Special Guest:
Dr. Venemans is a research staff scientist working at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany. His research topics include the discovery of black holes in the early Universe, the characterisation of the galaxies hosting these distant black holes, the Epoch of Reionisation and the galaxy environment of active galaxies.
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Which Came First, Supermassive Black Holes of their Galaxies?
There’s a supermassive black hole at the center of almost every galaxy in the Universe. How did they get there? What’s the relationship between these monster black holes and the galaxies that surround them?
Every time astronomers look farther out in the Universe, they discover new mysteries. These mysteries require all new tools and techniques to understand. These mysteries lead to more mysteries. What I’m saying is that it’s mystery turtles all the way down.
One of the most fascinating is the discovery of quasars, understanding what they are, and the unveiling of an even deeper mystery, where do they come from?
As always, I’m getting ahead of myself, so first, let’s go back and talk about the discovery of quasars.
Molecular clouds scattered by an intermediate black hole show very wide velocity dispersion in this artist’s impression. This scenario well explains the observational features of a peculiar molecular cloud CO-0.40-0.22. Credit: Keio University
Back in the 1950s, astronomers scanned the skies using radio telescopes, and found a class of bizarre objects in the distant Universe. They were very bright, and incredibly far away; hundreds of millions or even billion of light-years away. The first ones were discovered in the radio spectrum, but over time, astronomers found even more blazing in the visible spectrum.
The astronomer Hong-Yee Chiu coined the term “quasar”, which stood for quasi-stellar object. They were like stars, shining from a single point source, but they clearly weren’t stars, blazing with more radiation than an entire galaxy.
Over the decades, astronomers puzzled out the nature of quasars, learning that they were actually black holes, actively feeding and blasting out radiation, visible billions of light-years away.
But they weren’t the stellar mass black holes, which were known to be from the death of giant stars. These were supermassive black holes, with millions or even billions of times the mass of the Sun.
As far back as the 1970s, astronomers considered the possibility that there might be these supermassive black holes at the heart of many other galaxies, even the Milky Way.
The Whirlpool Galaxy (Spiral Galaxy M51, NGC 5194), a classic spiral galaxy located in the Canes Venatici constellation, and its companion NGC 5195. Credit: NASA/ESA
In 1974, astronomers discovered a radio source at the center of the Milky Way emitting radiation. It was titled Sagittarius A*, with an asterisk that stands for “exciting”, well, in the “excited atoms” perspective.
This would match the emissions of a supermassive black hole that wasn’t actively feeding on material. Our own galaxy could have been a quasar in the past, or in the future, but right now, the black hole was mostly silent, apart from this subtle radiation.
Astronomers needed to be certain, so they performed a detailed survey of the very center of the Milky Way in the infrared spectrum, which allowed them to see through the gas and dust that obscures the core in visible light.
They discovered a group of stars orbiting Sagittarius A-star, like comets orbiting the Sun. Only a black hole with millions of times the mass of the Sun could provide the kind of gravitational anchor to whip these stars around in such bizarre orbits.
Further surveys found a supermassive black hole at the heart of the Andromeda Galaxy, in fact, it appears as if these monsters are at the center of almost every galaxy in the Universe.
But how did they form? Where did they come from? Did the galaxy form first, and cause the black hole to form at the middle, or did the black hole form, and build up a galaxy around them?
Until recently, this was actually still one of the big unsolved mysteries in astronomy. That said, astronomers have done plenty of research, using more and more sensitive observatories, worked out their theories, and now they’re gathering evidence to help get to the bottom of this mystery.
Astronomers have developed two models for how the large scale structure of the Universe came together: top down and bottom up.
In the top down model, an entire galactic supercluster formed all at once out of a huge cloud of primordial hydrogen left over from the Big Bang. A supercluster’s worth of stars.
As the cloud came together it, it spun up, kicking out smaller spirals and dwarf galaxies. These could have combined later on to form the more complex structure we see today. The supermassive black holes would have formed as the dense cores of these galaxies as they came together.
Hubble image of Messier 54, a globular cluster located in the Sagittarius Dwarf Galaxy. Credit: ESA/Hubble & NASA
If you want to wrap your mind around this, think of the stellar nursery that formed our Sun and a bunch of other stars. Imagine a single cloud of gas and dust forming multiple stars systems within it. Over time, the stars matured and drifted away from each other.
That’s top down. One big event that leads to the structure we see today.
In the bottom up model, pockets of gas and dust collected together into larger and larger masses, eventually forming dwarf galaxies, and even the clusters and superclusters we see today. The supermassive black holes at the heart of galaxies were grown from collisions and mergers between black holes over eons.
In fact, this is actually how astronomers think the planets in the Solar System formed. By pieces of dust attracting one another into larger and larger grains until the planet-sized objects formed over millions of years.
Bottom up, small parts coming together.
Shortly after the Big Bang, the entire Universe was incredibly dense. But it wasn’t the same density everywhere. Tiny quantum fluctuations in density at the beginning evolved over billions of years of expansion into the galactic superclusters we see today. Colliding galaxies can force the supermassive black holes in their cores together (NCSA)
I want to stop and let this sink into your brain for a second. There were microscopic variations in density in the early Universe. And these variations became the structures hundreds of millions of light-years across we see today.
Imagine the two forces at play as the expansion of the Universe happened. On the one hand, you’ve got the mutual gravity of the particles pulling one another together. And on the other hand, you’ve got the expansion of the Universe separating the particles from one another. The size of the galaxies, clusters and superclusters were decided by the balance point of those opposing forces.
If small pieces came together, then you’d get that bottom up formation. If large pieces came together, you’d get that top down formation.
When astronomers look out into the Universe at the largest scales, they observe clusters and superclusters as far as they can see – which supports the top down model.
On the other hand, observations show that the first stars formed just a few hundred million years after the Big Bang, which supports bottom up.
The key is that gravity moves at the speed of light, which means that the gravitational interactions between particles spreading away from each other needed to catch up, going the speed of light.
In other words, you wouldn’t get a supercluster’s worth of material coming together, only a star’s worth of material. But these first stars were made of pure hydrogen and helium, and could grow much more massive than the stars we have today. They would live fast and die in supernova explosions, creating much more massive black holes than we get today.
This illustration shows the final stages in the life of a supermassive star that fails to explode as a supernova, but instead implodes to form a black hole. Credit: NASA/ESA/P. Jeffries (STScI)
The first protogalaxies came together, collecting together these first monster black holes and the massive stars surrounding them. And then, over millions and billions of years, these black holes merged again and again, accumulating millions and even billions of times the mass of the Sun. This was how we got the modern galaxies we see today.
There was a recent observation that supports this conclusion. Earlier this year, astronomers announced the discovery of supermassive black holes at the center of relatively tiny galaxies. In our own Milky Way, the supermassive black hole is 4.1 million times the mass of the Sun, but accounts for only .01% of the galaxy’s total mass.
But astronomers from the University of Utah found two ultra compact galaxies with black holes of 4.4 million and 5.8 million times the mass of the Sun respectively. And yet, the black holes account for 13 and 18 percent of the mass of their host galaxies.
The thinking is that these galaxies were once normal, but collided with other galaxies earlier on in the history of the Universe, were stripped of their stars and then were spat out to roam the cosmos.
They’re the victims of those early merging events, evidence of the carnage that happened in the early Universe when the mergers were happening.
We always talk about the unsolved mysteries in the Universe, but this is one that astronomers are starting to puzzle out.
It seems most likely that the structure of the Universe we see today formed bottom up. The first stars came together into protogalaxies, dying as supernova to form the first black holes. The structure of the Universe we see today is the end result of billions of years of formation and destruction. With the supermassive black holes coming together over time.
Once telescopes like James Webb get to work, we should be able to see these pieces coming together, at the very edge of the observable Universe.
Guests: Paul M. Sutter (pmsutter.com / @PaulMattSutter)
Paul, one of our WSH regular panelists, will be talking about Song of the Stars, a project he is leading that brings astronomy to life using modern dance.
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We record the Weekly Space Hangout every Friday at 12:00 pm Pacific / 3:00 pm Eastern. You can watch us live on Google+, Universe Today, or the Universe Today YouTube page.
Seen at the James and Barbara Moore Observatory in Punta Gorda, Florida: a scope worthy of a quasar hunt. Photo by author.
“How far can you see with that thing?”
It’s a common question overhead at many public star parties in reference to telescopes.
In the coming weeks as the Moon passes Full and moves out of the evening sky, we’d like to challenge you to hunt down a bright example of one of the most distant and exotic objects known: a quasar.
To carry out this feat, you’ll need a ‘scope with at least an aperture of 20 centimetres or greater, dark skies, and patience.
Although more than 200,000 of quasars are currently known and they’re some of the most luminous objects in the universe, they’re also tremendously distant. A very few are brighter than magnitude +14, about the brightness of Pluto. Most quasars have an absolute magnitude rivaling our Sun, though if you plopped one down 33 light years away, we’d definitely have other things to worry about. Continue reading “Peer Into the Distant Universe: How to See Quasars With Backyard Telescopes”
An illustration that shows the powerful winds driven by a supermassive black hole at the centre of a galaxy. The schematic figure in the inset depicts the innermost regions of the galaxy where a black hole accretes, that is, consumes, at a very high rate the surrounding matter (light grey) in the form of a disc (darker grey). At the same time, part of that matter is cast away through powerful winds. (Credits: XMM-Newton and NuSTAR Missions; NASA/JPL-Caltech;Insert:ESA)
The combined observations from two generations of X-Ray space telescopes have now revealed a more complete picture of the nature of high-speed winds expelled from super-massive black holes. Scientist analyzing the observations discovered that the winds linked to these black holes can travel in all directions and not just a narrow beam as previously thought. The black holes reside at the center of active galaxies and quasars and are surrounded by accretion discs of matter. Such broad expansive winds have the potential to effect star formation throughout the host galaxy or quasar. The discovery will lead to revisions in the theories and models that more accurately explain the evolution of quasars and galaxies.
This plot of data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency’s (ESA’s) XMM-Newton determines for the first time the shape of ultra-fast winds from supermassive black holes, or quasars. The winds blow in every direction, in a nearly spherical fashion, coming from both sides of a galaxy (Credit: NASA/JPL-Caltech/Keele Univ.;XMM-Newton and NuSTAR Missions, [Ref])The observations were by the XMM-Newton and NuSTAR x-ray space telescopes of the quasar PDS 456. The observations were combined into the graphic, above. PDS 456 is a bright quasar residing in the constellation Serpens Cauda (near Ophiuchus). The data graph shows both a peak and a trough in the otherwise nominal x-ray emission profile as shown by the NuSTAR data (pink). The peak represents X-Ray emissions directed towards us (i.e.our telescopes) while the trough is X-Ray absorption that indicates that the expulsion of winds from the super-massive black hole is in many directions – effectively a spherical shell. The absorption feature caused by iron in the high speed wind is the new discovery.
X-Rays are the signature of the most energetic events in the Cosmos but also are produced from some of the most docile bodies – comets. The leading edge of a comet such as Rosetta’s P67 generates X-Ray emissions from the interaction of energetic solar ions capturing electrons from neutral particles in the comet’s coma (gas cloud). The observations of a super-massive black hole in a quasar billions of light years away involve the generation of x-rays on a far greater scale, by winds that evidently has influence on a galactic scale.
A diagram of the ESA XMM-Newton X-Ray Telescope. Delivered to orbit by a Ariane 5 launch vehicle in 1999. (Illustration Credit: ESA/XMM-Newton)
The study of star forming regions and the evolution of galaxies has focused on the effects of shock waves from supernova events that occur throughout the lifetime of a galaxy. Such shock waves trigger the collapse of gas clouds and formation of new stars. This new discovery by the combined efforts of two space telescope teams provides astrophysicists new insight into how star and galaxy formation takes place. Super-massive blackholes, at least early in the formation of a galaxy, can influence star formation everywhere.
The NuStar Space Telescope launched into Earth orbit by a Orbital Science Corp. Pegasus rocket, 2012. The Wolter telescope design – optics in the foreground, 10 meter truss and detectors at back – images throughout a spectral range from 5 to 80 KeV. (Credit: NASA/Caltech-JPL)
Both the ESA built XMM-Newton and the NuSTAR X-Ray space telescope, a SMEX class NASA mission, use grazing incidence optics, not glass (refraction) or mirrors (reflection) as in conventional visible light telescopes. The incidence angle of the X-rays must be very shallow and consequently the optics are extended out on a 10 meter (33 foot) truss in the case of NuSTAR and over a rigid frame on the XMM-Newton.
Diagram of one of three x-ray telescopes of the XMM-Newton design. Only a few of the grazing angle concentric mirrors are shown. Inset: a simplified illustration of how a Wolter telescope works. (Credits: Wikimedia, ESA) [click to enlarge]The spectral ranges of the XMM-Newton and NuSTAR Telescopes. (Credits: NASA, ESA)
The ESA built XMM-Newton was launched in 1999, an older generation design that used a rigid frame and structure. All the fairing volume and lift capability of the Ariane 5 launch vehicle was needed to put the Newton in orbit. The latest X-Ray telescope – NuSTAR – benefits from tens years of technological advances. The detectors are more efficient and faster and the rigid frame was replaced with a compact truss which required all of 30 minutes to deploy. Consequently, NuSTAR was launched on a Pegasus rocket piggybacked on a L-1011, a significantly smaller and less expensive launch system.
So now these observations are effectively delivered to the theorists and modelers. The data is like a new ingredient in the batter from which a galaxy and stars are formed. The models of galaxy and star formation will improve and will more accurately describe how quasars, with their active super-massive black-holes, transition into more quiescent galaxies such as our own Milky Way.
Hubble Frontier Fields observing programme, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe. Credit: NASA.
Gravity’s a funny thing. Not only does it tug away at you, me, planets, moons and stars, but it can even bend light itself. And once you’re bending light, well, you’ve got yourself a telescope.
Everyone here is familiar with the practical applications of gravity. If not just from exposure to Loony Tunes, with an abundance of scenes with an anthropomorphized coyote being hurled at the ground from gravitational acceleration, giant rocks plummeting to a spot inevitably marked with an X, previously occupied by a member of the “accelerati incredibilus” family and soon to be a big squish mark containing the bodily remains of the previously mentioned Wile E. Coyote.
Despite having a very limited understanding of it, Gravity is a pretty amazing force, not just for decimating a infinitely resurrecting coyote, but for keeping our feet on the ground and our planet in just the right spot around our Sun. The force due to gravity has got a whole bag of tricks, and reaches across Universal distances. But one of its best tricks is how it acts like a lens, magnifying distant objects for astronomy.
This artist's rending shows "before" and "after" images of a changing look quasar. Credit: Yale University.
Last week, astronomers at Yale University reported seeing something unusual: a seemingly stedfast beacon from the far reaches of the Universe went quiet. This relic light source, a quasar located in the region of our sky known as the celestial equator, unexpectedly became 6-7 times dimmer over the first decade of the 21st century. Thanks to this dramatic change in luminosity, astronomers now have an unprecedented opportunity to study both the life cycle of quasars and the galaxies that they once called home.
A quasar arises from a distant (and therefore, very old) galaxy that once contained a central, rotating supermassive black hole – what astronomers call an active galactic nucleus. This spinning beast ravenously swallowed up large amounts of ambient gas and dust, kicking up surrounding material and sending it streaming out of the galaxy at blistering speeds. Quasars shine because these ancient jets achieved tremendous energies, thereby giving rise to a torrent of light so powerful that astronomers are still able to detect it here on Earth, billions of years later.
In their hey-day, some active galactic nuclei were also energetic enough to excite electrons farther away from the central black hole. But even in the very early Universe, electrons couldn’t withstand that kind of excitement forever; the laws of physics don’t allow it. Eventually, each electron would drop back down to its rest state, releasing a photon of corresponding energy. This cycle of excitation happened over and over and over again, in regular and predictable patterns. Modern astronomers can visualize those transitions – and the energies that caused them – by examining a quasar’s optical spectrum for characteristic emission lines at certain wavelengths.
A simple example of an atomic spectrum, showing emission lines at particular wavelengths. Broad humps correspond to brighter emission lines, while lines that arise from narrow, lower-intensity emissions appear dimmer. Credit: NASA
Not all quasars are created equal, however. While the spectra of some quasars reveal many bright, broad emission lines at different energies, other quasars’ spectra consist of only the dim, narrow variety. Until now, some astronomers thought that these variations in emission lines among quasars were simply due to differences in their orientation as seen from Earth; that is, the more face-on a quasar was relative to us, the broader the emission lines astronomers would be able to see.
But all of that has now been thrown into question, thanks to our friend J015957.64+003310.5, the quasar revealed by the team of astronomers at Yale. Indeed, it is now plausible that a quasar’s pattern of emission lines simply changes over its lifetime. After gathering ten years of spectral observations from the quasar, the researchers observed its original change in brightness in 2010. In July 2014, they confirmed that it was still just as dim, disproving hypotheses that suggested the effect was simply due to intervening gas or dust. “We’ve looked at hundreds of thousands of quasars at this point, and now we’ve found one that has switched off,” explained C. Megan Urry, the study’s co-author.
How would that happen, you ask? After observing the comparable dearth of broad emission lines in its spectrum, Urry and her colleagues believe that long ago, the black hole at the heart of the quasar simply went on a diet. After all, an active galactic nucleus that consumed less material would generate less energy, giving rise to fainter particle jets and fewer excited atoms. “The power source just went dim,” said Stephanie LaMassa, the study’s principal investigator.
LaMassa continued, “Because the life cycle of a quasar is one of the big unknowns, catching one as it changes, within a human lifetime, is amazing.” And since the life cycle of quasars is dependent on the life cycle of supermassive black holes, this discovery may help astronomers to explain how those that lie at the center of most galaxies evolve over time – including Sagittarius A*, the supermassive black hole at the center of our own Milky Way.
“Even though astronomers have been studying quasars for more than 50 years, it’s exciting that someone like me, who has studied black holes for almost a decade, can find something completely new,” added LaMassa.
The team’s research will be published in an upcoming issue of The Astrophysical Journal. A pre-print of the paper is available here.
An artist's conception of a black hole binary in a heart of a quasar, with the data showing the periodic variability superposed.
Credit: Santiago Lombeyda/Caltech Center for Data-Driven Discovery
Most large galaxies harbor central supermassive black holes with masses equivalent to millions, or even billions, of Suns. Some, like the one in the center of the Milky Way Galaxy, lie quiet. Others, known as quasars, chow down on so much gas they outshine their host galaxies and are even visible across the Universe.
Although their brilliant light varies across all wavelengths, it does so randomly — there’s no regularity in the peaks and dips of brightness. Now Matthew Graham from Caltech and his colleagues have found an exception to the rule.
Quasar PG 1302-102 shows an unusual repeating light signature that looks like a sinusoidal curve. Astronomers think hidden behind the light are two supermassive black holes in the final phases of a merger — something theoretically predicted but never before seen. If the theory holds, astronomers might be able to witness two black holes en route to a collision of incredible scale.
The light curve combines data from two CRTS telescopes (CSS and MLS) with historical data from the LINEAR and ASAS surveys. Image Credit: Graham et al.
Graham and his colleagues discovered the unusual quasar on a whim. They were aiming to study quasar variability using the Catalina Real-Time Transient Survey (CRTS), which uses three ground-based telescopes to monitor some 500 million objects strewn across 80 percent of the sky, when 20 or so periodic sources popped up.
Of those 20 periodic quasars, PG 1302-102 was the most promising. It had a strong signal that appeared to repeat every five years or so. But what causes the repeating signal?
The black holes that power quasars do not emit light. Instead the light originates from the hot accretion disk that feeds the black hole. Orbiting clouds of gas, which are heated and ionized by the disk, also contribute in the form of visible emission lines.
“When you look at the emission lines in a spectrum from an object, what you’re really seeing is information about speed — whether something is moving toward you or away from you and how fast. It’s the Doppler effect,” said study coauthor Eilat Glikman from Middlebury College in Vermont, in a news release. “With quasars, you typically have one emission line, and that line is a symmetric curve. But with this quasar, it was necessary to add a second emission line with a slightly different speed than the first one in order to fit the data. That suggests something else, such as a second black hole, is perturbing this system.”
So a tight supermassive black hole binary is the most likely explanation for this oddly periodic quasar.
“Until now, the only known examples of supermassive black holes on their way to a merger have been separated by tens or hundreds of thousands of light years,” said study coauthor Daniel Stern from NASA’s Jet Propulsion Laboratory. “At such vast distances, it would take many millions, or even billions, of years for a collision and merger to occur. In contrast, the black holes in PG 1302-102 are, at most, a few hundredths of a light year apart and could merge in about a million years or less.”
But astronomers remain unsure about what physical mechanism is responsible for the quasar’s repeating light signal. It’s possible that one quasar is funneling material from its accretion disk into jets, which are rotating like beams from a lighthouse. Or perhaps a portion of the accretion disk itself is thicker than the rest, causing light to be blocked at certain spots in its orbit. Or maybe the accretion disk is dumping material onto the black hole in a regular fashion, causing periodic bursts of energy.
“Even though there are a number of viable physical mechanisms behind the periodicity we’re seeing — either the precessing jet, warped accretion disk or periodic dumping — these are all still fundamentally caused by a close binary system,” said Graham.
Astronomers still don’t have a good handle on what happens in the final few light-years of a black hole merger. And of course these two black holes still won’t collide for thousands to millions of years. Even watching for the period to shorten as they spiral inward would dwarf human timescales. But the discovery of a system so late in the game proves promising for future work.
3D map of the cosmic web at a distance of 10.8 billion light years from Earth. The map was generated from imprints of hydrogen gas observed in the spectrum of 24 background galaxies, which are located behind the volume being mapped. This is the first time that large-scale structures in such a distant part of the Universe have been mapped directly. The coloring represents the density of hydrogen gas tracing the cosmic web, with brighter colors representing higher density.
Credit: Casey Stark (UC Berkeley) and Khee-Gan Lee (MPIA)
On the largest scales, networks of gaseous filaments span hundreds of millions of light-years, connecting massive galaxy clusters. But this gas is so rarified, it’s impossible to see directly.
For years, astronomers have used quasars — brilliant galactic centers fueled by supermassive black holes rapidly accreting material — to map the otherwise invisible matter.
But now, for the first time, a team of astronomers led by Khee-Gan Lee, a post-doc at the Max Planck Institute for Astronomy, has managed to create a three-dimensional map of the large-scale structure of the Universe using distant galaxies. And the advantages are numerous.
The science has always gone a little something like this: as the bright light from a distant quasar travels toward Earth, it encounters the intervening clouds of hydrogen gas and is partially absorbed. This leaves dark absorption lines in the quasar’s spectrum.
Artist’s impression illustrating how a distant quasar’s or galaxy’s spectrum becomes clouded with absorption lines from intervening hydrogen gas. Credit: Khee-Gan Lee (MPIA) and Casey Stark (UC Berkeley)
If the Universe were static, the dark absorption lines would always be located at the same spot (121 nanometers for the so-called Lyman-alpha line) in the quasar’s spectrum. But because the Universe is expanding, the distant quasar is flying away from the Earth at a rapid speed. This stretches the quasar’s light, such that each intervening hydrogen gas cloud imprints its absorption signature on a different region of the quasar’s spectrum, leaving a forest of lines.
Therefore detailed measurements of multiple quasars’ spectra close together can actually reveal the three-dimensional nature of the intervening hydrogen clouds. But galaxies are nearly 100 times more numerous than quasars. So in theory they should provide a much more detailed map.
The only problem is that galaxies are also about 15 times fainter than quasars. So astronomers thought they were simply not bright enough to see well in the distant universe. But Lee carried out calculations that suggested otherwise.
“I was surprised to find that existing large telescopes should already be able to collect sufficient light from these faint galaxies to map the foreground absorption, albeit at a lower resolution than would be feasible with future telescopes,” said Lee in a news release. “Still, this would provide an unprecedented view of the cosmic web which has never been mapped at such vast distances.”
Lee and his colleagues used the 10-meter Keck I telescope on Mauna Kea, Hawaii to take a look a closer look at the distant galaxies and the forest of hydrogen absorption embedded in their spectra. But even the weather in Hawaii can turn ugly.
“We were pretty disappointed as the weather was terrible and we only managed to collect a few hours of good data,” said coauthor Joseph Hennawi, also from the Max Planck Institute for Astronomy. “But judging by the data quality as it came off the telescope, it was already clear to me that the experiment was going to work.”
The team was only able to collect data for four hours. But it was still unprecedented. They looked at 24 distant galaxies, which provided sufficient coverage of a small patch of the sky and allowed them to combine the information into a three-dimensional map.
The map reveals the large-scale structure of the Universe when it was only a quarter of its current age. But the team hopes to soon parse the map for more information about the structure’s function — following the flows of cosmic gas as it funneled away from voids and onto distant galaxies. It will provide a unique historical record on how the galaxy clusters and voids grew from inhomogeneities in the Big Bang.
The results have been published in the Astrophysical Journal and are available online.
A computer model shows one scenario for how light is spread through the early universe on vast scales (more than 50 million light years across). Astronomers will soon know whether or not these kinds of computer models give an accurate portrayal of light in the real cosmos.
Credit: Andrew Pontzen/Fabio Governato
Most scientists can see, hear, smell, touch or even taste their research. But astronomers can only study light — photons traveling billions of light-years across the cosmos before getting scooped up by an array of radio dishes or a single parabolic mirror orbiting the Earth.
Luckily the universe is overflowing with photons across a spectrum of energies and wavelengths. But astronomers don’t fully understand where most of the light, especially in the early universe, originates.
Now, new simulations hope to uncover the origin of the ultraviolet light that bathes — and shapes — the early cosmos.
“Which produces more light? A country’s biggest cities or its many tiny towns?” asked lead author Andrew Pontzen in a press release. “Cities are brighter, but towns are far more numerous. Understanding the balance would tell you something about the organization of the country. We’re posing a similar question about the universe: does ultraviolet light come from numerous but faint galaxies, or from a smaller number of quasars?”
Answering this question will give us a valuable insight into the way the universe built its galaxies over time. It will also help astronomers calibrate their measurements of dark energy, the mysterious agent that is somehow accelerating the universe’s expansion.
The problem is that most of intergalactic space is impossible to see directly. But quasars — brilliant galactic centers fueled by black holes rapidly accreting material — shine brightly and illuminate otherwise invisible matter. Any intervening gas will absorb the quasar’s light and leave dark lines in the arriving spectrum.
“Because they can be seen at such great distances, quasars are a useful probe for finding out the properties of the universe,” said Pontzen. “Distant quasars can be used as a backlight, and the properties of the gas between them and us are imprinted on the light.
Multiple clouds of intervening hydrogen gas leave a “forest” of hydrogen absorption lines in the quasar’s spectrum. But, crucially, not all gas in the universe contributes to these dark lines. When hydrogen is bombarded by ultraviolet light, it becomes ionized — the electron separates from the proton — which renders it transparent.
So the pattern of absorption lines visible in a quasar’s spectrum map out the location of neutral and ionized regions in between the quasar and the Earth.
This pattern will tell astronomers the main contributing light source in the early universe. Quasars are fairly limited in number but individually extremely bright. If they caused most of the radiation, the pattern will be far from uniform, with some areas nearly transparent and others strongly opaque. But if galaxies, which are far more numerous but much dimmer, caused most of the radiation, the pattern will be very uniform, with evenly spaced absorption lines.
Current samples of quasars aren’t quite big enough for a robust analysis of the subtle differences between the two scenarios. But Pontzen and colleagues show that a number of new surveys should shed light on the question.
The team is hopeful the DESI (Dark Energy Spectroscopic Instrument) survey, which will look at about a million distant quasars in order to better understand dark energy, will also show the distribution of intervening gas.
“It’s amazing how little is known about the objects that bathed the universe in ultraviolet radiation while galaxies assembled into their present form,” said coauthor Hiranya Peiris. “This technique gives us a novel handle on the intergalactic environment during this critical time in the Universe’s history.”
The paper was published Aug. 27 in the Astrophysical Journal Letters and is available online.
