Sagittarius A* Archives

January 12, 2018May 11, 2018 by Matt Williams

Astronomers Figure Out How Black Holes Can Blast Out Relativistic Jets of Material Across Light Years of Space

Artist's impression of the relativistic jet emanating from a black hole. Credit: Northwestern University

Black holes have been an endless source of fascination ever since Einstein’s Theory of General Relativity predicted their existence. In the past 100 years, the study of black holes has advanced considerably, but the awe and mystery of these objects remains. For instance, scientists have noted that in some cases, black holes have massive jets of charged particles emanating from them that extend for millions of light years.

These “relativistic jets” – so-named because they propel charged particles at a fraction of the speed of light – have puzzled astronomers for years. But thanks to a recent study conducted by an international team of researchers, new insight has been gained into these jets. Consistent with General Relativity, the researchers showed that these jets gradually precess (i.e. change direction) as a result of space-time being dragged into the rotation of the black hole.

Their study, titled “Formation of Precessing Jets by Tilted Black Hole Discs in 3D General Relativistic MHD Simulations“, recently appeared in the Monthly Notices of the Royal Astronomical Society. The team consisted of members from the Anton Pannekoek Institute for Astronomy at the University of Amsterdam and a professor from the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University.

For the sake of their study, the team conducted simulations using the Blue Waters supercomputer at the University of Illinois. The simulations they conducted were the first ever to model the behavior of relativistic jets coming from Supermassive Black Holes (SMBHs). With close to a billion computational cells, it was also the highest-resolution simulation of an accreting black hole ever achieved.

As Alexander Tchekhovskoy, an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences, explained in a recent Northwestern Now press release:

“Understanding how rotating black holes drag the space-time around them and how this process affects what we see through the telescopes remains a crucial, difficult-to-crack puzzle. Fortunately, the breakthroughs in code development and leaps in supercomputer architecture are bringing us ever closer to finding the answers.”

Much like all Supermassive Black Holes, rapidly spinning SMBHs regularly engulf (aka. accrete) matter. However, rapidly spinning black holes are also known for the way they emit energy in the form of relativistic jets. The matter that feeds these black holes forms a rotating disk around them – aka. an accretion disk – which is characterized by hot, energized gas and magnetic field lines.

It is the presence of these field lines that allows black holes to propel energy in the form of these jets. Because these jets are so large, they are easier to study than the black holes themselves. In so doing, astronomers are able to understand how quickly the direction of these jets change, which reveals things about the rotation of the black holes themselves – such as the orientation and size of their rotating disks.

Advanced computer simulations are necessary when it comes to the study of black holes, largely because they are not observable in visible light and are typically very far away. For instance, the closest SMBH to Earth is Sagittarius A*, which is located about 26,000 light-years away at the center of our galaxy. As such, simulations are the only way to determine how a highly complex system like a black hole operates.

In previous simulations, scientists operated under the assumption that black hole disks were aligned. However, most SMBHs have been found to have tilted disks – i.e. the disks rotate around a separate axis than the black hole itself. This study was therefore seminal in that it showed how disks can change direction relative to their black hole, leading to precessing jets that periodically change their direction.

This was previously unknown because of the incredibly amount of computing power that is needed to construct 3-D simulations of the region surrounding a rapidly spinning black hole. With the support of a National Science Foundation (NSF) grant, the team was able to achieve this by using the Blue Waters, one of the largest supercomputers in the world.

With this supercomputer at their disposal, the team was able to construct the first black hole simulation code, which they accelerated using graphical processing units (GPUs). Thanks to this combination, the team was able to carry out simulations that had the highest level of resolution ever achieved – i.e. close to a billion computational cells. As Tchekhovskoy explained:

“The high resolution allowed us, for the first time, to ensure that small-scale turbulent disk motions are accurately captured in our models. To our surprise, these motions turned out to be so strong that they caused the disk to fatten up and the disk precession to stop. This suggests that precession can come about in bursts.”

The precession of relativistic jets could explain why light fluctuations have been observed coming from around black holes in the past – which are known as quasi-periodic oscillations (QPOs). These beams, which were first discovered by Michiel van der Klis (one of the co-authors on the study), operate in much the same way as a quasar’s beams, which appear to have a strobing effect.

This study is one of many that is being conducting on rotating black holes around the world, the purpose of which is to gain a better understanding about recent discoveries like gravitational waves, which are caused by the merger of black holes. These studies are also being applied to observations from the Event Horizon Telescope, which captured the first images of Sagittarius A*’s shadow. What they will reveal is sure to excite and amaze, and potentially deepen the mystery of black holes.

In the past century, the study of black holes has advanced considerably – from the purely theoretical, to indirect studies of the effects they have on surrounding matter, to the study of gravitational waves themselves. Perhaps one day, we might actually be able to study them directly or (if it’s not too much to hope for) peer directly inside them!

Further Reading: Northwestern Now, MNRAS

January 2, 2018 by Matt Williams

Supermassive Black Holes can Turn Star Formation On and Off in a Large Galaxy

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 study, titled “Black-Hole-Regulated Star Formation in Massive Galaxies“, recently appeared in the scientific journal Nature. Led by Ignacio Martín-Navarro, a Marie Curie Fellow at the University of California Observatories, the study team also consisted of members from the Max-Planck Institute for Astronomy and the Instituto de Astrofísica de Canarias.

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!

Further Reading: UCSC, MPIA, Nature

December 23, 2017 by Matt Williams

Mysterious Filament is Stretching Down Towards the Milky Way’s Supermassive Black Hole

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).

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.”

Further Reading: CfA, AJL

September 8, 2017September 8, 2017 by Matt Williams

Chinese Astronomers Spot Two New Hypervelocity Stars

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Further Reading: arXiv

September 5, 2017September 5, 2017 by Matt Williams

Another Monster Black Hole Found in the Milky Way

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

*Artist’s impression of two merging black holes, which has been theorized to be a source of gravitational waves. Credit: Bohn, Throwe, Hébert, Henriksson, Bunandar, Taylor, Scheel/SXS*

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

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

Further Reading: Nature Astronomy

August 11, 2017August 12, 2017 by Matt Williams

Stars Orbiting Supermassive Black Hole Show Einstein was Right Again!

Artist's impression of the orbits of three of the stars very close to the supermassive black hole at the center of the Milky Way. Credit: ESO/M. Parsa/L. Calçada

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Further Reading: ESO, Astrophysical Journal

February 22, 2017February 27, 2017 by Evan Gough

Get Ready for the First Pictures of a Black Hole’s Event Horizon

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

It might sound trite to say that the Universe is full of mysteries. But it’s true.

Chief among them are things like Dark Matter, Dark Energy, and of course, our old friends the Black Holes. Black Holes may be the most interesting of them all, and the effort to understand them—and observe them—is ongoing.

That effort will be ramped up in April, when the Event Horizon Telescope (EHT) attempts to capture our first image of a Black Hole and its event horizon. The target of the EHT is none other than Sagittarius A, the monster black hole that lies in the center of our Milky Way Galaxy. Though the EHT will spend 10 days gathering the data, the actual image won’t be finished processing and available until 2018.

The EHT is not a single telescope, but a number of radio telescopes around the world all linked together. The EHT includes super-stars of the astronomy world like the Atacama Large Millimeter Array (ALMA) as well as lesser known ‘scopes like the South Pole Telescope (SPT.) Advances in very-long-baseline-interferometry (VLBI) have made it possible to connect all these telescopes together so that they act like one big ‘scope the size of Earth.

The ALMA array in Chile. Once ALMA was added to the Event Horizon Telescope, it increased the EHT’s power by a factor of 10. Image: ALMA (ESO/NAOJ/NRAO), O. Dessibourg

The combined power of all these telescopes is essential because even though the EHT’s target, Sagittarius A, has over 4 million times the mass of our Sun, it’s 26,000 light years away from Earth. It’s also only about 20 million km across. Huge but tiny.

The EHT is impressive for a number of reasons. In order to function, each of the component telescopes is calibrated with an atomic clock. These clocks keep time to an accuracy of about a trillionth of a second per second. The effort requires an army of hard drives, all of which will be transported via jet-liner to the Haystack Observatory at MIT for processing. That processing requires what’s called a grid computer, which is a sort of virtual super-computer comprised of 800 CPUs.

But once the EHT has done its thing, what will we see? What we might see when we finally get this image is based on the work of three big names in physics: Einstein, Schwarzschild, and Hawking.

A simulation of what the EHT might show us. Image: Event Horizon Telescope Organization

As gas and dust approach the black hole, they speed up. They don’t just speed up a little, they speed up a lot, and that makes them emit energy, which we can see. That would be the crescent of light in the image above. The black blob would be a shadow cast over the light by the hole itself.

Einstein didn’t exactly predict the existence of Black Holes, but his theory of general relativity did. It was the work of one of his contemporaries, Karl Schwarzschild, that actually nailed down how a black hole might work. Fast forward to the 1970s and the work of Stephen Hawking, who predicted what’s known as Hawking Radiation.

Taken together, the three give us an idea of what we might see when the EHT finally captures and processes its data.

Einstein’s general relativity predicted that super massive stars would warp space-time enough that not even light could escape them. Schwarzschild’s work was based on Einstein’s equations and revealed that black holes will have event horizons. No light emitted from inside the event horizon can reach an outside observer. And Hawking Radiation is the theorized black body radiation that is predicted to be released by black holes.

The power of the EHT will help us clarify our understanding of black holes enormously. If we see what we think we’ll see, it confirms Einstein’s Theory of General Relativity, a theory which has been confirmed observationally over and over. If EHT sees something else, something we didn’t expect at all, then that means Einstein’s General Relativity got it wrong. Not only that, but it means we don’t really understand gravity.

In physics circles they say that it’s never smart to bet against Einstein. He’s been proven right time and time again. To find out if he was right again, we’ll have to wait until 2018.

February 14, 2017February 20, 2017 by Matt Williams

Distance & Speed Of Sun’s Orbit Around Galactic Centre Measured

Artist’s conception of the Gaia telescope backdropped by a photograph of the Milky Way taken at the European Southern Observatory. Credit: ESA/ATG medialab; background: ESO/S. Brunier

In 2013, the European Space Agency deployed the long-awaited Gaia space observatory. As one of a handful of next-generation space observatories that will be going up before the end of the decade, this mission has spent the past few years cataloging over a billion astronomical objects. Using this data, astronomers and astrophysicists hope to create the largest and most precise 3D map of the Milky Way to date.

Though it is almost to the end of its mission, much of its earliest information is still bearing fruit. For example, using the mission’s initial data release, a team of astrophysicists from the University of Toronto managed to calculate the speed at which the Sun orbits the Milky Way. From this, they were able to obtain a precise distance estimate between our Sun and the center of the galaxy for the first time.

For some time, astronomers have been unsure as to exactly how far our Solar System is from the center of our galaxy. Much of this has to do with the fact that it is impossible to view it directly, due to a combination of factors (i.e. perspective, the size of our galaxy, and visibility barriers). As a result, since the year 2000, official estimates have varied between 7.2 and 8.8 kiloparsecs (~23,483 to 28,700 light years).

Astronomy Image Gallery — *Infrared image from Spitzer Space Telescope, showing the stars at the center of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech)*

For the sake of their study, the team – which was led by Jason Hunt, a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto – combined Gaia’s initial release with data from the RAdial Velocity Experiment (RAVE). This survey, which was conducted between 2003 and 2013 by the Australian Astronomical Observatory (AAO), measured the positions, distances, radial velocities and spectra of 500,000 stars.

Over 200,000 of these stars were also observed by Gaia and information on them was included in its initial data release. As they explain in their study, which was published in the Journal of Astrophysical Letters in November 2016, they used this to examined the speeds at which these stars orbit the center of the galaxy (relative to the Sun), and in the process discovered that there was an apparent distribution in their relative velocities.

In short, our Sun moves around the center of the Milky Way at a speed of 240 km/s (149 mi/s), or 864,000 km/h (536,865 mph). Naturally, some of the more than 200,000 candidates were moving faster or slower. But for some, there was no apparent angular momentum, which they attributed to these stars being scattering onto “chaotic, halo-type orbits when they pass through the Galactic nucleus”.

As Hunt explained in Dunlap Institute press release:

“Stars with very close to zero angular momentum would have plunged towards the Galactic center where they would be strongly affected by the extreme gravitational forces present there. This would scatter them into chaotic orbits taking them far above the Galactic plane and away from the Solar neighbourhood… By measuring the velocity with which nearby stars rotate around our Galaxy with respect to the Sun, we can observe a lack of stars with a specific negative relative velocity. And because we know this dip corresponds to 0 km/sec, it tells us, in turn, how fast we are moving.”

The next step was to combine this information with proper motion calculations of Sagittarius A* – the supermassive black hole believed to be at the center of our galaxy. After correcting for its motion relative to background objects, they were able to effectively triangulate the Earth’s distance from the center of the galaxy. From this, they derived a refined distance of estimate of 7.6 to 8.2 kpc – which works out to about 24,788 to 26,745 light years.

This study builds upon previous work conducted by the study’s co-authors – Prof. Ray Calberg, the current chair of the Department of Astronomy & Astrophysics at the University of Toronto. Years ago, he and Prof. Kimmo Innanen of the Department of Physics and Astronomy at York University conducted a similar study using radial velocity measurement from 400 of the Milky Way’s stars.

But by incorporating data from the Gaia observatory, the UofT team was able to obtain a much more comprehensive data set and narrow the distance to galactic center by a significant amount. And this was based on only the initial data released by the Gaia mission. Looking ahead, Hunt anticipates that further data releases will allow his team and other astronomers to refine their calculations even more.

“Gaia’s final release in late 2017 should enable us to increase the precision of our measurement of the Sun’s velocity to within approximately one km/sec,” he said, “which in turn will significantly increase the accuracy of our measurement of our distance from the Galactic center.”

As more next-generation space telescopes and observatories are deployed, we can expect them to provide us with a wealth of new information about our Universe. And from this, we can expect that astronomers and astrophysicists will begin to shine the light on a number of unresolved cosmological questions.

Further Reading: University of Toronto, The Astrophysical Journal Letters

November 28, 2016 by Matt Williams

What is a Supermassive Black Hole?

In 1971, English astronomers Donald Lynden-Bell and Martin Rees hypothesized that a supermassive black hole (SMBH) resides at the center of our Milky Way Galaxy. This was based on their work with radio galaxies, which showed that the massive amounts of energy radiated by these objects was due to gas and matter being accreted onto a black hole at their center.

By 1974, the first evidence for this SMBH was found when astronomers detected a massive radio source coming from the center of our galaxy. This region, which they named Sagittarius A*, is over 10 million times as massive as our own Sun. Since its discovery, astronomers have found evidence that there are supermassive black holes at the centers of most spiral and elliptical galaxies in the observable Universe.

Description:

Supermassive black holes (SMBH) are distinct from lower-mass black holes in a number of ways. For starters, since SMBH have a much higher mass than smaller black holes, they also have a lower average density. This is due to the fact that with all spherical objects, volume is directly proportional to the cube of the radius, while the minimum density of a black hole is inversely proportional to the square of the mass.

In addition, the tidal forces in the vicinity of the event horizon are significantly weaker for massive black holes. As with density, the tidal force on a body at the event horizon is inversely proportional to the square of the mass. As such, an object would not experience significant tidal force until it was very deep into the black hole.

Formation:

How SMBHs are formed remains the subject of much scholarly debate. Astrophysicists largely believe that they are the result of black hole mergers and the accretion of matter. But where the “seeds” (i.e. progenitors) of these black holes came from is where disagreement occurs. Currently, the most obvious hypothesis is that they are the remnants of several massive stars that exploded, which were formed by the accretion of matter in the galactic center.

Another theory is that before the first stars formed in our galaxy, a large gas cloud collapsed into a “qausi-star” that became unstable to radial perturbations. It then turned into a black hole of about 20 Solar Masses without the need for a supernova explosion. Over time, it rapidly accreted mass in order to become an intermediate, and then supermassive, black hole.

In yet another model, a dense stellar cluster experienced core-collapse as the as a result of velocity dispersion in its core, which happened at relativistic speeds due to negative heat capacity. Last, there is the theory that primordial black holes may have been produced directly by external pressure immediately after the Big Bang. These and other theories remain theoretical for the time being.

Sagittarius A*:

Multiple lines of evidence point towards the existence of a SMBH at the center of our galaxy. While no direct observations have been made of Sagittarius A*, its presence has been inferred from the influence it has on surrounding objects. The most notable of these is S2, a star that flows an elliptical orbit around the Sagittarius A* radio source.

S2 has an orbital period of 15.2 years and reaches a minimal distance of 18 billion km (11.18 billion mi, 120 AU) from the center of the central object. Only a supermassive object could account for this, since no other cause can be discerned. And from the orbital parameters of S2, astronomers have been able to produce estimates on the size and mass of the object.

For instance, S2s motions have led astronomers to calculated that the object at the center of its orbit must have no less than 4.1 million Solar Masses (8.2 × 10³³ metric tons; 9.04 × 10³³ US tons). Furthermore, the radius of this object would have to be less than 120 AU, otherwise S2 would collide with it.

However, the best evidence to date was provided in 2008 by the Max Planck Institute for Extraterrestrial Physics and UCLAs Galactic Center Group. Using data obtained over a 16 year period by the ESO’s Very Large Telescope and Keck Telescope, they were able to not only accurately estimate the distance to the center of our galaxy (27,000 light years from Earth), but also track the orbits of the stars there with immense precision.

As Reinhard Genzel, the team leader from the Max-Planck-Institute for Extraterrestrial Physics said:

“Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”

Another indication of Sagittarius A*s presence came on January 5th, 2015, when NASA reported a record-breaking X-ray flare coming from the center of our galaxy. Based on readings from the Chandra X-ray Observatory, they reported emissions that were 400 times brighter than usual. These were thought to be the result of an asteroid falling into the black hole, or by the entanglement of magnetic field lines within the gas flowing into it.

Other Galaxies:

Astronomers have also found evidence of SMBHs at the center of other galaxies within the Local Group and beyond. These include the nearby Andromeda Galaxy (M31) and elliptical galaxy M32, and the distant spiral galaxy NGC 4395. This is based on the fact that stars and gas clouds near the center of these galaxies show an observable increase in velocity.

Another indication is Active Galactic Nuclei (AGN), where massive bursts of radio, microwave, infrared, optical, ultra-violet (UV), X-ray and gamma ray wavebands are periodically detected coming from the regions of cold matter (gas and dust) at the center of larger galaxies. While the radiation is not coming from the black holes themselves, the influence such a massive object would have on surrounding matter is believed to be the cause.

In short, gas and dust form accretion disks at the center of galaxies that orbit supermassive black holes, gradually feeding them matter. The incredible force of gravity in this region compresses the disk’s material until it reaches millions of degrees kelvin, generating bright radiation and electromagnetic energy. A corona of hot material forms above the accretion disc as well, and can scatter photons up to X-ray energies.

The interaction between the SMBH 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. at 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.

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. This interaction is likely to kick several stars out of our combined galaxy (producing rogue stars), and is also likely to cause our galactic nucleus (which is currently inactive) to become active one again.

The study of black holes is still in its infancy. And what we have learned over the past few decades alone has been both exciting and awe-inspiring. Whether they are lower-mass or supermassive, black holes are an integral part of our Universe and play an active role in its evolution.

Who knows what we will find as we peer deeper into the Universe? Perhaps some day we the technology, and sheer audacity, will exist so that we might attempt to peak beneath the veil of an event horizon. Can you imagine that happening?

We have written many interesting articles about black holes here at Universe Today. Here’s Beyond Any Reasonable Doubt: A Supermassive Black Hole Lives in Centre of Our Galaxy, X-Ray Flare Echo Reveals Supermassive Black Hole Torus, How Do You Weigh a Supermassive Black Hole? Take its Temperature, and What Happens When Supermassive Black Holes Collide?

Astronomy Cast also some relevant episodes on the subject. Here’s Episode 18: Black Holes Big and Small, and Episode 98: Quasars.

More to explore: Astronomy Cast’s episodes Quasars, and Black Holes Big and Small.

Sources:

November 8, 2016August 25, 2021 by Matt Williams

What are Active Galactic Nuclei?

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 — 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. The blue synchrotron radiation contrasts with the yellow starlight from the host galaxy. Credit: NASA/The Hubble Heritage Team (STScI/AURA) — *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.

We have written many articles about galaxies for Universe Today. Here’s What Fuels the Engine of a Supermassive Black Hole?, Could the Milky Way Become a Black Hole?, What is a Supermassive Black Hole?, Turning on a Supermassive Black Hole, What Happens when Supermassive Black Holes Collide?.

For more information, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

Astronomy Cast also has episodes about galactic nuclei and supermassive black holes. Here’s Episode 97: Galaxies and Episode 213: Supermassive Black Holes.

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