The Black Hole/Globular Cluster Correlation


Often in astronomy, one observable property traces another property which may be more difficult to observe directly; X-ray activity on stars can be used to trace turbulent heating of the photosphere. CO is used to trace cold H2. Sometimes these correlations make sense. Activities in stars produce the X-ray emissions. Other times, the tracer seems distantly related at best.

This is the case of a newly discovered correlation between the mass of the central black hole of galaxies and the number of globular clusters they contain. What can this relationship teach astronomers? Why does it hold for some types of galaxies better than others? And where does it come from in the first place.

The mass of a galaxy’s super massive black hole (SMBH) is known to have a strong relationship between many features of their host galaxies. It has identified to follow the range of velocities of stars in the galaxy, the mass and luminosity of the bulge of spiral galaxies, and the total amount of dark matter in galaxies. Because dark matter in the halo of galaxies and the luminosity have also been known to correspond to the number of globular clusters, Andreas Burkert of the Max-Planck-Institute for Extraterrestrial Physics in Germany, and Scott Tremaine at Princeton wondered if they could cut out the middlemen of dark matter and luminosity and still maintain a strong correlation between the central SMBH and the number of globular clusters.

Their initial investigation involved only 13 galaxies, but a follow-up study by Gretchen and William Harris and submitted to the Monthly Notices of the Royal Astronomical Society, increased the number of galaxies included in the survey to 33. The results of these studies indicated that for elliptical galaxies, the SMBH-GC relationship is evident. However, for lenticular galaxies there was no clear correlation. While there appeared to be a trend for classical spirals, the small number of data points (4) would not provide a strong statistical case independently, but did appear to follow the trend established by the elliptical galaxies.

Although the correlation appeared strong in most cases, about 10% of the galaxies included in the larger surveys were clear outliers. This included the Milky Way which has a SMBH mass that falls significantly short of the expectation from cluster number. One source of error the authors of the original study suspect is that it is possible that, in some cases, objects identified as globular clusters may have been misidentified and in actuality, be the cores of tidally stripped dwarf galaxies. Regardless, the relationship as it stands presently, seems to be quite strong and is even more tightly defined than that of the correlation between that of the SMBH mass and velocity dispersion that implied the potential relationship in the first place. The reason for the discordance in lenticular galaxies has not yet been explained and no reasons have yet been postulated.

But what of the cause of this unusual relation? Both sets of authors suggest the connection lies in the formation of the objects. While distinct in most respects, both are fed by major merger events; Black holes gain mass by accreting gas and globular clusters are often formed from the resulting shocks and interactions. Additionally, the majority of both types of objects formed at high redshifts.


A correlation between central supermassive black holes and the globular cluster systems of early-type galaxies

The Globular Cluster/Central Black Hole Connection in Galaxies

Powerhouse Black Hole Blows a Huge Bubble

Combining observations done with ESO's Very Large Telescope and NASA's Chandra X-ray telescope, astronomers have uncovered the most powerful pair of jets ever seen from a stellar black hole. The black hole blows a huge bubble of hot gas, 1,000 light-years across or twice as large and tens of times more powerful than the other such microquasars. The stellar black hole belongs to a binary system as pictured in this artist's impression. Credit: ESO/L. Calçada


A relatively small black hole is producing tremendously powerful jets while creating a huge bubble of hot gas. Both the jets and the bubble are the largest ever seen, meaning this mini black hole is a powerhouse. But the most unusual feature of this remarkable black hole is not its energy output, but how it is emitting energy.

“The energy output is impressive, but is comparable with the X-ray luminosity of so-called Ultraluminous X-ray sources,” said Manfred Pakull, the lead author of a new paper published today in Nature. “The notion that powerhouses exist that generate most of their energy in the form of jets (kinetic energy) and not as radiation (photons) is rather new.”

Black holes are known to release an incredible amount of energy when they swallow matter, and as Pakull told Universe Today, it was previously thought that most of the energy came out in the form of radiation, predominantly X-rays. But this new gas-blowing black hole, called S26, is showing that some black holes can release at least as much energy, and perhaps much more, in the form of collimated jets of fast moving particles.

“This black hole is just a few solar masses, but is a real miniature version of the most powerful quasars and radio galaxies,” said Pakull, “which contain black holes with masses of a few million times that of the Sun.”

This object is a microquasar, which are formed by two objects — either a white dwarf, neutron star or a black hole, along with a companion star. The X-rays are produced by matter falling from one component to the other, and can produce jets of high-speed particles. The fast jets slam into the surrounding interstellar gas, heating it and triggering an expanding bubble made of hot gas and ultra-fast particles colliding at different temperatures.

Of the dozen or so microquasars that have been found in the Milky Way Galaxy, most of the bubbles are fairly small, – less than 10 light-years across. But this one is 1,000 light-years wide. Plus this microquasar is tens of times more powerful than ones previously seen.

Using ESO’s Very Large Telescope and NASA’s Chandra X-ray telescope Pakull and his team were able to observe the areas where the jets smash into the interstellar gas around the black hole, and saw that the bubble of hot gas is inflating at a speed of almost one million kilometers per hour.

The jets are equally impressive, about 300 parsecs long, and although powerful jets have been seen from supermassive black holes, they were thought to be less frequent in the smaller microquasar variety. This new discovery may have astronomers looking more closely at other microquasars.

“The length of the jets in NGC 7793 is amazing, compared to the size of the black hole from which they are launched,” said co-author Robert Soria. “If the black hole were shrunk to the size of a soccer ball, each jet would extend from the Earth to beyond the orbit of Pluto.”

S26 is located 12 million light-years away, in the outskirts of the spiral galaxy NGC 7793. From the size and expansion velocity of the bubble the astronomers have found that the jet activity must have been ongoing for at least 200,000 years.

With all this incredible speed, size and activity, what do Pakull and his team project as the future of this microquasar?

“Yes, the expansion velocity (275 km/s) is quite impressive, but it will diminish with time,” Pakull told Universe Today. “If it was much lower at, say, 70 km/s the shocked gas would not emit so much optical light (for example the Balmer series of Hydrogen) and we would not have detected the bubble. The future of S26 depends on the evolution of the central microquasar which emits the jets. I expect that it could be active for another 100,000 to few million years.”

Pakull said it is interesting to imagine what would happen if the microqusar suddenly stopped emitting the jets. “Then the bubble would not suddenly disappear, but shine on like before for another few 100,000 years,” he said. “It would resemble a supernova remnant, albeit with a 100 times higher energy content.”

Pakull added that this new finding will help astronomers understand the similarity between small black holes formed from exploded stars and the supermassive black holes at the centers of galaxies, and he hopes this work will stimulate more theoretical work in how black holes produce energy.

Read the team’s paper (pdf file)

Sources: ESO, email exchange with Manfred Pakull.

Galaxy Mergers Make Black Holes ‘Light Up’

Only about 1% of supermassive black holes emit large amounts of energy, and astronomers have wondered for decades why so few exhibit this behavior. Data from Swift satellite, which normally studies gamma ray bursts, has allowed scientists to confirm that black holes “light up” when galaxies collide, and the data may offer insight into the future behavior of the black hole in our own Milky Way galaxy.

The intense emission from galaxy centers, or nuclei, arises near a supermassive black hole containing between a million and a billion times the sun’s mass. Giving off as much as 10 billion times the sun’s energy, some of these active galactic nuclei (AGN) are the most luminous objects in the universe. They include quasars and blazars.

“Theorists have shown that the violence in galaxy mergers can feed a galaxy’s central black hole,” said Michael Koss, the study’s lead author and a graduate student at the University of Maryland in College Park. “The study elegantly explains how the black holes switched on.”

Swift was launched in 2004, and while its Burst Alert Telescope (BAT) is waiting to detect the next gamma ray burst, it also has been mapping the sky using hard X-rays, said Neil Gehrels, Swift’s principal investigator. “In fact, it detected its 508th gamma ray burst about 30 minutes ago,” Gehrels said at the press conference the morning of May 26th at the 216th meeting of the American Astronomical Society. “But building up its exposure year after year, the Swift BAT Hard X-ray Survey is the largest, most sensitive and complete census of the sky at these energies.”

Until this hard X-ray survey, astronomers never could be sure they had counted the majority of the AGN. Thick clouds of dust and gas surround the black hole in an active galaxy, which can block ultraviolet, optical and low-energy, or soft X-ray, light. Infrared radiation from warm dust near the black hole can pass through the material, but it can be confused with emissions from the galaxy’s star-forming regions. Hard X-rays can help scientists directly detect the energetic black hole.


The survey, which is sensitive to AGN as far as 650 million light-years away, uncovered dozens of previously unrecognized systems.

“The Swift BAT survey is giving us a very different picture of AGN,” Koss said. The team finds that about a quarter of the BAT galaxies are in mergers or close pairs. “Perhaps 60 percent of these galaxies will completely merge in the next billion years. We think we have the ‘smoking gun’ for merger-triggered AGN that theorists have predicted.”

“A big problem in astronomy is understanding how black holes grow and are fed,” said Joel Bregman from the University of Michigan. “We know growth in the early stages of a black hole’s life is a combination of mergers plus accretion of gas and dust from nearby stars, and we think that the accretion is the more important process. But this shows us that the feeding of the gas and dust has been channeled into the center at a fairly early stage, and the disturbance from the mergers allows gas to be funneled into the center and flow into the black hole.”

“We’ve never seen the onset of AGN activity so clearly,” said Bregman, who was not involved in the study. “The Swift team must be identifying an early stage of the process with the Hard X-ray Survey.”

Other members of the study team include Richard Mushotzky and Sylvain Veilleux at the University of Maryland and Lisa Winter at the Center for Astrophysics and Space Astronomy at the University of Colorado in Boulder.

The study will appear in the June 20 issue of The Astrophysical Journal Letters.

Source: NASA, NASA press conference

Black Hole Gets Kicked Out of Galaxy


Supermassive black holes are thought to lie at the center of most large galaxies. But off in a distant remote galaxy, astronomers have possibly found a giant black hole that appears to be in the process of being expelled from the galaxy at high speed. This newly-discovered object was found by Marianne Heida, a student at Utrecht University in the Netherlands, and confirmed by an international team of astronomers who say the black hole was likely kicked out of its galaxy as a result of the merger of two smaller black holes.

Heida discovered the bizarre object, called CXO J122518.6+144545 during her final undergraduate project while doing research at the SRON Netherlands Institute for Space Research. To make the discovery she had to compare hundreds of thousands of X-ray sources, picked up by chance, with the positions of millions of galaxies. X-rays are also able to penetrate the dust and gas that surround black holes, with the bright source appearing as a starlike point. This object was very bright; however, it wasn’t at the center of a galaxy.

Super-massive black holes easily weigh more than 1 billion times the mass of the sun. So how could such a heavy object be hurled away from the galaxy at such high speeds? Astronomers say the expulsion can take place under special conditions when two black holes merge. The merger process creates a new black hole, and supercomputer models suggest that the larger black hole that results is shot out away at high speed, depending on the direction and speed in which the two black holes rotate before their collision.

And, the team of astronomers say, there could be more of these “recoiling” black holes out there. “We have found even more of this strange class of X-ray sources,” said Heida. “However, for these objects we first of all need accurate measurements from NASA’s Chandra satellite to pinpoint them more precisely.”

If this object is not a recoiling black hole, other possibilities are that it could possibly be either a very blue type IIn supernova or a ULX (ultra-luminous X-ray source) with a very bright optical counterpart.

Finding more of these expelled black holes will provide a better understanding of the characteristics of black holes before they merge. In the future, astronomers hope to even observe this process with the planned LISA satellite, which will be able to measure the gravity waves that the two merging black holes emit. Further research will provide more insight into how supermassive black holes are created.

Paper: “A bright off-nuclear X-ray source: a type IIn supernova, a bright ULX or a recoiling super-massive black hole in CXO J122518.6+144545”.

Sources: SRON, Royal Astronomical Society

Is Our Universe Inside Another Larger Universe?


A wormhole is a hypothetical “tunnel” connecting two different points in spacetime, and in theory, at each end of the wormhole there could be two universes. Theoretical physicist Nikodem Poplawski from Indiana University has taken things a step further by proposing that perhaps our universe could be located within the interior of a wormhole which itself is part of a black hole that lies within a much larger universe.

Whoa. I may have just lost my bearings.

As crazy as the concept of wormholes sounds, it does offer solutions to the equations of Einstein’s general theory of relativity. In fact, wormholes – also called an Einstein-Rosen Bridge — offer such a great solution that some theorists think that real wormholes may eventually be found or even created, and perhaps they could even be used for high-speed travel between two areas in space, or maybe even time travel.

However, a known property of wormholes is that they are highly unstable and would probably collapse instantly if even the tiniest amount of matter, such as a single photon, tried to travel though them.

But would it work – and could matter exist — if we were inside a wormhole inside a black hole inside another universe? Poplawski thinks so. He takes advantage of the Euclidean-based coordinate system called isotropic coordinates to describe the gravitational field of a black hole and to model the radial geodesic motion of a massive particle into a black hole.

“This condition would be satisfied if our universe were the interior of a black hole existing in a bigger universe,” Poplawski said. “Because Einstein’s general theory of relativity does not choose a time orientation, if a black hole can form from the gravitational collapse of matter through an event horizon in the future then the reverse process is also possible. Such a process would describe an exploding white hole: matter emerging from an event horizon in the past, like the expanding universe.”

So, a white hole would be connected to a black hole a wormhole, and is hypothetically the time reversal of a black hole. (Oh my, I’m now dizzy…)

Poplawski’s paper suggests that all astrophysical black holes, not just Schwarzschild and Einstein-Rosen black holes, may have Einstein-Rosen bridges, each with a new universe inside that formed simultaneously with the black hole.

“From that it follows that our universe could have itself formed from inside a black hole existing inside another universe,” he said.

IU theoretical physicist Nikodem Poplawski. Credit: Indiana University

By continuing to study the gravitational collapse of a sphere of dust in isotropic coordinates, and by applying the current research to other types of black holes, views where the universe is born from the interior of an Einstein-Rosen black hole could avoid problems seen by scientists with the Big Bang theory and the black hole information loss problem which claims all information about matter is lost as it goes over the event horizon (in turn defying the laws of quantum physics).

Poplawski theorizes that this model in isotropic coordinates of the universe as a black hole could explain the origin of cosmic inflation.

Could this be tested? Well, there is the issue that to see if an object could travel through a wormhole, the observer would have to be inside the wormhole as well, since the interior cannot be observed unless an observer enters or resides within.

A possible solution is that exotic matter wouldn’t collapse the wormhole, so we’d have to create – and be made of – exotic matter to keep the it open. But perhaps, as Poplawski proposes, if the wormhole is inside a black hole inside another universe it would work.

Anyone ready to give it a try?

Radial motion into an Einstein-Rosen bridge,” Physics Letters B, by Nikodem J. Poplawski. (Volume 687, Issues 2-3, 12 April 2010, Pages 110-113.

Sources: Indiana University
, Internet Encyclopedia of Science

Astronomers Find Black Holes Do Not Absorb Dark Matter


There’s the common notion that black holes suck in everything in the nearby vicinity by exerting a strong gravitational influence on the matter, energy, and space surrounding them. But astronomers have found that the dark matter around black holes might be a different story. Somehow dark matter resists ‘assimilation’ into a black hole.

About 23% of the Universe is made up of mysterious dark matter, invisible material only detected through its gravitational influence on its surroundings. In the early Universe clumps of dark matter are thought to have attracted gas, which then coalesced into stars that eventually assembled the galaxies we see today. In their efforts to understand galaxy formation and evolution, astronomers have spent a good deal of time attempting to simulate the build up of dark matter in these objects.

Dr. Xavier Hernandez and Dr. William Lee from the National Autonomous University of Mexico (UNAM) calculated the way in which the black holes found at the center of galaxies absorb dark matter. These black holes have anything between millions and billions of times the mass of the Sun and draw in material at a high rate.

The researchers modeled the way in which the dark matter is absorbed by black holes and found that the rate at which this happens is very sensitive to the amount of dark matter found in the black holes’ vicinity. If this concentration were larger than a critical density of 7 Suns of matter spread over each cubic light year of space, the black hole mass would increase so rapidly, hence engulfing such large amounts of dark matter, that soon the entire galaxy would be altered beyond recognition.

“Over the billions of years since galaxies formed, such runaway absorption of dark matter in black holes would have altered the population of galaxies away from what we actually observe,” said Hernandez

Their work therefore suggests that the density of dark matter in the centers of galaxies tends to be a constant value. By comparing their observations to what current models of the evolution of the Universe predict, Hernandez and Lee conclude that it is probably necessary to change some of the assumptions that underpin these models – dark matter may not behave in the way scientists thought it did.

There work appears in the journal Monthly Notices of the Royal Astronomical Society.

The team’s paper can be found here.

Can a Really, Really Fast Spacecraft Turn Into A Black Hole?

This question was posed in an Astronomy Cast episode a while back. It offers an interesting thought experiment, although a reasonably definitive answer to the question can be arrived at. 

Imagine a scenario where a spacecraft gains relativistic mass as it approaches the speed of light, while at the same time its volume is reduced via relativistic length contraction. If these changes can continue towards infinite values (which they can) – it seems you have the perfect recipe for a black hole

Of course, the key word here is relativistic. Back on Earth, it can appear that a spacecraft which is approaching the speed of light, is indeed both gaining mass and shrinking in volume. Also, light from the spacecraft will become increasingly red-shifted – potentially into almost-blackness. This can be partly Doppler effect for a receding spacecraft, but is also partly a time dilation effect where the sub-atomic particles of the spacecraft seem to oscillate slower and hence emit light at lower frequencies. 

So, back on Earth, ongoing measurements may indicate the spacecraft is becoming more massive, more dense and much darker as its velocity increases. 

But of course, that’s just back on Earth. If we sent out two such spacecraft flying in formation – they could look across at each other and see that everything was quite normal. The captain might call a red alert when they look back towards Earth and see that it is starting to turn into a black hole – but hopefully the future captains of our starships will have enough knowledge of relativistic physics not to be too concerned. 

So, one answer to the Astronomy Cast question is that yes, a very fast spacecraft can appear to be almost indistinguishable from a black hole – from a particular frame (or frames) of reference. 

But it’s never really a black hole. 

Centaurus A with jets powered by a supermassive black hole within - the orange jets are as seen in submillimetre by the Atacama Pathfinder and the blue lobes are as seen by the Chandra X-ray space telescope.

Special relativity allows you to calculate transformations from your proper mass (as well as proper length, proper volume, proper density etc) as your relative velocity changes. So, it is certainly possible to find a point of reference from which your relativistic mass (length, volume, density etc) will seem to mimic the parameters of a black hole. 

But a real black hole is a different story. Its proper mass and other parameters are already those of a black hole – indeed you won’t be able to find a point of reference where they aren’t. 

A real black hole is a real black hole – from any frame of reference. 

(I must acknowledge my Dad – Professor Graham Nerlich, Emeritus Professor of Philosophy, University of Adelaide and author of The Shape of Space, for assistance in putting this together).

GRB Central Engines Observed in Nearby Supernovae?

Are the relativistic jets of long gamma ray bursts (GRBs) produced by brand new black holes? Do some core-collapse supernovae result in black holes and relativistic jets?

The answer to both questions is ‘very likely, yes’! And what recent research points to those answers? Study of an Ic supernova (SN 2007gr), and an Ibc one (SN 2009bb), by two different teams, using archived Gamma-Ray Burst Coordination Network data, and trans-continental Very Long Baseline Interferometry (VLBI) radio observations.

“In every respect, these objects look like gamma-ray bursts – except that they produced no gamma rays,” said Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Soderberg led a team that studied SN 2009bb, a supernova discovered in March 2009. It exploded in the spiral galaxy NGC 3278, located about 130 million light-years away.

SN 2007gr (Image Credit: Z. Paragi, Joint Institute for VLBI in Europe (JIVE))

The other object is SN 2007gr, which was first detected in August 2007 in the spiral galaxy NGC 1058, some 35 million light-years away (it’s one of the closest Ic supernovae detected in the radio waveband). The team which studied this supernova using VLBI was led by Zsolt Paragi at the Netherlands-based Joint Institute for Very Long Baseline Interferometry in Europe, and included Chryssa Kouveliotou, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

The researchers searched for gamma-rays associated with the supernovae using archived records in the Gamma-Ray Burst Coordination Network located at NASA’s Goddard Space Flight Center in Greenbelt, Md. This project distributes and archives observations of gamma-ray bursts by NASA’s SWIFT spacecraft, the Fermi Gamma-ray Space Telescope and many others. However, no bursts coincided with the supernovae.

“The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent the speed of light,” explained Kouveliotou, co-author of one of the new studies. “Yet, in these new objects, we’re tracking gas moving some 20 times faster than this.”

Unlike typical core-collapse supernovae, the stars that produce long gamma-ray bursts possess a “central engine” – likely a nascent black hole – that drives particle jets clocked at more than 99 percent the speed of light (short GRBs are likely produced by the collision/merger of two neutron stars, or a neutron star and a stellar mass black hole).

By contrast, the fastest outflows detected from SN 2009bb reached 85 percent of the speed of light and SN 2007gr reached more than 60 percent of light speed; this is “mildly relativistic”.

“These observations are the first to show some supernovae are powered by a central engine,” Soderberg said. “These new radio techniques now give us a way to find explosions that resemble gamma-ray bursts without relying on detections from gamma-ray satellites.”

The VLBI radio observations showcase how the new electronic capabilities of the European VLBI Network empower astronomers to react quickly when transient events occur. The team led by Paragi included 14 members from 12 institutions spread over seven countries, the United States, the Netherlands, Hungary, the United Kingdom, Canada, Australia and South Africa.

“Using the electronic VLBI technique eliminates some of the major issues,” said Huib Jan van Langevelde, the director of JIVE “Moreover it allows us to produce immediate results necessary for the planning of additional measurements.”

Perhaps as few as one out of every 10,000 supernovae produce gamma rays that we detect as a long gamma-ray burst. In some cases, the star’s jets may not be angled in a way to produce a detectable burst; in others, the energy of the jets may not be enough to allow them to blast through the overlying bulk of the dying star.

“We’ve now found evidence for the unsung crowd of supernovae – those with relatively dim and mildly relativistic jets that only can be detected nearby,” Kouveliotou said. “These likely represent most of the population.”

The 28 January, 2010 issue of Nature contains two papers reporting these discoveries: A relativistic type Ibc supernova without a detected γ-ray burst (arXiv:0908.2817 is the preprint), and A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr (arXiv:1001.5060 is the preprint).

Sources: Newborn Black Holes May Add Power to Many Exploding Stars, Newborn Black Holes Boost Explosive Power of Supernovae

Extra-Galactic Whopper Black Hole Breaks Distance Record


Not only is a newly found black huge, it also is the most far-away stellar-mass black hole ever detected. “This is the most distant stellar-mass black hole ever weighed, and it’s the first one we’ve seen outside our own galactic neighborhood, the Local Group,” said Paul Crowther, from the University of Sheffield. Using ESO’s Very Large Telescope, astronomers peered six million light-years from Earth into a spiral galaxy called NGC 300 and found a black hole with a mass above fifteen times that of the Sun. This makes it the second most massive stellar-mass black hole ever found. But soon it could get bigger. The black hole appears to have a nearby partner, a massive Wolf–Rayet star which likely will become a black hole itself, and the two black holes could merge into an even more massive object.

This image obtained with the FORS2 instrument on the VLT is centred on the position of the black hole. The image covers a field of view of about 2x2 arcminutes, or about 4000 light-years at the distance of NGC 300. Credit: ESO/P. Crowther

In 2007, an X-ray source in NGC 300 was discovered with the XMM-Newton X-ray observatory and the Swift Observatory. “We recorded periodic, extremely intense X-ray emission, a clue that a black hole might be lurking in the area,” said team member Stefania Carpano from ESA.

Subsequent observations with the VLT’s FORS2 instrument (a visual and near UV FOcal Reducer and low dispersion Spectrograph) confirmed their hunch, but also showed that the black hole and the Wolf–Rayet star circled each other every 32 hours. The astronomers also found that the black hole is stripping matter away from the star as they orbit each other.

“This is indeed a very ‘intimate’ couple,” said collaborator Robin Barnard. “How such a tightly bound system has been formed is still a mystery.”

Artists impression of the black hole and Wolf-Rayet star in NGC 300. Credit: ESO

Stellar-mass black holes are the extremely dense, final remnants of the collapse of very massive stars. These black holes have masses up to around twenty times the mass of the Sun, as opposed to supermassive black holes, found in the center of most galaxies, which can weigh a million to a billion times as much as the Sun. So far, around 20 stellar-mass black holes have been found.

Only one other system of this type has previously been seen, but other systems comprising a black hole and a companion star are not unknown to astronomers. Based on these systems, the astronomers see a connection between black hole mass and galactic chemistry.

“We have noticed that the most massive black holes tend to be found in smaller galaxies that contain less ‘heavy’ chemical elements,” said Crowther. “Bigger galaxies that are richer in heavy elements, such as the Milky Way, only succeed in producing black holes with smaller masses.”

Astronomers believe that a higher concentration of heavy chemical elements influences how a massive star evolves, increasing how much matter it sheds, resulting in a smaller black hole when the remnant finally collapses.

In less than a million years, it will be the Wolf–Rayet star’s turn to go supernova and become a black hole. “If the system survives this second explosion, the two black holes will merge, emitting copious amounts of energy in the form of gravitational waves as they combine,” said Crowther.

But this won’t happen for a few billion years. “Our study does however show that such systems might exist, and those that have already evolved into a binary black hole might be detected by probes of gravitational waves, such as LIGO or Virgo.”

Paper: NGC 300 1-X is a Wolf-Rayet/Black Hole Binary

Source: ESO

Dual Black Holes Spinning in a Cosmic Dance – Complete with Disco Ball

Caption: An image of the galaxy COSMOS J100043.15+020637.2 taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Image courtesy Dr. Julia Comerford.

Astronomers have discovered 33 pairs of merging black holes in cosmic dances around each other, a finding that was predicted or ‘choreographed’ by Isaac Newton. “These results are significant because we now know that these ‘waltzing’ black holes are much more common than previously known,” said Dr. Julia Comerford of the University of California, Berkeley, at the American Astronomical Society meeting in Washington, DC. “Galaxy mergers are causing the waltzing, can use this finding to determine how often mergers occur. The black holes dancing towards us are shifted towards blue light, and those moving away from us are shifted toward the red. So it is like a cosmic disco ball showing us where the black holes are dancing.”

The dances are occurring in dual black holes, which are different from binary black holes in that the distance between the two object is much larger for dual black holes.

“These black holes have a separation of a kilo parsec,” said Comerford. “You haven’t heard about lots of small binary black holes, because no one has definitively found any yet. But this is the next best thing. We know these duals are going to merge and can use models to find out how often they merge.”

The team was able to observe the black holes that have gas collapsing onto them, and this gas releases energy and powers each black hole as an active galactic nucleus (AGN), which lights up the black hole like a Christmas tree.

Astronomical observations have shown that nearly every galaxy has a central supermassive black hole (with a mass of a million to a billion times the mass of the Sun), and also that galaxies commonly collide and merge to form new, more massive galaxies. As a consequence of these two observations, a merger between two galaxies should bring two supermassive black holes to the new, more massive galaxy formed from the merger. The two black holes gradually in-spiral toward the center of this galaxy, engaging in a gravitational tug-of-war with the surrounding stars. The result is a black hole dance. Such a dance is expected to occur in our own Milky Way Galaxy in about 3 billion years, when it collides with the Andromeda Galaxy.

The team of astronomers used two new techniques to discover the waltzing black holes. First, they identified waltzing black holes and their velocities by the disco ball of the red-shift or blue-shift.

The second technique for identifying waltzing black holes through a chance discovery of a curious-looking galaxy. While visually inspecting images of galaxies taken with the Advanced Camera for Surveys on the Hubble Space Telescope, the team noticed a galaxy with a tidal tail of stars, gas, and dust, an unmistakable sign that the galaxy had recently merged with another galaxy, and the galaxy also featured two bright nuclei near its center. The team recognized that the two bright nuclei might be the AGNs of two waltzing black holes, a hypothesis seemingly supported by the recent galaxy merger activity evinced by the tidal tail. To test this hypothesis, the very next night the team obtained a spectrum of the galaxy with the DEIMOS spectrograph on the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii.

The spectrum showed that the two central nuclei in the galaxy were indeed both AGNs, supporting the team’s hypothesis that the galaxy has two supermassive black holes. The black holes may be waltzing within the host galaxy, or the galaxy may have a recoiling black hole kicked out of the galaxy by gravity wave emission; additional observations are necessary to distinguish between these explanations.

Comerford said these new techniques can be used to find many more waltzing pairs in the future.

Source: AAS, Dr. Julia Comerford’s website