese Academy of Sciences.
Artist's impression of merging binary black holes. 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.
“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.“
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:
