# Does 2 merging black holes necessarily make a quasar?

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2 black holes are about to merge to become a larger black hole. Does this mean it'll become a quasar?

From what I know, quasars are supermassive black holes or a collective amount of them. So, if it merges and becomes a supermassive black hole, does this mean that it is a quasar? If the size is smaller than what would be considered as a supermassive black hole would it not be a quasar?

Does 2 merging black holes necessarily make a quasar?

Basically no. While the merging of 2 black holes is a very interesting event, a quasar is what you get when 1 very large black hole eats a whole bunch of matter and the light from the quasar comes from the intense heat and interactions from that tightly bound, rapidly spiraling and very excited matter.

Quasar's were probably most common when galaxies are young but there are a few more recent ones (see examples in comments). Quasar

Related This Site Question

If the size is smaller than what would be considered as a supermassive black hole would it not be a quasar?

Almost all large galaxies have a super-massive black hole in their center. Source. The sizes vary with the size of the galaxy. Quasars are much more rare, only in a few galaxies.

As for the specific size of black hole that can form a quasar, credit to Rob Jeffries below.

## Hubble Spots Double Quasars In Merging Galaxies

This artist's conception shows the brilliant light of two quasars residing in the cores of two galaxies that are in the chaotic process of merging. The gravitational tug-of-war between the two galaxies stretches them, forming long tidal tails and igniting a firestorm of starbirth. Quasars are brilliant beacons of intense light from the centers of distant galaxies. They are powered by supermassive black holes voraciously feeding on infalling matter. This feeding frenzy unleashes a torrent of radiation that can outshine the collective light of billions of stars in the host galaxy. In a few tens of millions of years, the black holes and their galaxies will merge, and so will the quasar pair, forming an even more massive black hole. A similar sequence of events will happen a few billion years from now when our Milky Way galaxy merges with the neighboring Andromeda galaxy. CREDIT NASA, ESA, and J. Olmsted (STScI)

NASA's Hubble Space Telescope is "seeing double." Peering back 10 billion years into the universe's past, Hubble astronomers found a pair of quasars that are so close to each other they look like a single object in ground-based telescopic photos, but not in Hubble's crisp view.

The researchers believe the quasars are very close to each other because they reside in the cores of two merging galaxies. The team went on to win the "daily double" by finding yet another quasar pair in another colliding galaxy duo.

A quasar is a brilliant beacon of intense light from the center of a distant galaxy that can outshine the entire galaxy. It is powered by a supermassive black hole voraciously feeding on inflating matter, unleashing a torrent of radiation.

"We estimate that in the distant universe, for every 1,000 quasars, there is one double quasar. So finding these double quasars is like finding a needle in a haystack," said lead researcher Yue Shen of the University of Illinois at Urbana-Champaign.

The discovery of these four quasars offers a new way to probe collisions among galaxies and the merging of supermassive black holes in the early universe, researchers say.

Quasars are scattered all across the sky and were most abundant 10 billion years ago. There were a lot of galaxy mergers back then feeding the black holes. Therefore, astronomers theorize there should have been many dual quasars during that time.

"This truly is the first sample of dual quasars at the peak epoch of galaxy formation with which we can use to probe ideas about how supermassive black holes come together to eventually form a binary," said research team member Nadia Zakamska of Johns Hopkins University in Baltimore, Maryland.

The team's results appeared in the April 1 online issue of the journal Nature Astronomy.

Shen and Zakamska are members of a team that is using Hubble, the European Space Agency's Gaia space observatory, and the Sloan Digital Sky Survey, as well as several ground-based telescopes, to compile a robust census of quasar pairs in the early universe.

The observations are important because a quasar's role in galactic encounters plays a critical part in galaxy formation, the researchers say. As two close galaxies begin to distort each other gravitationally, their interaction funnels material into their respective black holes, igniting their quasars.

Over time, radiation from these high-intensity "light bulbs" launch powerful galactic winds, which sweep out most of the gas from the merging galaxies. Deprived of gas, star formation ceases, and the galaxies evolve into elliptical galaxies.

"Quasars make a profound impact on galaxy formation in the universe," Zakamska said. "Finding dual quasars at this early epoch is important because we can now test our long-standing ideas of how black holes and their host galaxies evolve together."

Astronomers have discovered more than 100 double quasars in merging galaxies so far. However, none of them is as old as the two double quasars in this study.

The Hubble images show that quasars within each pair are only about 10,000 light-years apart. By comparison, our Sun is 26,000 light-years from the supermassive black hole in the center of our galaxy.

The pairs of host galaxies will eventually merge, and then the quasars also will coalesce, resulting in an even more massive, single solitary black hole.

Finding them wasn't easy. Hubble is the only telescope with vision sharp enough to peer back to the early universe and distinguish two close quasars that are so far away from Earth. However, Hubble's sharp resolution alone isn't good enough to find these dual light beacons.

Astronomers first needed to figure out where to point Hubble to study them. The challenge is that the sky is blanketed with a tapestry of ancient quasars that flared to life 10 billion years ago, only a tiny fraction of which are dual. It took an imaginative and innovative technique that required the help of the European Space Agency's Gaia satellite and the ground-based Sloan Digital Sky Survey to compile a group of potential candidates for Hubble to observe.

Located at Apache Point Observatory in New Mexico, the Sloan telescope produces three-dimensional maps of objects throughout the sky. The team pored through the Sloan survey to identify the quasars to study more closely.

The researchers then enlisted the Gaia observatory to help pinpoint potential double-quasar candidates. Gaia measures the positions, distances, and motions of nearby celestial objects very precisely. But the team devised a new, innovative application for Gaia that could be used for exploring the distant universe. They used the observatory's database to search for quasars that mimic the apparent motion of nearby stars. The quasars appear as single objects in the Gaia data. However, Gaia can pick up a subtle, unexpected "jiggle" in the apparent position of some of the quasars it observes.

The quasars aren't moving through space in any measurable way, but instead their jiggle could be evidence of random fluctuations of light as each member of the quasar pair varies in brightness. Quasars flicker in brightness on timescales of days to months, depending on their black hole's feeding schedule.

This alternating brightness between the quasar pair is similar to seeing a railroad crossing signal from a distance. As the lights on both sides of the stationary signal alternately flash, the sign gives the illusion of "jiggling."

When the first four targets were observed with Hubble, its crisp vision revealed that two of the targets are two close pairs of quasars. The researchers said it was a "light bulb moment" that verified their plan of using Sloan, Gaia, and Hubble to hunt for the ancient, elusive double powerhouses.

Team member Xin Liu of the University of Illinois at Urbana-Champaign called the Hubble confirmation a "happy surprise." She has long hunted for double quasars closer to Earth using different techniques with ground-based telescopes. "The new technique can not only discover dual quasars much further away, but it is much more efficient than the methods we've used before," she said.

Their Nature Astronomy article is a "proof of concept that really demonstrates that our targeted search for dual quasars is very efficient," said team member Hsiang-Chih Hwang, a graduate student at Johns Hopkins University and the principal investigator of the Hubble program. "It opens a new direction where we can accumulate a lot more interesting systems to follow up, which astronomers weren't able to do with previous techniques or datasets."

The team also obtained follow-up observations with the National Science Foundation NOIRLab's Gemini telescopes. "Gemini's spatially-resolved spectroscopy can unambiguously reject interlopers due to chance superpositions from unassociated star-quasar systems, where the foreground star is coincidentally aligned with the background quasar," said team member Yu-Ching Chen, a graduate student at the University of Illinois at Urbana-Champaign.

Although the team is convinced of their result, they say there is a slight chance that the Hubble snapshots captured double images of the same quasar, an illusion caused by gravitational lensing. This phenomenon occurs when the gravity of a massive foreground galaxy splits and amplifies the light from the background quasar into two mirror images. However, the researchers think this scenario is highly unlikely because Hubble did not detect any foreground galaxies near the two quasar pairs.

Galactic mergers were more plentiful billions of years ago, but a few are still happening today. One example is NGC 6240, a nearby system of merging galaxies that has two and possibly even three supermassive black holes. An even closer galactic merger will occur in a few billion years when our Milky Way galaxy collides with neighboring Andromeda galaxy. The galactic tussle would likely feed the supermassive black holes in the core of each galaxy, igniting them as quasars.

Future telescopes may offer more insight into these merging systems. NASA's James Webb Space Telescope, an infrared observatory scheduled to launch later this year, will probe the quasars' host galaxies. Webb will show the signatures of galactic mergers, such as the distribution of starlight and the long streamers of gas pulled from the interacting galaxies.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Artist's Illustration: NASA, ESA, and J. Olmsted (STScI)

Science: NASA, ESA, Y. Shen and X. Liu (University of Illinois, Urbana-Champaign), and H.-C. Hwang and N. Zakamska (Johns Hopkins University)

## Merging black holes responsible for mysterious flickering quasar

Columbia researchers predict that a pair of converging supermassive black holes in the Virgo constellation will collide sooner than expected. Above, an artist’s conception of a merger. Illustration credit: P. Marenfeld/NOAO/AURA/NSF. Earlier this year, astronomers discovered what appeared to be a pair of supermassive black holes circling toward a collision so powerful it would send a burst of gravitational waves surging through the fabric of space-time itself.

Now, in a study in the journal Nature, astronomers at Columbia University provide additional evidence that a pair of closely orbiting black holes is causing the rhythmic flashes of light coming from quasar PG 1302-102.

Based on calculations of the pair’s mass &mdash together, and relative to each other &mdash the researchers go on to predict a smashup 100,000 years from now, an impossibly long time to humans but the blink of an eye to a star or black hole. Spiraling together 3.5 billion light-years away, deep in the Virgo constellation, the pair is separated by a mere light-week. By contrast, the closest previously confirmed black hole pair is separated by 20 light-years.

“This is the closest we’ve come to observing two black holes on their way to a massive collision,” said the study’s senior author, Zoltan Haiman, an astronomer at Columbia. “Watching this process reach its culmination can tell us whether black holes and galaxies grow at the same rate, and ultimately test a fundamental property of space-time: its ability to carry vibrations called gravitational waves, produced in the last, most violent, stage of the merger.”

At the centre of most giant galaxies, including our own Milky Way, lies a supermassive black hole so dense that not even light can escape. Over time, black holes grow bigger &mdash millions to billions times more massive than the Sun &mdash by gobbling up stars, galaxies and even other black holes.

A supermassive black hole about to cannibalise its own can be detected by the mysterious flickering of a quasar &mdash the beacon of light produced by black holes as they burn through gas and dust swirling around them. Normally, quasars brighten and dim randomly, but when two black holes are on the verge of uniting, the quasar appears to flicker at regular intervals, like a light bulb on timer.

Recently, a team led by Matthew Graham, a computational astronomer at the California Institute of Technology, designed an algorithm to pick out repeating light signals from 247,000 quasars monitored by telescopes in Arizona and Australia. Of the 20 pairs of black hole candidates discovered, they focused on the most compelling bright quasar &mdash PG 1302-102. In a January study in Nature, they showed that PG 1302-102 appeared to brighten by 14 percent every five years, indicating the pair was less than a tenth of a light-year apart.

Intrigued, Haiman and his colleagues wondered if they could build a theoretical model to explain the repeating signal. If the black holes were as close as predicted, one had to be circling a much larger counterpart at nearly a tenth of the speed of light, they hypothesised. At that speed, the smaller black hole would appear to brighten as it approached Earth’s line of sight under the relativistic Doppler beaming effect.

If correct, they predicted they would find a five-year cycle in the quasar’s ultraviolet emissions &mdash only two-and-a-half times more variable in its intensity. Analysing UV observations collected by NASA’s Hubble and GALEX space telescopes they found exactly that.

Previous explanations for the repeating signal include a warp in the debris discs orbiting the black holes, a wobble in the axis of one black hole and a lopsided debris disc formed as one black hole draws material off the other &mdash all creating the impression of a periodic flicker from Earth. A black hole merger is expected to release the gravitational waves predicted by Einstein, but not yet detected. Above, an artist’s conception of waves rippling through space-time. Illustration credit: NASA. The new study also offers a new technique for investigating other converging black holes, the researchers said. By estimating the combined and relative mass of PG 1302-102’s black holes, they narrow down the pair’s predicted crash time to between 20,000 and 350,000 years from now with a best estimate of 100,000 years. (The predicted crash time by Graham’s team was 10,000 to several million years from now with a best estimate of 250,000 years).

“We can start to put numbers on the rates that black holes come together and build up into larger black holes, and use what we’re learning to search for more black holes pairs,” said study co-author David Schiminovich, an astronomer at Columbia.

An uptick in the number of black hole binary discoveries has made astronomers hopeful that a collision could be detected in the next decade. This summer, Graham and his colleagues reported another 90 candidates, while astronomers at Columbia expect to soon unveil discoveries of their own from data collected at California’s Palomar Observatory.

With more black holes to watch, the chance of witnessing a crash and the gravitational waves predicted, but not yet detected, by Einstein’s general theory of relativity, grows.

“The detection of gravitational waves lets us probe the secrets of gravity and test Einstein’s theory in the most extreme environment in our universe &mdash black holes,” said the study’s lead author, Daniel D’Orazio, a graduate student at Columbia. “Getting there is a holy grail of our field.”

## Quasar Pairs Show One Good Black Hole Deserves Another

Two quasar pairs seen in the early Universe are the oldest, most-distant bodies objects of their kind yet seen in the Cosmos.

Quasars are extremely energetic galaxies, powered by highly-active supermassive black holes near their centers. Matter falling into the behemoth void in the galactic core radiates vast amounts of energy out to space, forming a quasar. While this process is active, these supermassive black holes can outshine entire galaxies.

The quasars in each pair examined in this new study are just 10,000 light years from each other. This may sound like quite a distance, but this is just one-tenth the distance from one side of our galaxy to the other. This proximity suggests the quasars are found within merging galaxies.

“This truly is the first sample of dual quasars at the peak epoch of galaxy formation that we can use to probe ideas about how supermassive black holes come together to eventually form a binary,” explained Dr. Nadia Zakamska of Johns Hopkins University.

#### Paring Down the Pairs

The two quasar pairs examined in this study, as seen in visible light by the Hubble Space Telescope. Image credit: NASA, ESA, H. Hwang and N. Zakamska (Johns Hopkins University), and Y. Shen (University of Illinois, Urbana-Champaign)

Finding these quasars was the result of collecting data from several telescopes on and above the Earth, including observations from the Gemini Observatory.

One of the quasar pairs are seen as they existed 10 billion years before our time. Quasars are not exceptionally rare, but finding these objects in pairs remains a rarity.

Studying the Sloan Digital Sky Survey, a 3D map of the Cosmos, astronomers selected 15 quasars for closer examination. These candidates were then matched with data from the Gaia spacecraft, narrowing the search to four potential quasar pairs. The Hubble Space Telescope was then able to resolve two of the pairs, showing separations between the pairs of enigmatic objects.

Using the Gemini North telescope in Hawaii, researchers broke light from the quasars into component colors, allowing measurements of the movements of the bodies. This examination also provided proof of their positions, as well as information regarding their chemical compositions.

“Quasars make a profound impact on galaxy formation in the universe. Finding dual quasars at this early epoch is important because we can now test our long-standing ideas of how black holes and their host galaxies evolve together,” Zakamska said.

Although researchers believe these quasar pairs are in the positions they calculated, there remains a small chance that these pairs of images may be double images of lone quasars. This could occur due to gravitational lensing — the warping of light caused by a massive galaxy in-between an astronomical target and the Earth. However, researchers were unable to find any object in their view capable of causing this effect.

#### A Long Time Ago, in Galaxies Far, Far Away…

Finding quasars in the ancient Universe. Video credit: The Cosmic Companion.

Billions of years from now, our own Milky Way will, itself, become a highly-luminescent quasar. Every galaxy, including the Milky Way and our neighbor Andromeda, contains a supermassive black hole at their center. Currently, both of these are “sleeping giants,” only consuming small amounts of matter.

However, our neighboring galaxy and the Milky Way are approaching each other, and will one day collide. As the galaxies draw closer, the supermassive black holes at the cores of each galaxy will awaken. When this happens, highly-energetic radiation will bathe each galaxy, potentially killing off life on any worlds unfortunate enough to be too close to the galactic centers.

“Twinkle, twinkle, quasi-star, biggest puzzle from afar
How unlike the other ones, brighter than a billion suns
Twinkle, twinkle, quasi-star, how I wonder what you are” — George Gamow

A look at the discovery of these two rare quasar pairs. Video credit: NOIRLab

Quasars are found throughout the Universe, but quasar pairs are rare.

“We estimate that in the distant universe, for every 1,000 quasars, there is one double quasar. So finding these double quasars is like finding a needle in a haystack,” Dr. Yue Shen of the University of Illinois Urbana-Champaign, explains.

These newly-recognized pairs are the oldest of the 100 or so quasar pairs currently known to astronomers.

The quasar pairs examined in this study are seen as they were the distant past, less than four billion years after the Big Bang. As the quasars drifted toward each other, energy would have raced from the bodies, radiating into space. Eventually, the quasars would have merged into single black holes of enormous size.

Analysis of the study was published in Nature Astronomy.

#### James Maynard

James Maynard is the founder and publisher of The Cosmic Companion. He is a New England native turned desert rat in Tucson, where he lives with his lovely wife, Nicole, and Max the Cat.

## Hubble spots double quasars in merging galaxies

This artist's conception shows the brilliant light of two quasars residing in the cores of two galaxies that are in the chaotic process of merging. The gravitational tug-of-war between the two galaxies stretches them, forming long tidal tails and igniting a firestorm of starbirth. Quasars are brilliant beacons of intense light from the centers of distant galaxies. They are powered by supermassive black holes voraciously feeding on infalling matter. This feeding frenzy unleashes a torrent of radiation that can outshine the collective light of billions of stars in the host galaxy. In a few tens of millions of years, the black holes and their galaxies will merge, and so will the quasar pair, forming an even more massive black hole. A similar sequence of events will happen a few billion years from now when our Milky Way galaxy merges with the neighboring Andromeda galaxy. Credit: NASA, ESA, and J. Olmsted (STScI)

NASA's Hubble Space Telescope is "seeing double." Peering back 10 billion years into the universe's past, Hubble astronomers found a pair of quasars that are so close to each other they look like a single object in ground-based telescopic photos, but not in Hubble's crisp view.

The researchers believe the quasars are very close to each other because they reside in the cores of two merging galaxies. The team went on to win the "daily double" by finding yet another quasar pair in another colliding galaxy duo.

A quasar is a brilliant beacon of intense light from the center of a distant galaxy that can outshine the entire galaxy. It is powered by a supermassive black hole voraciously feeding on inflating matter, unleashing a torrent of radiation.

"We estimate that in the distant universe, for every 1,000 quasars, there is one double quasar. So finding these double quasars is like finding a needle in a haystack," said lead researcher Yue Shen of the University of Illinois at Urbana-Champaign.

The discovery of these four quasars offers a new way to probe collisions among galaxies and the merging of supermassive black holes in the early universe, researchers say.

Quasars are scattered all across the sky and were most abundant 10 billion years ago. There were a lot of galaxy mergers back then feeding the black holes. Therefore, astronomers theorize there should have been many dual quasars during that time.

"This truly is the first sample of dual quasars at the peak epoch of galaxy formation with which we can use to probe ideas about how supermassive black holes come together to eventually form a binary," said research team member Nadia Zakamska of Johns Hopkins University in Baltimore, Maryland.

The team's results appeared in the April 1 online issue of the journal Nature Astronomy.

Shen and Zakamska are members of a team that is using Hubble, the European Space Agency's Gaia space observatory, and the Sloan Digital Sky Survey, as well as several ground-based telescopes, to compile a robust census of quasar pairs in the early universe.

These two Hubble Space Telescope images reveal two pairs of quasars that existed 10 billion years ago and reside at the hearts of merging galaxies. Each of the four quasars resides in a host galaxy. These galaxies, however, cannot be seen because they are too faint, even for Hubble. The quasars within each pair are only about 10,000 light-years apart -- the closest ever seen at this cosmic epoch. Quasars are brilliant beacons of intense light from the centers of distant galaxies that can outshine their entire galaxies. They are powered by supermassive black holes voraciously feeding on infalling matter, unleashing a torrent of radiation. The quasar pair in the left-hand image is catalogued as J0749+2255 and the pair on the right as J0841+4825. The two pairs of host galaxies inhabited by each double quasar will eventually merge. The quasars will then tightly orbit each other until they eventually spiral together and coalesce, resulting in an even more massive, but solitary black hole. The image for J0749+2255 was taken Jan. 5, 2020. The J0841+4825 snapshot was taken Nov. 30, 2019. Both images were taken in visible light with Wide Field Camera 3. Credit: NASA, ESA, H. Hwang and N. Zakamska (Johns Hopkins University), and Y. Shen (University of Illinois, Urbana-Champaign)

The observations are important because a quasar's role in galactic encounters plays a critical part in galaxy formation, the researchers say. As two close galaxies begin to distort each other gravitationally, their interaction funnels material into their respective black holes, igniting their quasars.

Over time, radiation from these high-intensity "light bulbs" launch powerful galactic winds, which sweep out most of the gas from the merging galaxies. Deprived of gas, star formation ceases, and the galaxies evolve into elliptical galaxies.

"Quasars make a profound impact on galaxy formation in the universe," Zakamska said. "Finding dual quasars at this early epoch is important because we can now test our long-standing ideas of how black holes and their host galaxies evolve together."

Astronomers have discovered more than 100 double quasars in merging galaxies so far. However, none of them is as old as the two double quasars in this study.

The Hubble images show that quasars within each pair are only about 10,000 light-years apart. By comparison, our Sun is 26,000 light-years from the supermassive black hole in the center of our galaxy.

The pairs of host galaxies will eventually merge, and then the quasars also will coalesce, resulting in an even more massive, single solitary black hole.

Finding them wasn't easy. Hubble is the only telescope with vision sharp enough to peer back to the early universe and distinguish two close quasars that are so far away from Earth. However, Hubble's sharp resolution alone isn't good enough to find these dual light beacons.

Astronomers first needed to figure out where to point Hubble to study them. The challenge is that the sky is blanketed with a tapestry of ancient quasars that flared to life 10 billion years ago, only a tiny fraction of which are dual. It took an imaginative and innovative technique that required the help of the European Space Agency's Gaia satellite and the ground-based Sloan Digital Sky Survey to compile a group of potential candidates for Hubble to observe.

Located at Apache Point Observatory in New Mexico, the Sloan telescope produces three-dimensional maps of objects throughout the sky. The team pored through the Sloan survey to identify the quasars to study more closely.

The researchers then enlisted the Gaia observatory to help pinpoint potential double-quasar candidates. Gaia measures the positions, distances, and motions of nearby celestial objects very precisely. But the team devised a new, innovative application for Gaia that could be used for exploring the distant universe. They used the observatory's database to search for quasars that mimic the apparent motion of nearby stars. The quasars appear as single objects in the Gaia data. However, Gaia can pick up a subtle, unexpected "jiggle" in the apparent position of some of the quasars it observes.

The quasars aren't moving through space in any measurable way, but instead their jiggle could be evidence of random fluctuations of light as each member of the quasar pair varies in brightness. Quasars flicker in brightness on timescales of days to months, depending on their black hole's feeding schedule.

This alternating brightness between the quasar pair is similar to seeing a railroad crossing signal from a distance. As the lights on both sides of the stationary signal alternately flash, the sign gives the illusion of "jiggling."

When the first four targets were observed with Hubble, its crisp vision revealed that two of the targets are two close pairs of quasars. The researchers said it was a "light bulb moment" that verified their plan of using Sloan, Gaia, and Hubble to hunt for the ancient, elusive double powerhouses.

Team member Xin Liu of the University of Illinois at Urbana-Champaign called the Hubble confirmation a "happy surprise." She has long hunted for double quasars closer to Earth using different techniques with ground-based telescopes. "The new technique can not only discover dual quasars much further away, but it is much more efficient than the methods we've used before," she said.

Their Nature Astronomy article is a "proof of concept that really demonstrates that our targeted search for dual quasars is very efficient," said team member Hsiang-Chih Hwang, a graduate student at Johns Hopkins University and the principal investigator of the Hubble program. "It opens a new direction where we can accumulate a lot more interesting systems to follow up, which astronomers weren't able to do with previous techniques or datasets."

The team also obtained follow-up observations with the National Science Foundation NOIRLab's Gemini telescopes. "Gemini's spatially-resolved spectroscopy can unambiguously reject interlopers due to chance superpositions from unassociated star-quasar systems, where the foreground star is coincidentally aligned with the background quasar," said team member Yu-Ching Chen, a graduate student at the University of Illinois at Urbana-Champaign.

Although the team is convinced of their result, they say there is a slight chance that the Hubble snapshots captured double images of the same quasar, an illusion caused by gravitational lensing. This phenomenon occurs when the gravity of a massive foreground galaxy splits and amplifies the light from the background quasar into two mirror images. However, the researchers think this scenario is highly unlikely because Hubble did not detect any foreground galaxies near the two quasar pairs.

Galactic mergers were more plentiful billions of years ago, but a few are still happening today. One example is NGC 6240, a nearby system of merging galaxies that has two and possibly even three supermassive black holes. An even closer galactic merger will occur in a few billion years when our Milky Way galaxy collides with neighboring Andromeda galaxy. The galactic tussle would likely feed the supermassive black holes in the core of each galaxy, igniting them as quasars.

Future telescopes may offer more insight into these merging systems. NASA's James Webb Space Telescope, an infrared observatory scheduled to launch later this year, will probe the quasars' host galaxies. Webb will show the signatures of galactic mergers, such as the distribution of starlight and the long streamers of gas pulled from the interacting galaxies.

## Astronomers see a Rare “Double Quasar” in a Pair of Merging Galaxies

What’s better than a quasar? That’s right, two quasars. Astronomers have spotted for the first time two rare double-quasars, and the results show us the dynamic, messy consequences of galaxy formation.

Every galaxy is thought to host a supermassive black hole in its center. When galaxies merge, their black holes merge along with them. During the height of the merger process, huge volumes of gas and dust swirl down to the center of the galaxy. As all that gas and dust compresses down into the black hole, it heats up.

The forces are so intense that the cores of these galaxies become “quasars”, blazing brighter than millions of normal galaxies put together and beaming massive jets of radiation thousands of lightyears into space.

Astronomers have long observed many of these quasars, and many more normal galaxies. But double quasars? These would represent galaxies in a state of mid-merger, where gas and dust has compressed onto the core but the black holes themselves have not yet merged. Since it’s such a brief phase of the merger evolution, astronomers had not yet observed any.

“We estimate that in the distant universe, for every 1,000 quasars, there is one double quasar. So finding these double quasars is like finding a needle in a haystack,” said lead researcher Yue Shen of the University of Illinois at Urbana-Champaign.

The astronomers devised a technique that looked for subtle flickers in the brightness of quasars, a sign that the light we see may be from two competing cores rather than a single unified black hole. With the two sets of doubles on hand, astronomers can begin to directly probe this extremely violent phase of galaxy evolution.

“This truly is the first sample of dual quasars at the peak epoch of galaxy formation with which we can use to probe ideas about how supermassive black holes come together to eventually form a binary,” said research team member Nadia Zakamska of Johns Hopkins University in Baltimore, Maryland.

“Quasars make a profound impact on galaxy formation in the universe,” Zakamska said. “Finding dual quasars at this early epoch is important because we can now test our long-standing ideas of how black holes and their host galaxies evolve together.”

With the new technique, hopefully astronomers can catch a lot more quasars in the act. Their Nature Astronomy article is a “proof of concept that really demonstrates that our targeted search for dual quasars is very efficient,” said team member Hsiang-Chih Hwang, a graduate student at Johns Hopkins University and the principal investigator of the Hubble program. “It opens a new direction where we can accumulate a lot more interesting systems to follow up, which astronomers weren’t able to do with previous techniques or datasets.”

## Hubble Space Telescope Seeing Double Quasars in Merging Galaxies

The Hubble Space Telescope of NASA is’seeing double.’ Hubble astronomers discovered a pair of quasars that are so close to each other that they appear to be a single object in ground-based telescopic photos, but not in Hubble’s crisp view. The quasars, according to the researchers, are very close to each other because they are located in the cores of two merging galaxies. The team then won the “daily double” by discovering yet another quasar pair in another colliding galaxy pair.

A quasar is a brilliant beacon of intense light that shines from the center of a distant galaxy and can outshine the entire galaxy. It is propelled by a supermassive black hole that devours inflating matter, releasing a torrent of radiation.

“We estimate that in the distant universe, there is one double quasar for every 1,000 quasars. Finding these double quasars is akin to looking for a needle in a haystack “Yue Shen of the University of Illinois at Urbana-Champaign was the lead researcher.

According to the researchers, the discovery of these four quasars provides a new way to investigate galaxies colliding and supermassive black holes merging in the early universe. Quasars are found all over the sky and were most common 10 billion years ago. There were many galaxy mergers back then, which fed the black holes. As a result, astronomers believe there should have been a lot of dual quasars around at the time.

NASA’s Hubble Space Telescope is “seeing double.” Peering back 10 billion years into the universe’s past, Hubble astronomers found a pair of quasars that are so close to each other they look like a single object in ground-based telescopic photos, but not in Hubble’s crisp view.

“This truly is the first sample of dual quasars at the peak epoch of galaxy formation that we can use to probe ideas about how supermassive black holes come together to eventually form a binary,” said Johns Hopkins University’s Nadia Zakamska.

The team’s findings were published in the online edition of Nature Astronomy on April 1st. Shen and Zakamska are part of a team that is compiling a comprehensive list of quasar pairs in the early universe using Hubble, the European Space Agency’s Gaia space observatory, the Sloan Digital Sky Survey, and several ground-based telescopes.

The findings are significant because the role of quasars in galactic encounters is critical to galaxy formation, according to the researchers. As two nearby galaxies begin to gravitationally distort each other, the material is funneled into their respective black holes, igniting their quasars.

Over time, the radiation from these high-intensity “light bulbs” creates powerful galactic winds that sweep away the majority of the gas from the merging galaxies. Star formation ceases when galaxies are deprived of gas, and the galaxies evolve into elliptical galaxies.

“Quasars make a profound impact on galaxy formation in the universe,” Zakamska said. “Finding dual quasars at this early epoch is important because we can now test our long-standing ideas of how black holes and their host galaxies evolve together.”

Hubble spots double quasars in merging galaxies

So far, astronomers have discovered over 100 double quasars in merging galaxies. None of them, however, are as old as the two double quasars studied in this study. According to the Hubble images, the quasars in each pair are only about 10,000 light-years apart. Our Sun, by comparison, is 26,000 light-years away from the supermassive black hole at the center of our galaxy.

The host galaxies will eventually merge, followed by the quasars, resulting in an even more massive, single solitary black hole. It wasn’t easy to track them down. Hubble is the only telescope capable of peering back into the early universe and distinguishing two close quasars that are so far away from Earth. However, Hubble’s sharp resolution alone isn’t good enough to find these dual light beacons.

Astronomers had to first figure out where to point Hubble in order to study them. The difficulty is that the sky is a tapestry of ancient quasars that flared to life 10 billion years ago, only a tiny fraction of which are dual. To compile a group of potential candidates for Hubble to observe, it took an imaginative and innovative technique that required the assistance of the European Space Agency’s Gaia satellite and the ground-based Sloan Digital Sky Survey.

The Sloan telescope, located at Apache Point Observatory in New Mexico, creates three-dimensional maps of objects in the sky. The team combed through the Sloan survey to find quasars to investigate further.

The researchers then enlisted the assistance of the Gaia observatory to help them identify potential double-quasar candidates. Gaia precisely measures the positions, distances, and motions of nearby celestial objects. However, the team created a new, innovative application for Gaia that could be used to explore the far reaches of the universe. They searched the observatory’s database for quasars that resemble the apparent motion of nearby stars. In the Gaia data, the quasars appear as single objects. Gaia, on the other hand, can detect a subtle, unexpected “jiggle” in the apparent position of some of the quasars it observes.

The quasars aren’t moving through space in any discernible way, but their jiggle could be due to random fluctuations in light as each quasar pair varies in brightness. Quasars flicker in brightness over timescales ranging from days to months, depending on the feeding schedule of their black hole.

The quasar pair’s alternating brightness is analogous to seeing a railroad crossing signal from a distance. The sign appears to “jiggle” as the lights on both sides of the stationary signal alternately flash.

When Hubble observed the first four targets, its sharp vision revealed that two of the targets are two close pairs of quasars. It was a “light bulb moment” for the researchers, who said it validated their plan to use Sloan, Gaia, and Hubble to hunt for the ancient, elusive double powerhouses.

The Hubble confirmation was a “happy surprise,” according to team member Xin Liu of the University of Illinois at Urbana-Champaign. She has long been on the lookout for double quasars closer to Earth, employing various techniques with ground-based telescopes. “Not only can the new technique detect dual quasars much further away, but it is also much more efficient than previous methods,” she explained.

Their Nature Astronomy paper is a “proof of concept that really demonstrates that our targeted search for dual quasars is very efficient,” according to team member Hsiang-Chih Hwang, a graduate student at Johns Hopkins University and the Hubble program’s principal investigator. “It opens up a new avenue where we can collect a lot more interesting systems to follow up on, which astronomers were unable to do with previous techniques or datasets.”

The team also obtained follow-up observations using the Gemini telescopes at the National Science Foundation NOIRLab. “Gemini’s spatially-resolved spectroscopy can unambiguously reject interlopers from unassociated star-quasar systems, where the foreground star is coincidentally aligned with the background quasar,” said team member Yu-Ching Chen, a graduate student at the University of Illinois at Urbana-Champaign.

Although the team is confident in their findings, they acknowledge that there is a slight possibility that the Hubble snapshots captured two images of the same quasar, an illusion caused by gravitational lensing. When the gravity of a massive foreground galaxy splits and amplifies the light from a background quasar, it creates two mirror images. The researchers believe this scenario is unlikely because Hubble did not detect any foreground galaxies near the two quasar pairs.

Galactic mergers were more common billions of years ago, but a few still occur today. NGC 6240, a nearby system of merging galaxies with two, possibly three, supermassive black holes, is one example. Our Milky Way galaxy will collide with the neighboring Andromeda galaxy in a few billion years, resulting in an even closer galactic merger. The galactic conflict would most likely feed the supermassive black holes at the heart of each galaxy, igniting them as quasars.

Future telescopes may provide more information about these merging systems. NASA’s James Webb Space Telescope, an infrared observatory set to launch later this year, will investigate the host galaxies of the quasars. Webb will display galactic merger signatures such as starlight distribution and long streamers of gas pulled from interacting galaxies.

## A new way to find gigantic black holes paired up at the dawn of the Universe

Using what's really a pretty dang clever new technique, astronomers have discovered what appear to be two double-quasars (and possibly a third pair as well) billions of light years from Earth. This may help constrain how many of these ferocious objects existed when the Universe was young.

All big galaxies, and many smaller ones, have a supermassive black hole at their heart. As I have described approximately eleventy bazillion times before, if matter from the surrounding galaxy is falling into the core, it can pile up into a disk that orbits the black hole, slowly feeding it. The disk is incredibly hot and can shine incredibly luminously, easily outshining the rest of the galaxy. Sometimes, through forces not terribly well understood (though the magnetic properties of the disk are the likely culprit), twin beams of matter are launched up and down, away from the disk, with the matter moving at extremely high velocity, sometimes just a smidge slower than the speed of light.

Generically, such an object is called an active galaxy. If one of those beams happens to be pointed at Earth, we can see lots of light from across almost the entire electromagnetic spectrum, from radio waves to X-rays. We call an object like that a quasar.

Artwork depicting binary quasars, two actively galaxies orbiting one another. Credit: NASA/ESA/Hubble/ESO/M. Kornmesser, adapted by Phil Plait

We know that there are extremely massive black holes out there which may have grown to such enormous size when two big galaxies merge. The black holes fall toward each other, eventually orbiting each other and then, after billions of years, they can merge together into one bigger black hole. This implies we should find binary supermassive black holes, or at least two that are close together (say, within a few thousand light years of each other). However very few are seen, mainly because they're hard to discover.

The new research figured out a way to find some of them though, and it's super clever. Quasars are known to fluctuate in brightness, getting brighter and dimmer on a timescale of days, weeks or months. If two quasars are close together, one may get brighter than the other over time and vice-versa.

If they're so close together they appear as one blob, this effect can betray their dual nature. Say one is to the left of the other. If the left one is brighter, the blob will appear to be shifted a tiny bit to the left. If it then fades and the other gets brighter, the blob will shift a little bit to the right. The science of measuring the positions of objects on the sky is called astrometry, and so the scientists who worked on this double-quasar research call this idea astrometric jitter.

Artwork showing a double quasar, where each one varies in brightness over time. Credit: NASA, ESA, and J. Olmsted (STScI)

The satellite observatory Gaia has been surveying the sky for years, measuring the positions of billions of objects with incredible accuracy. If the center of what it thinks is a single object is seen to move back and forth over time, perhaps it's one of these rare double quasars.

The team first made a list of known quasars farther away than about 10 billion light years — anything closer than that and the extended light from the stars in the galaxy surrounding the black hole might interfere with the measurements. They found about 11,000 such quasars.

They then searched the Gaia database to see if the positions of any of these quasars suffered from recurrent jitter. Out of that list they found 15 (noting that there could be more that were missed because they were too close together to see any shift in the center).

The team submitted these 15 objects to be observed by Hubble as part of the “Snapshot” program: Very short exposures (typically 5 – 8 minutes) of targets that can be obtained between regularly scheduled observations. Four of the 15 were observed. Of these, one is clearly a quasar with a star very close by, so it's not a double quasar. A second was not resolved even by Hubble — it still appears as a single object — so its qualifications are uncertain.

Hubble images of two pairs of potential double quasars. J0749+2255 (left) is a candidate double, while J0841+4825 (right) is highly likely to be a pair of quasars. Credit: NASA, ESA, H. Hwang and N. Zakamska (Johns Hopkins University), and Y. Shen (University of Illinois, Urbana-Champaign)

But the other two pairs both appear to be double quasars, with two clearly resolved components each. However, we have to be careful here. It's possible that each is actually a single quasar but is gravitationally lensed: The gravity of a foreground galaxy between us and them can distort the light, creating multiple images of the same object. The team cannot rule this out, even after taking spectra of one of the double quasars (and finding it's about 11.5 billion light years away). Still, looking at the number of lensed quasars known they find the odds of it being a lensed single quasar is only about 5%, meaning they have 95% confidence it really is a double quasar.

To be fair it could also just be two active galaxies that happen to be close to one another, or a single extremely clumpy still-forming galaxy where two clumps happen to have massive black holes in them.

Artwork showing a pair of quasars just a few thousand light years apart, each the center of a relatively normal galaxy that are merging with one another. Such objects should have been more common in the very young Universe. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva

What's needed are more follow-up observations (including the 11 potential pairs not yet observed by Hubble) to try to rule out some of these possible confounding factors. However, the bigger point here is that the astrometric jitter idea worked. Over time Gaia may see more of these as well, since the longer it observes the sky the better its measurements get. Also, future sky surveys may improve on this technique, so even though it only yields a handful of double quasars now it may prove even more useful later.

The early Universe should have more double quasars, twin monsters roaring in the dark. Here's hoping we find more of them.

## Does 2 merging black holes necessarily make a quasar? - Astronomy

This is a great question! There are two reasons why we never see matter fall into a black hole. The first reason is because of gravitational time dilation if we dropped a clock into a black hole, we would see it tick slower and slower. The second reason is because of gravitational redshift as a luminous object falls into a black hole, the photons emitted, lose energy climbing out of the gravitational field, causing the entire spectrum to be shifted into the infrared, which is outside of the range of visible light that we can detect.

However, both of these effects are only strong and significant very close to the black hole. For a black hole the mass of the Sun, the size of the black hole is only a few kilometers. So for you to be affected by the time dilation effects of the solar-mass black hole (ignoring the gravity!), the black hole would have to be in the same city as you! So black hole are allowed to get very close to each other before time dilation becomes strong. Of course, black holes do get close, and merge even, so at some point we must have very strong time dilation. What happens at this point?

Remember that black hole themselves are not actually solid objects that are crashing into each other. But there is another way we can think of black holes to make sense of them:

Whirlpools are places in the ocean where the current pulls us in toward the center where we can't escape. This is similar to the effect of a black hole. In fact, we can even understand time dilation in this way:

Imagine you and a friend get are in separate boats and the two of you have found a large whirlpool in the ocean. Your friend decides to explore the whirlpool while you stay safely far away. To communicate with you, you friend brought messenger fishes they release the fish into the ocean and they swim towards your boat, carrying a little message. Of course, as you friend gets deeper and deeper into the whirlpool, the messenger fishes take longer and longer to get out and reach you. Eventually, your friend has fallen deep enough into the whirlpool that all the messenger fishes they release are trapped with them you stop receiving messages.

The analogy to be made here is that black holes can be thought of as whirlpools in spacetime, and the "messenger fishes" your friend released are the photons (and gravitational waves) that are emitted when objects fall into black holes. That the messages are eventually trapped within the whirlpool represents that the event horizon, the point of no return, has been crossed. Notice how there's no physical surface around the whirlpools that represent this point of no return with black holes the situation is the same.

So now we can answer the question, what does it look like when two black holes merge? It's similar to two whirlpools merging, We can imagine releasing many messenger fishes to make sense of what is happening in this highly dynamical region of the ocean. Thankfully we don't have to imagine too much, one of our alumni, Will Throwe, did this very calculation! The results of the calculation have been made into a video that you can find by following the link:Two Black Holes Merge into One

Note that the black spheres in the video are not the black holes themselves, but rather their shadows, meaning their appearance when we factor in how they deflect and trap light. Even though there is severe time dilation near the surface of a black hole, the surfaces themselves are very dynamic objects and so we must use simulations instead to answer our questions regarding what things will look like if we had a telescope. Thankfully we have many determined and passionate people working on it!

There might be one lingering question: How do we see black holes merge if I will never see an object dropped into a black hole pass the horizon? How do I reconcile this? One thing to keep is that when we picture dropping a clock into a black hole, we imagine the clock approaching more and more slowly, and the black hole just sitting there, not reacting. From binary black hole mergers we should realize that black holes have lives of their own! As in, the "stationary" black hole that we had in our dropping-a-clock thought experiment it not so stationary after all. The moving clock means we are in a dynamical spacetime, and in a dynamical spacetime the black hole horizon actually changes shape! At some point, when the clock gets close enough, the surface of the black hole "puckers" out slightly and "kisses" the dropped clock as it falls in. This is a more accurate picture than the one in which the surface of the black hole does nothing at all. But to arrive at the full scientific answer and arrive at what the full story is, we have to do simulations like Will did! Hopefully we have more cool videos of simulations in the future!

## A hidden population of high-redshift double quasars unveiled by astrometry

A new technique leads to the discovery of two closely separated pairs of supermassive black holes in the early universe.

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If you thought that the Nobel-Prize-winning LIGO discovery of stellar mass black hole mergers was cool, just think about the possibility of merging black holes with masses millions or billions of times that of the Sun! The enormous masses of black holes in centers of galaxies almost defy imagination. One would think that such energetic merger events or their progenitors -- supermassive black holes orbiting one another on tight orbits -- would be easy to identify. Yet they have so far eluded researchers, and discovering such objects remains a tantalizing goal of modern observational astronomy. The detection of gravitational wave emission from such supermassive black hole binaries is the principal goal of ongoing and upcoming low-frequency gravitational wave searches with pulsar timing arrays (NANOGrav, http://nanograv.org/ ) and planned space interferometer LISA ( https://lisa.nasa.gov/ ).

First of all, what reason do we have to think that supermassive black hole binaries exist? Astronomers have known for over two decades that nearly every massive galaxy hosts a supermassive black hole at its center. (The 2020 Nobel Prize in physics was awarded for the discovery and the measurements of Sgr A*, the supermassive black hole in the center of the Milky Way.) Galactic mergers are known to be an important feature of galaxy growth throughout the history of the Universe. So what then would happen to the central supermassive black holes when two galaxies merge? And what observations could we perform to track these processes?

Artist's conception of quasars in merging galaxies. Image credit: NASA, ESA, and J. Olmstead (STScI)

Our paper "A hidden population of high-redshift double quasars unveiled by astrometry" (Shen et al. 2021) that appeared in Nature Astronomy on April 1 presents the discovery of two supermassive black hole pairs in the early universe. The black holes in our study are in a quasar' phase -- they are swallowing surrounding material, which emits copious radiation before its final plunge into the black hole. Within each pair, the quasars are separated by

10,000 light years, meaning that the galaxy mergers that host the pairs are well underway.

Color-composite HST images of dual quasars from this study.
Image credit: NASA, ESA, H.-C. Hwang (JHU), N.L. Zakamska (JHU), Y.Shen (UIUC), X.Liu (UIUC)

Two aspects of this study are important. This is the first detection of such small separation quasar pairs in the early universe. When two galaxies first come together, for hundreds of millions of years they conduct an orbital dance before they start blending into one single galactic merger product. There are many missing pieces in our understanding of this process, and especially of what happens to the two supermassive black holes. Therefore, the objects we have discovered offer the first probe of this mysterious stage.

But even more importantly, our study validates a novel search technique for dual supermassive black holes which may allow us to find a large number of these objects over the entire sky with high efficiency. The new technique (Hwang et al. 2020) takes advantage of the unprecedented astrometric sensitivity of the Gaia satellite. A single quasar in the early universe is much too far away for Gaia to be able to measure its parallax (reflex motion as the Earth is moving around the Sun) or its proper motion (velocity across the plane of the sky). However, if an apparently single quasar is instead a closely separated pair, the stochastic brightness variations of the two components lead to an `astrometric jitter", in which the centroid of the light appears to move slightly back and forth: when the quasar on the left is brighter the total light shifts to the left, and when the quasar on the right is brighter the total light shifts to the right.

Illustration of the astrometric jitter method: as the quasars vary, light centroid shifts.
Image credit: H.-C. Hwang (JHU)

Guided by this idea, we used Gaia and Sloan Digital Sky Survey data to identify quasars that appeared single, but had much too much astrometric noise. We then proposed to conduct follow-up observations of some of these candidates with the Hubble Space Telescope, as its spatial resolution -- the ability to distinguish closely separated point sources -- is superior to any other available optical facility. These observations allowed us to see that out of four targets, three were made up of two different point sources. We further obtained Gemini spectroscopy and Very Long Baseline Array radio imaging, confirming two of the targets as quasar pairs.

After years of hard work and trial and error, several dozen pairs of active black holes have been identified in relatively nearby galaxies using a variety of observationally-expensive methods (Liu et al. 2018). But finding these objects at high redshift -- where the action should be -- has until now been prohibitively difficult: in ground-based survey data, high-redshift host galaxies are too faint to be detected and identified as candidate mergers, and the two quasars in the pair cannot be resolved into two individual sources when they are too close together. Our new technique based on the astrometric noise offers a possibility of finding dual quasars with a wide range of separations, and the observations presented by Shen et al. (2021) confirm the high efficiency of the new method. A large sample of such pairs, in turn, will enable new probes of all stages of the merger of galaxies and their black holes.

As the merger process continues, the two black holes will spiral closer and closer to the nucleus of the merger remnant galaxy, until they will be so close together that even our new astrometric noise technique cannot distinguish them from a point source. What direct evidence do we have for the existence of such binaries? The answer is -- surprisingly little, and many groups are working hard on developing methods and obtaining data to chase these elusive candidates. Moreover, it is not known how long supermassive black hole binaries spend in the gravitational wave stage of their careers. This and other questions about the dual supermassive black hole evolution will undoubtedly be the subject of continuing research in the upcoming decade.

1. Ten Eych