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Is there any correlation between the size or spin - or any other properties - of a galaxy's central supermassive black hole and the speed of galaxy rotation (which I think would also be proportional to the calculated amount of dark matter in the galaxy [?])?
No relation between a supermassive black hole and its host galaxy?
Using the Atacama Large Millimeter/submillimeter Array (ALMA) to observe an active galaxy with a strong ionized gas outflow from the galactic center, a team led by Dr. Yoshiki Toba of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan) has obtained a result making astronomers even more puzzled -- the team clearly detected carbon monoxide (CO) gas that is associated with the galactic disk, yet they have also found that the CO gas which settles in the galaxy is not affected by the strong ionized gas outflow launched from the galactic center.
According to a popular scenario explaining the formation and evolution of galaxies and supermassive black holes, radiation from galactic centers -- where supermassive black holes locate -- can significantly influence the molecular gas (such as CO) and the star formation activities of the galaxies. With an ALMA result showing that the ionized gas outflow driven by the supermassive black hole does not necessarily affect its host galaxy, "it has made the co-evolution of galaxies and supermassive black holes more puzzling," Yoshiki explains, "the next step is looking into more data of this kind of galaxies. That is crucial for understanding the full picture of the formation and evolution of galaxies and supermassive black holes."
Answering the question "How did galaxies form and evolve during the 13.8-billion-year history of the universe?" has been one top issue in modern astronomy. Studies already revealed that almost all massive galaxies harbor a supermassive black hole at their centers. In recent findings, studies further revealed that the masses of black holes are tightly correlated with those of their host galaxies. This correlation suggests that supermassive black holes and their host galaxies have evolved together and closely interacted each other as they grow, also known as the co-evolution of galaxies and supermassive black holes.
The gas outflow driven by a supermassive black hole at the galactic center recently has become the focus of attention as it possibly is playing a key role in the co-evolution of galaxies and black holes. A widely accepted idea has described this phenomenon as: the strong radiation from the galactic center in which the supermassive black hole locates ionizes the surrounding gas and affects even molecular gas that is the ingredient of star formation the strong radiation activates or suppresses the star formation of galaxies. However, "we astronomers do not understand the real relation between the activity of supermassive black holes and star formation in galaxies," says Tohru Nagao, Professor at Ehime University. "Therefore, many astronomers including us are eager to observe the real scene of the interaction between the nuclear outflow and the star-forming activities, for revealing the mystery of the co-evolution."
The team has focused on a particular type of objects called Dust-Obscured Galaxy (DOG) that has a prominent feature: despite being very faint in the visible light, it is very bright in the infrared.
Astronomers are believing that DOGs harbor actively growing supermassive black holes in their nuclei. In particular, one DOG (WISE1029+0501, hereafter WISE1029) is outflowing gas ionized by the strong radiation from its supermassive black hole. WISE1029 is known as an extreme case in terms of ionized gas outflow, and this particular factor has motivated the researchers to see what happens to its molecular gas.
By making use of ALMA's outstanding sensitivity which is excellent in investigating properties of molecular gas and star forming activities in galaxies, the team conducted their research by observing the CO and the cold dust of galaxy WISE1029. After detailed analysis, surprisingly they found, there is no sign of significant molecular gas outflow. Furthermore, star forming activity is neither activated nor suppressed. This indicates that a strong ionized gas outflow launched from the supermassive black hole in WISE1029 neither significantly affect the surrounding molecular gas nor the star formation.
There have been many reports saying that the ionized gas outflow driven by the accretion power of a supermassive black hole has a great impact on surrounding molecular gas. However, it is a very rare case that there is no tight interaction between ionized and molecular gas as the researchers are reporting this time. Yoshiki and the team's result suggests that the radiation from a supermassive black hole does not always affect the molecular gas and star formation of its host galaxy.
While their result is making the co-evolution of galaxies and supermassive black holes more puzzling, Yoshiki and his team are exciting about revealing the full picture of the scenario. He says, "understanding such co-evolution is crucial for astronomy. By collecting statistical data of this kind of galaxies and continuing in more follow-up observations using ALMA, we hope to reveal the truth."
Researchers clarify dynamics of black hole rotational energy
Fig. 1. A supermassive black hole surrounded by an accretion disk, shown in red, emits jets — the vertical beams. Credit: MIPT
Astrophysicists at MIPT have developed a model for testing a hypothesis about supermassive black holes at the centers of galaxies. The new model enables scientists to predict how much rotational energy a black hole loses when it emits beams of ionized matter known as astrophysical jets. The energy loss is estimated based on measurements of a jet's magnetic field. The paper was published in the journal Frontiers in Astronomy and Space Sciences.
Astrophysicists have observed hundreds of relativistic jets—enormous outflows of matter emitted by active galactic nuclei harboring supermassive black holes. The matter in a jet is accelerated nearly to the speed of light, hence the term "relativistic." Jets are colossal, even by astronomical standards—their length can be up to several percent of the radius of the host galaxy, or about 300,000 times larger than the associated black hole.
That said, there is still a lot researchers do not know about jets. Astrophysicists are not even sure what they are made of because jet observations yield no spectral lines. Current consensus holds that jets are likely made of electrons and positrons or protons, but they remain quite a mystery. As researchers obtain new data, a more comprehensive and self-consistent model of this phenomenon is gradually emerging.
The matter orbiting and falling onto a black hole is referred to as the accretion disk. It plays a crucial role in jet formation. A black hole, together with its accretion disk and jets (fig. 1), are thought to be the most effective "machine" for converting energy. If we define the efficiency of such a system as the ratio of the energy carried away by the jets to the energy of the accreted matter, it may even exceed 100 percent.
Nevertheless, a closer look at the system reveals that the second law of thermodynamics still holds. This is no perpetual motion machine. It turns out that part of the energy of the jet comes from the rotation of the black hole. That is, by powering a jet, a black hole spins progressively slower.
In a way, this seeming perpetual motion is more like an electric bike. There is an apparent mismatch between the input energy of the accreting matter—muscle work, in the case of the biker—and the output energy of the jet, or the motion of the bicycle. In both cases, though, there is an additional hidden energy source—namely, the battery powering the bike's electric motor and the rotation of the black hole.
Via accretion, a black hole gains angular momentum—that is, it starts spinning faster. Jets carry away some of this excess angular momentum in what is known as rotational energy extraction. Similar effects are observed in young stars. During the formation of a star, it captures the matter of the accretion disk, which has enormous angular momentum. However, observations show such stars actually rotate rather slowly. All of the missing angular momenta is used up to power the narrow jets emitted by these stars.Fig. 2. Transverse structure of the magnetic field of a jet. Credit: MIPT
Scientists have recently developed a new method for measuring the magnetic fields in the jets emitted by the active galactic nuclei. In her paper, astrophysicist Elena Nokhrina showed that this method can be used to estimate the contribution of black hole rotation to jet power. Up until now, the formula for the channeling of rotational energy into the energy of the jet has not been tested empirically. Unfortunately, no reliable observations so far have captured black hole rotation rate, which is important for estimating the loss of rotational energy.
A black hole does not have a magnetic field of its own. However, a vertical magnetic field is generated around it by the ionized matter in the accretion disk. To estimate the loss of rotational energy by a black hole, scientists need to find the magnetic flux through the boundary around a black hole known as the event horizon.
"Because the magnetic flux is conserved, by measuring its magnitude in the jet, we also learn the magnetic flux near the black hole. Knowing the black hole's mass, we can calculate the distance from its rotation axis to the event horizon—its notional boundary. This allows us to estimate the electric potential difference between the axis of rotation and the boundary of the black hole. By accounting for the electric field screening in plasma, it is possible to find the electric current near the black hole. Knowing both the current and the difference of potentials, we can estimate the amount of energy lost by the black hole due to the slowing down of its rotation," says Elena Nokhrina, the author of the paper and deputy head of the relativistic astrophysics laboratory at MIPT.
The calculations point toward a correlation between the total power of a jet emitted by a black hole and the loss of rotational energy by the black hole. Notably, this study makes use of a recent model of jet structure (fig. 2). Before this model was advanced, researchers assumed jets to have homogeneous transverse structure, which is a simplification. In the new model, the magnetic field of a jet is not homogeneous, enabling more accurate predictions.
Most of the galaxies hosting jets are too remote for the transverse structure of their magnetic fields to be discerned. So the experimentally measured magnetic field is compared with its model transverse structure to estimate the magnitude of the field's components. Only by taking transverse structure into account is it possible to test the mechanism of black hole rotation powering jets. Otherwise, it would be necessary to know the rotation rate.
The hypothesis that was put to the test in the study states that jet power depends on the magnetic flux and the rotation rate of the black hole. This makes it possible to gauge to what extent a jet is powered by rotational energy. Notably, this theoretical work enables us to estimate how much rotational energy is lost by a black hole without knowing its rotation rate—using only the magnetic field measurements of the jet.
WISDOM project - VI. Exploring the relation between supermassive black hole mass and galaxy rotation with molecular gas
Empirical correlations between the masses of supermassive black holes (SMBHs) and properties of their host galaxies are well-established. Among these is the correlation with the flat rotation velocity of each galaxy measured either at a large radius in its rotation curve or via a spatially-integrated emission line width. We propose here the use of the de-projected integrated CO emission line width as an alternative tracer of this rotation velocity, that has already been shown useful for the Tully-Fisher (luminosity-rotation velocity) relation. We investigate the correlation between CO line widths and SMBH masses for two samples of galaxies with dynamical SMBH mass measurements, with respectively spatially-resolved and unresolved CO observations. The tightest correlation is found using the resolved sample of 25 galaxies as log (MBH/M⊙) = (7.5 ± 0.1) + (8.5 ± 0.9)[log (W50/sin i km s−1) − 2.7], where MBH is the central SMBH mass, W50 the full-width at half-maximum of a double-horned emission line profile, and i the inclination of the CO disc. This relation has a total scatter of 0.6 dex, comparable to those of other SMBH mass correlations, and dominated by the intrinsic scatter of 0.5 dex. A tight correlation is also found between the de-projected CO line widths and the stellar velocity dispersions averaged within one effective radius. We apply our correlation to the COLD GASS sample to estimate the local SMBH mass function.
|Schools:||Physics and Astronomy|
|Publisher:||Oxford University Press|
|Date of First Compliant Deposit:||9 November 2020|
|Date of Acceptance:||15 October 2020|
|Last Modified:||20 Jan 2021 10:08|
Cited times time in Web of Science. View in Web of Science.
ALMA has made the most precise measurements of cold gas swirling around a supermassive black hole -- the cosmic behemoth at the center of the giant elliptical galaxy NGC 3258. The multi-color ellipse reflects the motion of the gas orbiting the black hole, with blue indicating motion toward us and red motion away from us. The inset box represents how the orbital velocity changes with distance from the black hole. [Credit: ALMA (ESO/NAOJ/NRAO), B. Boizelle NRAO/AUI/NSF, S. Dagnello Hubble Space Telescope (NASA/ESA) Carnegie-Irvine Galaxy Survey.]
Astronomers Confirm Link between Galaxy Mergers and Supermassive Black Holes
This artist’s impression illustrates how high-speed jets from supermassive black holes would look. These outflows of plasma are the result of the extraction of energy from a supermassive black hole’s rotation as it consumes the disc of swirling material that surrounds it. These jets have very strong emissions at radio wavelengths.
Astronomers used the Hubble Space Telescope to conduct a large survey to investigating the relationship between galaxies that have undergone mergers and the activity of the supermassive black holes at their cores, revealing a link between the presence of supermassive black holes that power high-speed, radio-signal-emitting jets and the merger history of their host galaxies.
In the most extensive survey of its kind ever conducted, a team of scientists have found an unambiguous link between the presence of supermassive black holes that power high-speed, radio-signal-emitting jets and the merger history of their host galaxies. Almost all of the galaxies hosting these jets were found to be merging with another galaxy, or to have done so recently. The results lend significant weight to the case for jets being the result of merging black holes and will be presented in the Astrophysical Journal.
A team of astronomers using the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3) have conducted a large survey to investigate the relationship between galaxies that have undergone mergers and the activity of the supermassive black holes at their cores.
The team studied a large selection of galaxies with extremely luminous centers — known as active galactic nuclei (AGNs) — thought to be the result of large quantities of heated matter circling around and being consumed by a supermassive black hole. Whilst most galaxies are thought to host a supermassive black hole, only a small percentage of them are this luminous and fewer still go one step further and form what are known as relativistic jets . The two high-speed jets of plasma move almost with the speed of light and stream out in opposite directions at right angles to the disc of matter surrounding the black hole, extending thousands of light-years into space. The hot material within the jets is also the origin of radio waves.
This artist’s impression illustrates how high-speed jets from supermassive black holes would look. These outflows of plasma are the result of the extraction of energy from a supermassive black hole’s rotation as it consumes the disc of swirling material that surrounds it. These jets have very strong emissions at radio wavelengths. Credit: ESA/Hubble, L. Calçada (ESO)
It is these jets that Marco Chiaberge from the Space Telescope Science Institute, USA (also affiliated with Johns Hopkins University, USA and INAF-IRA, Italy) and his team hoped to confirm were the result of galactic mergers .
The team inspected five categories of galaxies for visible signs of recent or ongoing mergers — two types of galaxies with jets, two types of galaxies that had luminous cores but no jets, and a set of regular inactive galaxies .
“The galaxies that host these relativistic jets give out large amounts of radiation at radio wavelengths,” explains Marco. “By using Hubble’s WFC3 camera we found that almost all of the galaxies with large amounts of radio emission, implying the presence of jets, were associated with mergers. However, it was not only the galaxies containing jets that showed evidence of mergers!” .
“We found that most merger events in themselves do not actually result in the creation of AGNs with powerful radio emission,” added co-author Roberto Gilli from Osservatorio Astronomico di Bologna, Italy. “About 40% of the other galaxies we looked at had also experienced a merger and yet had failed to produce the spectacular radio emissions and jets of their counterparts.”
Although it is now clear that a galactic merger is almost certainly necessary for a galaxy to host a supermassive black hole with relativistic jets, the team deduce that there must be additional conditions which need to be met. They speculate that the collision of one galaxy with another produces a supermassive black hole with jets when the central black hole is spinning faster — possibly as a result of meeting another black hole of a similar mass — as the excess energy extracted from the black hole’s rotation would power the jets.
“There are two ways in which mergers are likely to affect the central black hole. The first would be an increase in the amount of gas being driven towards the galaxy’s center, adding mass to both the black hole and the disc of matter around it,” explains Colin Norman, co-author of the paper. “But this process should affect black holes in all merging galaxies, and yet not all merging galaxies with black holes end up with jets, so it is not enough to explain how these jets come about. The other possibility is that a merger between two massive galaxies causes two black holes of a similar mass to also merge. It could be that a particular breed of merger between two black holes produces a single spinning supermassive black hole, accounting for the production of jets.”
Future observations using both Hubble and the Atacama Large Millimeter/submillimeter Array (ALMA) are needed to expand the survey set even further and continue to shed light on these complex and powerful processes.
 Relativistic jets travel at close to the speed of light, making them one of the fastest astronomical objects known.
 The new observations used in this research were taken in collaboration with the 3CR-HST team. This international team of astronomers is currently led by Marco Chiaberge and has conducted a series of surveys of radio galaxies and quasars from the 3CR catalog using the Hubble Space Telescope.
 The team compared their observations with the swathes of archival data from Hubble. They directly surveyed twelve very distant radio galaxies and compared the results with data from a large number of galaxies observed during other observing programs.
 Other studies had shown a strong relationship between the merger history of a galaxy and the high levels of radiation at radio wavelengths that suggests the presence of relativistic jets lurking at the galaxy’s center. However, this survey is much more extensive, and the results very clear, meaning it can now be said with almost certainty that radio-loud AGNs, that is, galaxies with relativistic jets, are the result of galactic mergers.
Event Horizon Telescope: Black hole produces twisting jet
This time we're being shown the base of a colossal jet of excited gas, or plasma, screaming away from another black hole at near light-speed.
The scene was actually in the "background" of the original target.
The scientists who operate the Event Horizon Telescope describe the jet in the journal Astronomy & Astrophysics.
They say their studies of the region of space known as 3C 279 will help them better understand the physics that drives behaviour in the vicinity of black holes.
3C 279 is what astronomers term a quasar - the extremely bright core of a very distant galaxy. This one is about 5.5 billion light-years from Earth.
It is well known, and was used as the calibration target to align the performance of the EHT's eight individual radio telescopes when they simultaneously made their astonishing map of the supermassive black hole at the centre of Galaxy M87.
The remarkable resolution achieved by the EHT - put to such great effect with M87 - pays dividends again with 3C 279, because we see previously unrecognised features.
3C 279 also has a supermassive black hole at its heart. It's about one billion times the mass of our Sun and its gravity is pulling in and shredding any stars or gas that get too close. This material is likely being accreted on to a disc that winds around the hole, but some of it is being shot back out into space along two jets moving in opposite directions.
In previous images of 3C 279, we've been able to detect the outline of the jet that moves towards us (the one moving in the opposite direction is not detected). But in the new EHT picture, we can resolve detail close to the point where this jet leaves the black hole. What's more, this base area seems twisted and somewhat offset from the main axis of the jet.
"It's curious," said EHT Collaboration member Dr Ziri Younsi. "We're seeing a region that's actually pretty close to the black hole. It could be an interaction layer where the jet couples to the accretion disc and extracts all of its energy from the black hole.
"We don't really understand how jets are powered by black holes. Black holes, when they rotate rapidly, are the most efficient liberators of energy in the Universe, but the mechanism by which the jet can extract that energy is unknown. There are a few ideas, but we're not sure yet which one is the right one," the University College London, UK, researcher told BBC News.
The data in the images of M87 and 3C 279 was collected by the ENT's widely dispersed array of radio telescopes in 2017. The project has gone on to collect data on the supermassive black hole that exists at the centre of our own galaxy, the Milky Way.
"We have that data - of a region we call Sagittarius A*," said Dr Younsi. "We are working on it right now and although we have some preliminary results, these can't be shared just yet. We hope to have something perhaps before the end of this year." The team finds itself in a position to concentrate on this analysis because the observational time it had booked on the EHT array for this year got cancelled in the coronavirus outbreak.
A PDF of the A&A paper describing 3C 279 is available here. Its lead author is Dr Jae-Young Kim from the Max Planck Institute for Radio Astronomy in Bonn, Germany.
The Event Horizon Telescope is a "virtual telescope" that links a large array of radio receivers - from the South Pole, to Hawaii, to the Americas and Europe. It uses a technique called very long baseline array interferometry (VLBI). This combines the observations from the dispersed network to mimic a telescope aperture that can produce the resolution necessary to perceive a pinprick on the sky. For the EHT, this pinprick is measured in microarcseconds.
To convey such performance to the general public, EHT team-members talk about the sharpness of vision as being the equivalent of seeing from Earth something the size of a grapefruit on the surface of the Moon.
Monster Black Hole Spins at Half the Speed of Light
For the first time, astronomers have directly measured how fast a black hole spins, clocking its rotation at nearly half the speed of light.
The distant supermassive black hole would ordinarily be too faint to measure, but a rare lineup with a massive elliptical galaxy created a natural telescope known as a gravitational lens that allowed scientists to study the faraway object.
"The gravitational lens is crucial," study co-author Mark Reynolds of the University of Michigan told Space.com via email.."Without this, we would not be able to collect X-ray photons to measure the spin of a black hole that is so distant." [The Strangest Black Holes in the Universe]
Nature's free telescope
Just more than 6 billion light-years from Earth, a supermassive black hole powers the quasar . Quasars, the most luminous objects in the universe, shine brightly across vast distances, fed by material that falls into their black holes.
Black holes are massive objects whose gravitational pull is so powerful that even light cannot escape their grasp. Most form when a star at the end of its lifetime explodes, its outer core collapsing into a tiny dense ball.
Supermassive black holes have masses millions of times that of the sun and are found at the center of most galaxies, including the Milky Way. Their origins are still unknown.
The only features scientists are able to measure about the voracious objects are their mass and spin. Astronomers can determine the mass of a black hole by measuring its interactions with gas and other objects, but characterizing its rotation has remained a challenge, especially for more distant supermassive black holes.
In the new study, a team led by Rubens Reis of the University of Michigan used NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton — the largest X-ray space telescopes currently available — to observe the X-rays generated in the innermost regions of the disk of material circling and feeding the supermassive black hole that powers the quasar J1131.
Measuring the radius of the disk allowed the astronomers to calculate the black hole's spin speed, which was almost half the speed of light.
The team would have been unable to measure the spin without a rare lineup in space. A giant elliptical galaxy lies between Earth and the quasar J1131. The huge galaxy acts as a gravitational lens to bend and magnify objects that lie behind it — in this case, the supermassive black hole.
"It acts like a telescope, but a free one provided by nature," Reynolds said.
"Such a quadruple lens of a quasar is a very rare object," Guido Risaliti, of the Harvard-Smithsonian Center for Astrophysics, told Space.com in an email. "Until a few years ago, none of them was known."
Risaliti, who was not involved in the research, also studies supermassive black holes. Last year, he made the first reliable measurement of the spin of a nearby supermassive black hole. He authored a News & Views article that appeared along with the research in the journal Nature today (March 5). [No Escape: Dive Into a Black Hole (Infographic)]
The spin of a supermassive black hole can reveal information about how it accretes the material it consumes. To achieve a rapid spin, material must fall into the black hole in a direction similar to its rotation, ultimately revving it up like a child spinning a merry-go-round.
A slower spin indicates that the gas and dust supplying the black hole fall into it from multiple directions, spinning the black hole up or down depending on whether it comes in with or against the rotation. In this case, the random influx of material acts like a child alternating pushing and pulling the merry-go-round.
The quick spin of J1131 indicates that the black hole is being fed by a bountiful supply of gas and dust. These large volumes could be provided by collisions and mergers between galaxies, among other sources, Reynolds said.
A slower spin and more haphazard feeding process would be caused by material arriving in spurts, from interstellar gas clouds and stars wandering too close from a variety of directions.
"Observational studies over the past 20 years have shown a clear link between the mass of the supermassive black hole at the center of a galaxy and the properties of the galaxy in which it resides," Reynolds said. "These relations suggest a symbiotic relationship between the central black hole and its host galaxy."
By studying the black hole, astronomers can learn more about the origin and evolution of galaxies — and spin plays a very important role.
"The growth history of a supermassive black hole is encoded in its spin," Reynolds said.
"The next immediate step is to obtain a few more black hole spins in the nearby AGN, but it will be difficult to repeat observations like the one of Reis' team due to the rarity of these sources," Risaliti said. "The big step forward will be the measurements of the black hole spins with the next generation of high sensitivity X-ray telescopes, such as the ESA's Athena."
This Huge Black Hole Is Spinning at Half the Speed of Light!
The crumbs left over from a supermassive black hole's recent meal have allowed scientists to calculate the monster's rotation rate, and the results are mind-boggling.
The huge black hole, known as ASASSN-14li, is spinning at least 50 percent the speed of light, research team members said.
"This black hole’s event horizon is about 300 times bigger than the Earth," study co-author Ron Remillard, of the Massachusetts Institute of Technology (MIT), said in a statement. (The event horizon is the limit beyond which nothing, not even light, can escape a black hole's gravitational clutches.) [Images: Black Holes of the Universe]
"Yet the black hole is spinning so fast it completes one rotation in about two minutes, compared to the 24 hours it takes our planet to rotate," Remillard added.
ASASSN-14li lies at the heart of a galaxy 290 million light-years away from Earth and harbors between 1 million and 10 million times the mass of the sun. So, it's about as hefty as the black hole at the Milky Way's core, known as Sagittarius A*, which contains about 4 million solar masses. (Supermassive black holes can get much weightier some tip the scales at tens of billions of solar masses.)
ASASSN-14li was discovered in November 2014, after it tore apart a star that strayed too close. This dramatic event caused a flash of bright light, which was spotted by a system of optical telescopes called the All-Sky Automated Survey for Supernovae (hence the black hole's name).
In the new study, a team led by Dheeraj Pasham, also of MIT, observed the X-ray light coming from the ASASSN-14li system. The researchers analyzed data gathered by a number of instruments, including NASA's Chandra X-ray Observatory and Neil Gehrels Swift space telescopes, as well as the European Space Agency's XMM-Newton spacecraft.
These datasets revealed a consistent flickering: ASASSN-14li's X-ray emissions rise and fall every 131 seconds. This clockwork signal is likely caused by a clump of the torn-apart star circling the black hole very close to the event horizon, study team members said.
"The fact that we can track this region of bright X-ray emission as it circles the black hole lets us track just how quickly material in the disk is spinning," Pasham said in the same statement. "That gives us information about the spin rate of the supermassive black hole itself."
That spin speed is impressive but not unprecedented. The few supermassive black holes whose rotation rates have been clocked to date are in the same extreme neighborhood, generally whipping around between 33 percent and 84 percent the speed of light.
The new results &mdash which Pasham presented Wednesday (Jan. 9) at the 233rd meeting of the American Astronomical Society (AAS) in Seattle &mdash could help astronomers better understand how supermassive black holes evolve.
These behemoths can grow in two main ways, Pasham said &mdash by galaxy-scale mergers, and/or by steadily accreting smaller bits of surrounding material. A relatively low rotation rate would implicate mergers as the primary factor, because these random smashups likely wouldn't keep spinning the growing black hole up in the same direction.
However, "if you have a high-spin black hole, supermassive black hole, that's telling us that maybe steady accretion was dominant," Pasham said during a news conference at AAS Wednesday.
Did a Supermassive Black Hole Influence the Evolution of Life on Earth?
In 1939, Albert Einstein published a paper in Annals of Mathematics, arguing that black holes do not exist in nature. A quarter of a century later, Maarten Schmidt discovered quasars as powerful sources of light at cosmological distances. These enigmatic point-like sources were explained in the mid-1960s by Yakov Zel&rsquodovich in the East and Ed Salpeter in the West as supermassive black holes that are fed with gas from their host galaxies. When gas flows towards the black hole, it swirls like water going down the drain. As the gas approaches a fraction of the speed of light at the innermost stable circular orbit (ISCO) around the black hole, it heats-up by rubbing against itself through turbulent viscosity.
Consequently, its accretion disk glows brightly, radiating away about a tenth of its rest mass and exceeding by orders of magnitude the total luminosity from stars in its host galaxy. High feeding rates make quasars visible all the way out to the edge of the visible Universe. Decades later, astronomers found that almost every galaxy hosts a supermassive black hole at its center, which is starved most of the time but bursts sporadically for merely tens of millions of years during each burst. The quasars resemble a baby that tends to remove food off the dining table as soon as it is fed by virtue of becoming too energetic.
This year, the Nobel Prize in Physics was awarded to Andrea Ghez and Reinhard Genzel for providing conclusive evidence that a black hole, albeit starved at the present time, lurks also at the center of our own Milky Way galaxy. This monster, weighing four million Suns, is dormant right now, glowing as the feeble radio source Sagittarius A* (abbreviated SgrA*), which is a billion times fainter than it would have been if it was fed as generously as a quasar.
Even though SgrA* is dim right now, we have clues that it must have experienced episodes of vigorous feeding in the past. This is not a surprise, given that a gas cloud approaching the Galactic center or a star passing within ten times the horizon scale of SgrA* (which translates to roughly the Earth-Sun separation), would get spaghettified by the strong gravitational tide there and turn into a stream of gas that triggers a quasar-like flare.
The &ldquosmoking gun&rdquo evidence for recent feeding episodes of SgrA* by massive quantities of gas is that young stars around SgrA* orbit in preferred planes. This implies that these stars formed out of planar gas disks, just like the planets in the Solar system plane or the stars in the Milky Way disk. Since the age of the stars near SgrA* is less than a percent of the age of the Milky Way galaxy, major accretion episodes from disruption of gas clouds must have occurred at least a hundred of times around SgrA*, based on the Copernican principle that the present time is not special. Indeed, a pair of giant blobs of hot gas, called the Fermi bubbles, are observed to emanate from the Galactic center along the rotation axis of the Milky Way, implying a recent accretion episode around SgrA* that could have powered them. Theoretical calculations imply that in addition to disruption of massive gas clouds, individual stars are also scattered into the vicinity of the black hole and get tidally disrupted once every ten thousand years. The intense feeding from the resulting debris streams could lead to the brightest flares from SgrA*. Such tidal disruption events of stars are indeed observed in other galaxies at the expected rate.
Would the resulting flares of SgrA* have any implications for life on Earth? In principle, they could, since they carry damaging X-ray and Ultraviolet (XUV) radiation. In collaboration with my former postdoc, John Forbes, we showed in 2018 that the XUV radiation emitted during such flares has the capacity to evaporate the atmospheres of Mars or Earth if the Solar system had only been ten times closer to the center of the Milky Way. But even at larger distances, the XUV radiation could suppress the growth of complex life, creating an effect similar to stepping on a lawn so frequently that you inhibit its growth.
At the current location of the Sun, terrestrial life is safe from XUV flares of SgrA*. However, recent studies indicate that the birthplace of the Sun may have been significantly closer to the Galactic center and that the Sun migrated to its current location through gravitational kicks. The exposure to past XUV flares from SgrA* at closer distances, could have harmed complex life during the early evolution of the Earth. This might explain why the oxygen level in the Earth&rsquos atmosphere rose to its currently high level only after two billion years, perhaps only after the Earth was sufficiently far away from SgrA*. In collaboration with Manasvi Lingam, I am currently exploring this possible connection between terrestrial life and the migration of the Sun away from the Galactic center.
Traditionally, the Sun was thought to be the only astronomical source of light that affected life on Earth. But it is also possible that the black hole, SgrA* played an important role in shaping the history of terrestrial life. A surprising realization of this sort is similar to figuring out that a stranger might have impacted your family history before you were born. If a link between SgrA* and terrestrial life can be established, then this supermassive black hole might trigger a second Nobel Prize.
ABOUT THE AUTHOR(S)
Avi Loeb is former chair (2011-2020) of the astronomy department at Harvard University, founding director of Harvard's Black Hole Initiative and director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. He also chairs the Board on Physics and Astronomy of the National Academies and the advisory board for the Breakthrough Starshot project, and is a member of President's Council of Advisors on Science and Technology. Loeb is the bestselling author of Extraterrestrial: The First Sign of Intelligent Life Beyond Earth (Houghton Mifflin Harcourt).