Against all Odds: Astronomers Baffled by Discovery of Rare Quasar Quartet
14. Mai 2015
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Hitting the jackpot is one thing, but if you hit the jackpot four times in a row you might wonder if the odds were somehow stacked in your favor. A group of astronomers led by Joseph Hennawi of the Max Planck Institute for Astronomy have found themselves in exactly this situation. They discovered the first known quasar quartet: four quasars, each one a rare object in its own right, in close physical proximity to each other.
Quasars constitute a brief phase of galaxy evolution, powered by the infall of matter onto a supermassive black hole at the center of a galaxy. During this phase, they are the most luminous objects in the Universe, shining hundreds of times brighter than their host galaxies, which themselves contain hundreds of billions of stars. But these hyper-luminous episodes last only a tiny fraction of a galaxy’s lifetime, which is why astronomers need to be very lucky to catch any given galaxy in the act. As a result, quasars are exceedingly rare on the sky, and are typically separated by hundreds of millions of light years from one another. The researchers estimate that the odds of discovering a quadruple quasar by chance is one in ten million. How on Earth did they get so lucky?
Clues come from peculiar properties of the quartet’s environment. The four quasars are surrounded by a rare giant nebula of cool dense hydrogen gas - which the astronomers dubbed the "Jackpot nebula", given their surprise at discovering it around the already unprecedented quadruple quasar. The nebula emits light because it is irradiated by the intense glare of the quasars. In addition, both the quartet and the surrounding nebula reside in a rare corner of the universe with a surprisingly large amount of matter. “There are several hundred times more galaxies in this region than you would expect to see at these distances” explains J. Xavier Prochaska, professor at the University of California Santa Cruz and the principal investigator of the Keck observations.
Given the exceptionally large number of galaxies, this system resembles the massive agglomerations of galaxies, known as galaxy clusters, that astronomers observe in the present-day universe. But because the light from this cosmic metropolis has been travelling for 10 billion years before reaching Earth, the images show the region as it was 10 billion years ago, less than 4 billion years after the big bang. It is thus an example of a progenitor or ancestor of a present-day galaxy cluster, or proto-cluster for short.
Piecing all of these anomalies together, the researchers tried to understand what appears to be their incredible stroke of luck. Hennawi explains “if you discover something which, according to current scientific wisdom, should be extremely improbable, you can come to one of two conclusions: either you just got very lucky, or you need to modify your theory.”
The researchers speculate that some physical process might make quasar activity much more likely in specific environments. One possibility is that quasar episodes are triggered when galaxies collide or merge, because these violent interactions efficiently funnel gas onto the central black hole. Such encounters are much more likely to occur in a dense proto-cluster filled with galaxies, just as one is more likely to encounter traffic when driving through a big city.
"The giant emission nebula is an important piece of the puzzle," says Fabrizio Arrigoni-Battaia, a PhD student at the Max Planck Institute for Astronomy who was involved in the discovery, “since it signifies a tremendous amount of dense cool gas.” Supermassive black holes can only shine as quasars if there is gas for them to swallow, and an environment that is gas rich could provide favorable conditions for fueling quasars.
On the other hand, given the current understanding of how massive structures in the universe form, the presence of the giant nebula in the proto-cluster is totally unexpected. According to Sebastiano Cantalupo of ETH Zurich, a co-author of the study: "Our current models of cosmic structure formation based on supercomputer simulations predict that massive objects in the early universe should be filled with rarefied gas that is about ten million degrees, whereas this giant nebula requires gas thousands of times denser and colder."
"Extremely rare events have the power to overturn long-standing theories," says Hennawi. As such, the discovery of the first quadruple quasar may force cosmologists to rethink their models of quasar evolution and the formation of the most massive structures in the universe.
The results described here will be published as Hennawi et al., "Quasar Quartet Embedded in Giant Nebulae Reveals Rare Massive Structure in Distant Universe" in the May 15, 2015 edition of the journal Science.
More information, including a copy of the paper, can be found online at the Science press package at http://www.eurekalert.org/jrnls/sci. You will need your EurekAlert user ID and password to access this information.
The members of the group are Joseph F. Hennawi (Max Planck Institute for Astronomy), J. Xavier Prochaska (University of California at Santa Cruz), Sebastiano Cantalupo (University of California at Santa Cruz; ETH Zurich) and Fabrizio Arrigoni-Battaia (Max Planck Institute for Astronomy).
The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.
The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.
What is new / unusual / interesting about the discovery?
First, the extremely low probability of a discovery like this occurring by chance. Even close pairs of quasars are very rare: Out of the nearly 500,000 quasars that astronomers have cataloged to date, only about a hundred such binary quasars are known. It came as a big surprise in 2007, when a team of American and Swiss astronomers announced the discovery of the first triple quasar. But now the newly discovered quadruple quasar by Hennawi and collaborators (cf. “How was the quadruple quasar discovered?”, below) dramatically ups the ante, since the probability of finding such an object by chance is estimated to be one in ten million.
Such extremely low odds forced the scientists to consider that they are perhaps not dealing with an incredibly unlikely chance discovery, but rather that some physical process makes quasars much more likely to occur in specific environments, making the odds more favorable. This is where the unusual combination of properties for this region of space comes into play. The quasar quartet is embedded in an exceptionally bright and large emission nebula, and at the same time, resides in a massive proto-cluster of galaxies (as mentioned in the main text, our images show this region at a time when the universe was less than a third its current age).
This combination could be the key to explaining why the unusual quadruple quasar was discovered – it may be that quasar episodes are more likely to be triggered in such an unusual environment, which is rich in both gas and galaxies, as the researchers speculate. But the combination also poses new questions: Given our current understanding of the formation of such massive structures, giant emission nebulae and proto-clusters are not expected to occupy the same regions of space.
What are quasars, and why are they so rare?
Quasars constitute a brief phase of galaxy evolution, powered by the infall of matter onto a supermassive black hole at the center of a galaxy. During this phase, they are the most luminous objects in the Universe, shining hundreds of times brighter than their host galaxies, which themselves contain up to hundreds of billions of stars. Astronomers believe that every galaxy has a supermassive black hole embedded in its center, with typical masses between a few million and a few billion times the mass of our Sun. As matter swirls around these supermassive black holes, it travels at velocities near the speed of light, and is heated to temperatures of a million degrees, emitting copious amounts of light before being inevitably swallowed by the black hole.
The peak of quasar activity in galaxies occurred when the Universe was about one fifth of its current age, whereas today all massive galaxies are observed to have supermassive black holes at their centers that are dormant, which is to say that there is no significant inflow of matter onto them. However, in order to reach their present masses, these black holes must have swallowed enormous amounts of matter in the past, and current models link this growth to the galaxies' quasar episodes. The physical processes that determine when and why supermassive black holes shine as quasars are poorly understood, but it probably has to do with the supply of fuel: in order to ignite a quasar, a large amount of gas must find its way deep into the core of a galaxy, sufficiently close to experience the gravitational pull of the black hole.
While all supermassive black holes in massive galaxies underwent a quasar phase at some point in their evolution, this phase lasts only around ten million years, a thousand times shorter than the much longer ages of galaxies (about ten billion years and counting). Thus when we observe a quasar, we are catching a galaxy during a very brief period in its life, which explains why quasars are so rare on the sky, and hence typically separated by hundreds of millions of light years from one another.
How was the quadruple quasar discovered?
Hennawi and his colleagues were searching for quasars surrounded by so-called Lyman-α (pronounced “Lyman-alpha”) nebulae. If a quasar is surrounded by a large reservoir of cool hydrogen gas, the intense radiation emitted by the quasar can act like a ‘cosmic flashlight’, illuminating gas in its neighborhood and thereby revealing its structure. Under the quasar flashlight’s intense glare, the hydrogen gas emits light via the same mechanism at work in an ordinary fluorescent lamp, namely because it is being constantly bombarded with energy. In the case of ordinary lamps this energy is provided by an electrical current, whereas in Lyman-α nebulae the fluorescence is powered by energy from the quasar radiation (cf. MPIA Science Release 1/2014).
In their search for new Lyman-α nebulae, the researchers visually examined the spectra of 29 quasars to look for signatures of diffuse extended emission characteristic of fluorescing gas. One of their candidates, with the catalogue number SDSSJ0841+3921, appeared promising, and was then subjected to detailed observations using the LRIS imaging spectrometer at the 10m Keck Telescope on the summit of Maunakea in Hawaii. The object was observed with Keck/LRIS for 3 hours in late 2012, using a custom-built narrow-band filter that was tuned to capture only the light emitted by cool hydrogen gas (i.e. using a Lyman-α filter customized for the objects particular redshift).
These observations revealed one of the largest and brightest Lyman-α nebulae known to astronomy. The object is so distant that its light has taken nearly 10.5 billion years to reach us (cosmological redshift z = 2.041). The nebula has an extent of one million light-years across (310 kpc, corresponding to an angular size of 37 arcseconds). In the process of examining these images, the astronomers realized that there was not just one quasar, but four of them embedded in the nebula in one large physical structure. Examination of the four quasar spectra confirmed that these were indeed four distinct quasars (rather than multiple images of a single quasar; a phenomenon that can occur through gravitational lensing, when light is bent by the gravity of a massive object in the foreground). After their surprising find, the astronomers began referring to the giant Lyman-α nebula as the "Jackpot nebula".
What is a proto-cluster?
The largest gravitationally bound structures in the present-day universe are not individual galaxies, but rather huge agglomerations of up to a thousand galaxies, known as galaxy clusters, which extend several millions of light years across. In our current picture of structure formation, a cluster of galaxies continuously grows over cosmic time as more matter and galaxies collapse onto it due to its attractive gravitational force. Proto-cluster is the name given to the ancient progenitors of present-day clusters. Astronomers can directly observe such proto-clusters if they look sufficiently far into the distance - after all, in astronomy, the further you look into space, the further you look into the past.
For example, the light from the proto-cluster discovered around the quadruple quasar took 10.5 billion years to reach Earth (cosmological redshift z =2.041), and thus provides a view of what clusters of galaxies looked like just 4 billion years after the big bang. The proto-cluster has a size of several hundred thousand-light years, and within this region there are hundreds of times more galaxies than expected at a typical location in the distant universe.