Schematic representation of the tilted accretion disk model. Credit: Cui et al. (2023), Intouchable Lab@Openverse and Zhejiang Lab
Fifty-five million light-years away, in the galaxy known as M87, lies a supermassive black hole. It is a powerfully active black hole with a mass of 6.5 billion Suns, and in 2019 it was the first black hole to be imaged directly. The radio image captured by the Event Horizon Telescope (EHT) shows a halo of ambient light warped by the black hole’s gravity and directed our way. On one side of the halo, the light is brighter, which according to general relativity is due to the rotation or spin of the black hole. It was the first direct confirmation that the black hole rotates. A new study published in Nature has given us more rotational evidence.
The Drake Equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Credit: University of Rochester
Did humanity miss the party? Are SETI, the Drake Equation, and the Fermi Paradox all just artifacts of our ignorance about Advanced Life in the Universe? And if we are wrong, how would we know?
A new study focusing on black holes and their powerful effect on star formation suggests that we, as advanced life, might be relics from a bygone age in the Universe.
A disk of hot gas swirls around a black hole in this illustration. The stream of gas stretching to the right is what remains of a star that was pulled apart by the black hole. A cloud of hot plasma (gas atoms with their electrons stripped away) above the black hole is known as a corona.
Credits: NASA/JPL-Caltech
Black holes are confounding objects that stretch physics to its limits. The most massive ones lurk in the centers of large galaxies like ours. They dominate the galactic center, and when a star gets too close, the black hole’s powerful gravitational force tears the star apart as they feed on it. Not even the most massive stars can resist.
But supermassive black holes (SMBHs) didn’t start out that massive. They attained their gargantuan mass by accreting material over vast spans of time and by merging with other black holes.
There are large voids in our understanding of how SMBHs grow and evolve, and one way astrophysicists fill those voids is by watching black holes as they consume stars.
Artist view of an active supermassive black hole. Credit: ESO/L. Calçada
Black holes are gluttonous behemoths that lurk in the center of galaxies. Almost everybody knows that nothing can escape them, not even light. So when anything made of simple matter gets too close, whether a planet, a star or a gas cloud, it’s doomed.
But the black hole doesn’t eat it at once. It plays with its food like a fussy kid. Sometimes, it spews out light.
When the black hole is not only at the center of a galaxy but the center of a cluster of galaxies, these burps and jets carve massive cavities out of the hot gas at the center of the cluster called radio bubbles.
Using NASA’s Chandra X-ray Observatory, astronomers have seen that the famous giant black hole in Messier 87 is propelling particles at speeds greater than 99% of the speed of light. Image Credit: NASA/CXC/SAO/B. Snios et al.
Can black holes be famous? If they can, then the one at the heart of the M87 galaxy qualifies. And this famous black hole is emitting jets of material that travel at near the speed of light.
An artist's impression of the accretion disc around the supermassive black hole that powers an active galaxy. Astronomers want to know if the energy radiated from a black hole is caused by jets of material shooting away from the hole, or by the accretion disk of swirling material near the hole. Credit: NASA/Dana Berry, SkyWorks Digital
In the 1970s, astronomers became aware of a compact radio source at the center of the Milky Way Galaxy – which they named Sagittarius A. After many decades of observation and mounting evidence, it was theorized that the source of these radio emissions was in fact a supermassive black hole (SMBH). Since that time, astronomers have come to theorize that SMBHs at the heart of every large galaxy in the Universe.
Most of the time, these black holes are quiet and invisible, thus being impossible to observe directly. But during the times when material is falling into their massive maws, they blaze with radiation, putting out more light than the rest of the galaxy combined. These bright centers are what is known as Active Galactic Nuclei, and are the strongest proof for the existence of SMBHs.
Description:
It should be noted that the enormous bursts in luminosity observed from Active Galactic Nuclei (AGNs) are not coming from the supermassive black holes themselves. For some time, scientists have understood that nothing, not even light, can escape the Event Horizon of a black hole.
Instead, the massive burst of radiations – which includes emissions in the radio, microwave, infrared, optical, ultra-violet (UV), X-ray and gamma ray wavebands – are coming from cold matter (gas and dust) that surround the black holes. These form accretion disks that orbit the supermassive black holes, and gradually feeding them matter.
The incredible force of gravity in this region compresses the disk’s material until it reaches millions of degrees kelvin. This generates bright radiation, producing electromagnetic energy that peaks in the optical-UV waveband. A corona of hot material forms above the accretion disc as well, and can scatter photons up to X-ray energies.
A large fraction of the AGN’s radiation may be obscured by interstellar gas and dust close to the accretion disc, but this will likely be re-radiated at the infrared waveband. As such, most (if not all) of the electromagnetic spectrum is produced through the interaction of cold matter with SMBHs.
The interaction between the supermassive black hole’s rotating magnetic field and the accretion disk also creates powerful magnetic jets that fire material above and below the black hole at relativistic speeds (i.e. a significant fraction of the speed of light). These jets can extend for hundreds of thousands of light-years, and are a second potential source of observed radiation.
Types of AGN:
Typically, scientists divide AGN into two categories, which are referred to as “radio-quiet” and “radio-loud” nuclei. The radio-loud category corresponds to AGNs that have radio emissions produced by both the accretion disk and the jets. Radio-quiet AGNs are simpler, in that any jet or jet-related emission are negligible.
Carl Seyfert discovered the first class of AGN in 1943, which is why they now bear his name. “Seyfert galaxies” are a type of radio-quiet AGN that are known for their emission lines, and are subdivided into two categories based on them. Type 1 Seyfert galaxies have both narrow and broadened optical emissions lines, which imply the existence of clouds of high density gas, as well as gas velocities of between 1000 – 5000 km/s near the nucleus.
Type 2 Seyferts, in contrast, have narrow emissions lines only. These narrow lines are caused by low density gas clouds that are at greater distances from the nucleus, and gas velocities of about 500 to 1000 km/s. As well as Seyferts, other sub classes of radio-quiet galaxies include radio-quiet quasars and LINERs.
Low Ionisation Nuclear Emission-line Region galaxies (LINERs) are very similar to Seyfert 2 galaxies, except for their low ionization lines (as the name suggests), which are quite strong. They are the lowest-luminosity AGN in existence, and it is often wondered if they are in fact powered by accretion on to a supermassive black hole.
Artist’s representation of an active galactic nucleus (AGN) at the center of a galaxy. Credit: NASA/CXC/M.Weiss
Radio-loud galaxies can also be subdivded into categories like radio galaxies, quasars, and blazars. As the name suggests, radio galaxies are elliptical galaxies that are strong emitters of radiowaves. Quasars are the most luminous type of AGN, which have spectra similar to Seyferts.
However, they are different in that their stellar absorption features are weak or absent (meaning they are likely less dense in terms of gas) and the narrow emission lines are weaker than the broad lines seen in Seyferts. Blazars are a highly variable class of AGN that are radio sources, but do not display emission lines in their spectra.
Detection:
Historically speaking, a number of features have been observed within the centers of galaxies that have allowed for them to be identified as AGNs. For instance, whenever the accretion disk can be seen directly, nuclear-optical emissions can be seen. Whenever the accretion disk is obscured by gas and dust close to the nucleus, an AGN can be detected by its infra-red emissions.
Then there are the broad and narrow optical emission lines that are associated with different types of AGN. In the former case, they are produced whenever cold material is close to the black hole, and are the result of the emitting material revolving around the black hole with high speeds (causing a range of Doppler shifts of the emitted photons). In the former case, more distant cold material is the culprit, resulting in narrower emission lines.
Image taken by the Hubble Space Telescope of a 5000-light-year-long jet ejected from the active galaxy M87. Credit: NASA/The Hubble Heritage Team (STScI/AURA)
Next up, there are radio continuum and x-ray continuum emissions. Whereas radio emissions are always the result of the jet, x-ray emissions can arise from either the jet or the hot corona, where electromagnetic radiation is scattered. Last, there are x-ray line emissions, which occur when x-ray emissions illuminate the cold heavy material that lies between it and the nucleus.
These signs, alone or in combination, have led astronomers to make numerous detections at the center of galaxies, as well as to discern the different types of active nuclei out there.
The Milky Way Galaxy:
In the case of the Milky Way, ongoing observation has revealed that the amount of material accreted onto Sagitarrius A is consistent with an inactive galactic nucleus. It has been theorized that it had an active nucleus in the past, but has since transitioned into a radio-quiet phase. However, it has also been theorized that it might become active again in a few million (or billion) years.
When the Andromeda Galaxy merges with our own in a few billion years, the supermassive black hole that is at its center will merge with our own, producing a much more massive and powerful one. At this point, the nucleus of the resulting galaxy – the Milkdromeda (Andrilky) Galaxy, perhaps? – will certainly have enough material for it to be active.
The discovery of active galactic nuclei has allowed astronomers to group together several different classes of galaxies. It’s also allowed astronomers to understand how a galaxy’s size can be discerned by the behavior at its core. And last, it has also helped astronomers to understand which galaxies have undergone mergers in the past, and what could be coming for our own someday.
This artist's rending shows "before" and "after" images of a changing look quasar. Credit: Yale University.
Last week, astronomers at Yale University reported seeing something unusual: a seemingly stedfast beacon from the far reaches of the Universe went quiet. This relic light source, a quasar located in the region of our sky known as the celestial equator, unexpectedly became 6-7 times dimmer over the first decade of the 21st century. Thanks to this dramatic change in luminosity, astronomers now have an unprecedented opportunity to study both the life cycle of quasars and the galaxies that they once called home.
A quasar arises from a distant (and therefore, very old) galaxy that once contained a central, rotating supermassive black hole – what astronomers call an active galactic nucleus. This spinning beast ravenously swallowed up large amounts of ambient gas and dust, kicking up surrounding material and sending it streaming out of the galaxy at blistering speeds. Quasars shine because these ancient jets achieved tremendous energies, thereby giving rise to a torrent of light so powerful that astronomers are still able to detect it here on Earth, billions of years later.
In their hey-day, some active galactic nuclei were also energetic enough to excite electrons farther away from the central black hole. But even in the very early Universe, electrons couldn’t withstand that kind of excitement forever; the laws of physics don’t allow it. Eventually, each electron would drop back down to its rest state, releasing a photon of corresponding energy. This cycle of excitation happened over and over and over again, in regular and predictable patterns. Modern astronomers can visualize those transitions – and the energies that caused them – by examining a quasar’s optical spectrum for characteristic emission lines at certain wavelengths.
A simple example of an atomic spectrum, showing emission lines at particular wavelengths. Broad humps correspond to brighter emission lines, while lines that arise from narrow, lower-intensity emissions appear dimmer. Credit: NASA
Not all quasars are created equal, however. While the spectra of some quasars reveal many bright, broad emission lines at different energies, other quasars’ spectra consist of only the dim, narrow variety. Until now, some astronomers thought that these variations in emission lines among quasars were simply due to differences in their orientation as seen from Earth; that is, the more face-on a quasar was relative to us, the broader the emission lines astronomers would be able to see.
But all of that has now been thrown into question, thanks to our friend J015957.64+003310.5, the quasar revealed by the team of astronomers at Yale. Indeed, it is now plausible that a quasar’s pattern of emission lines simply changes over its lifetime. After gathering ten years of spectral observations from the quasar, the researchers observed its original change in brightness in 2010. In July 2014, they confirmed that it was still just as dim, disproving hypotheses that suggested the effect was simply due to intervening gas or dust. “We’ve looked at hundreds of thousands of quasars at this point, and now we’ve found one that has switched off,” explained C. Megan Urry, the study’s co-author.
How would that happen, you ask? After observing the comparable dearth of broad emission lines in its spectrum, Urry and her colleagues believe that long ago, the black hole at the heart of the quasar simply went on a diet. After all, an active galactic nucleus that consumed less material would generate less energy, giving rise to fainter particle jets and fewer excited atoms. “The power source just went dim,” said Stephanie LaMassa, the study’s principal investigator.
LaMassa continued, “Because the life cycle of a quasar is one of the big unknowns, catching one as it changes, within a human lifetime, is amazing.” And since the life cycle of quasars is dependent on the life cycle of supermassive black holes, this discovery may help astronomers to explain how those that lie at the center of most galaxies evolve over time – including Sagittarius A*, the supermassive black hole at the center of our own Milky Way.
“Even though astronomers have been studying quasars for more than 50 years, it’s exciting that someone like me, who has studied black holes for almost a decade, can find something completely new,” added LaMassa.
The team’s research will be published in an upcoming issue of The Astrophysical Journal. A pre-print of the paper is available here.
An artist's conception of a supermassive black hole's jets. Credit: NASA / Dana Berry / SkyWorks Digital
The supermassive black holes in the cores of most massive galaxies wreak havoc on their immediate surroundings. During their most active phases — when they ignite as luminous quasars — they launch extremely powerful and high-velocity outflows of gas.
These outflows can sweep up and heat material, playing a pivotal role in the formation and evolution of massive galaxies. Not only have astronomers observed them across the visible Universe, they also play a key ingredient in theoretical models.
But the physical nature of the outflows themselves has been a longstanding mystery. What physical mechanism causes gas to reach such high speeds, and in some cases be expelled from the galaxy?
A new study provides the first direct evidence that these outflows are accelerated by energetic jets produced by the supermassive black hole.
Using the Very Large Telescope in Chile, a team of astronomers led by Clive Tadhunter from Sheffield University, observed the nearby active galaxy IC 5063. At locations in the galaxy where its jets are impacting regions of dense gas, the gas is moving at extraordinary speeds of over 600,000 miles per hour.
“Much of the gas in the outflows is in the form of molecular hydrogen, which is fragile in the sense that it is destroyed at relatively low energies,” said Tadhunter in a press release. “I find it extraordinary that the molecular gas can survive being accelerated by jets of highly energetic particles moving at close to the speed of light.
As the jets travel through the galactic matter, they disrupt the surrounding gas and generate shock waves. These shock waves not only accelerate the gas, but also heat it. The team estimates the shock waves heat the gas to temperatures high enough to ionize the gas and dissociate the molecules. Molecular hydrogen is only formed in the significantly cooler post-shock gas.
“We suspected that the molecules must have been able to reform after the gas had been completely upset by the interaction with a fast plasma jet,” said Raffaella Morganti from the Kapteyn Institute Groningen University. “Our direct observations of the phenomenon have confirmed that this extreme situation can indeed occur. Now we need to work at describing the exact physics of the interaction.”
In interstellar space, molecular hydrogen forms on the surface of dust grains. But in this scenario, the dust is likely to have been destroyed in the intense shock waves. While it is possible for molecular hydrogen to form without the aid of dust grains (as seen in the early Universe) the exact mechanism in this case is still unknown.
The research helps answer a longstanding question — providing the first direct evidence that jets accelerate the molecular outflows seen in active galaxies — and asks new ones.
Black hole with disc and jets visualization courtesy of ESA
The concept of a black hole jet isn’t a new one, but we still have a lot to learn about the mixture of particles found in the vicinity of them. Through the use of ESA’s XMM-Newton Observatory, astronomers have been taking a look at a black hole in our galaxy and found some surprising results.
As we know, stellar mass black holes take on materials from nearby stars. Matter from these companion stars is pulled away from the parent body toward the black hole and radiates a temperture so intense that it emits X-rays. However, a black hole doesn’t always ingest everything that comes its way. Sometimes they reject small portions of this incoming mass, pushing it away in the form of a set of powerful jets. These jets also feed the surroundings, releasing both mass and energy… robbing the black hole of fuel.
Through the study of jet composition, researchers are able to better determine what gets taken into a black hole and what doesn’t. Through observations taken at the radio wavelength of the electromagnetic spectrum, we have seen electrons crusing along at nearly the speed of light. However, it hasn’t been clearly determined whether the negative charge of the electrons is complemented by their anti-particles, positrons, or rather by heavier positively-charged particles in the jets, like protons or atomic nuclei.” With XMM-Newton’s power behind them, astronomers have had the opportunity to examine a black hole binary system called 4U1630–47 – a candidate known to have unexpected outbursts of X-rays for segments of time which last between months and years.
“In our observations, we found signs of highly ionised nuclei of two heavy elements, iron and nickel,” says María Díaz Trigo of the European Southern Observatory in Munich, Germany, lead author of the paper published in the journal Nature. “The discovery came as a surprise – and a good one, since it shows beyond doubt that the composition of black hole jets is much richer than just electrons.”
During September 2012, a team of astronomers headed up by Dr. Díaz Trigo and collaborators, observed 4U1630–47 with XMM-Newton. They also backed up their observations with near-simultaneous radio observations taken from the Australia Telescope Compact Array. Even though the studies were done close to each other – within just a couple of weeks – the results couldn’t have been more different.
According to Trigo’s team, the initial set of observations picked up X-ray signatures from the accretion disc, but there was no activity in the radio band. This is an indicator that the jets weren’t active at that time. However, in the second set of observations, there was activity in both X-ray and radio… the jets had turned back on! While examining the X-ray data from the second set, they also found iron nuclei in motion. These particles were moving both toward and away from XMM-Newton – proof the ions were part of twin jets aimed in opposite directions. However, that’s not all. There was also evidence of nickel nuclei pointing toward the observatory.
“From these ‘fingerprints’ of iron and nickel, we could show that the speed of the jet is very high, about two-thirds of the speed of light,” says co-author James Miller-Jones from the Curtin University node of the International Centre for Radio Astronomy Research in Perth, Australia.
“Moreover, the presence of heavy atomic nuclei in black hole jets means that mass and energy are being carried away from the black hole in much larger amounts than we previously thought, which may have an impact on the mechanism and rate by which the black hole accretes matter,” adds co-author Simone Migliari from the University of Barcelona, Spain.
Astounding new findings? Well… yeah. For a typical stellar-mass black hole, this is the first time that heavy nuclei has been detected within the jets. As of the present, there is only “one other X-ray binary that shows similar signatures from atomic nuclei in its jets – a source known as SS 433. This black hole system, however, is characterised by an unusually high accretion rate, which makes it difficult to compare its properties to those of more ordinary black holes.” Through these new observations of 4U1630–47, astronomers will be able to fill in information gaps about what causes jets to occur in black hole accretion disks and what drives them.
“While we now know a great deal about black holes and what happens around them, the formation of jets is still a big puzzle, so this observation is a major step forward in understanding this fascinating phenomenon,” says Norbert Schartel, ESA’s XMM-Newton Project Scientist.
This detailed view shows the central parts of the nearby active galaxy NGC 1433. The dim blue background image, showing the central dust lanes of this galaxy, comes from the NASA/ESA Hubble Space Telescope. The coloured structures near the centre are from recent ALMA observations that have revealed a spiral shape, as well as an unexpected outflow, for the first time. Credit: ALMA (ESO/NAOJ/NRAO)/NASA/ESA/F. Combes
Did you ever wonder what it would be like to observe what happens to a galaxy near a black hole? For all of us who remember that wonderful Disney movie, it would be a remarkable – if not hypnotic – experience. Now, thanks to the powerful observational tools of the Atacama Large Millimeter/submillimeter Array (ALMA), two international astronomy teams have had the opportunity to study the jets of black holes near their galactic cores and see just how they impact their neighborhood. The researchers have captured the best view so far of a molecular gas cloud surrounding a nearby, quiescent black hole and were gifted with a surprise look at the base of a massive jet near a distant one.
These aren’t lightweights. The black holes the astronomers are studying weigh in a several billion solar masses and make their homes at the center of nearly all the galaxies in the Universe – including the Milky Way. Once upon a time, these enigmatic galactic phenomena were busy creatures. They absorbed huge amounts of matter from their surroundings, shining like bright beacons. These early black holes thrust small amounts of the matter they took in through highly powerful jets, but their current counterparts aren’t quite as active. While things may have changed a bit with time, the correlation of black hole jets and their surroundings still play a crucial role in how galaxies evolve. In the very latest of studies, both published today in the journal Astronomy & Astrophysics, astronomers employed ALMA to investigate black hole jets at very different scales: a nearby and relatively quiet black hole in the galaxy NGC 1433 and a very distant and active object called PKS 1830-211.
“ALMA has revealed a surprising spiral structure in the molecular gas close to the center of NGC 1433,” says Françoise Combes (Observatoire de Paris, France), who is the lead author of the first paper. “This explains how the material is flowing in to fuel the black hole. With the sharp new observations from ALMA, we have discovered a jet of material flowing away from the black hole, extending for only 150 light-years. This is the smallest such molecular outflow ever observed in an external galaxy.”
Need feedback? Well, that’s exactly what this process is called. “Feedback” may enlighten us to the relationship between black hole mass and the mass of the surrounding galactic bulge. The black hole consumes gas and becomes active, but then it creates jets which purge gas from its proximity. This halts star formation and controls the growth of the central bulge. In PKS 1830-211, Ivan Marti-Vidal (Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden) and his team witnessed a supermassive black hole with a jet, “but a much brighter and more active one in the early universe. It is unusual because its brilliant light passes a massive intervening galaxy on its way to Earth, and is split into two images by gravitational lensing.”
Are supermassive black holes messy eaters? You bet. There have been occasions when a supermassive black hole will unexpectedly consume a staggering amount of mass which, in turn, turbo-charges the power of the jets and lights up the radiation output to the very pinnacle of energy output. This energy is emitted as gamma rays, the shortest wavelength and highest energy form of electromagnetic radiation. And now ALMA has, by chance, caught one of these events as it happened in PKS 1830-211.
“The ALMA observation of this case of black hole indigestion has been completely serendipitous. We were observing PKS 1830-211 for another purpose, and then we spotted subtle changes of color and intensity among the images of the gravitational lens. A very careful look at this unexpected behavior led us to the conclusion that we were observing, just by a very lucky chance, right at the time when fresh new matter entered into the jet base of the black hole,” says Sebastien Muller, a co-author of the second paper.
The main image, showing the nearby active galaxy NGC 1433, comes from the NASA/ESA Hubble Space Telescope. The coloured structures near the centre shown in the insert are from recent ALMA observations that have revealed a spiral shape, as well as an unexpected outflow, for the first time. Credit: ALMA (ESO/NAOJ/NRAO)/NASA/ESA/F. CombesAs with all astronomical observations, the key to discovery is confirmation. Did the ALMA findings show up on other telescopic observations? The answer is yes. Thanks to monitoring observations with NASA’s Fermi Gamma-ray Space Telescope, there was a definite gamma ray signature exactly where it should be. Whatever was responsible for the scaling up of radiation at ALMA’s long wavelengths was also responsible for making the light of the black hole jet flare impressively.
“This is the first time that such a clear connection between gamma rays and submillimeter radio waves has been established as coming from the real base of a black hole’s jet,” adds Sebastien Muller.
It isn’t the end of the story, however. It’s just the beginning. ALMA will continue to probe into the mysterious workings of supermassive black hole jets – both near and far. Combes and her investigative team are already observing close active galaxies with ALMA, and even a unique object cataloged as PKS 1830-211. The research will continue, and with it we may one day have answers to many questions.
“There is still a lot to be learned about how black holes can create these huge energetic jets of matter and radiation,” concludes Ivan Marti-Vidal. “But the new results, obtained even before ALMA was completed, show that it is a uniquely powerful tool for probing these jets — and the discoveries are just beginning!”
