Thanks to a record haul of new, ultra-distant quasars—powerhouses of light from the farthest reaches of the universe—astrophysicists can now piece together the rise of mighty objects in the early cosmos.
The discovery of more than 60 quasars — stupendously bright regions in the cores of galaxies, powered by gargantuan black holes — is a windfall for astrophysicists probing the early universe. At more than 13 billion light-years away, these brilliant beacons rank among the farthest objects ever glimpsed by humans.
That’s important because they take us way back in time, to the first billion years after the Big Bang, and may help explain how the first galaxies and supermassive black holes arose. Guided by their light, astrophysicist hope to understand how the universe transitioned from a dark, featureless expanse into a rich, starry realm loaded with luminous galaxies.
The Kavli Foundation recently spoke with three astrophysicists about how this haul of ultra-distant quasars will transform what we know about the early universe.
The participants were:
- ROBERTO MAIOLINO – is a professor of experimental astrophysics at the Cavendish Laboratory of the University of Cambridge and director of the Kavli Institute for Cosmology, Cambridge (KICC). He studies distant quasars to learn about how galaxies and black holes have evolved together throughout cosmic history.
- LINHUA JIANG – is the Youth Qianren Research Professor at the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University. An author of two recent studies that discovered dozens of new and extremely distant quasars, Jiang is interested in how the first galaxies changed the universe hundreds of millions of years after the Big Bang.
- MARTA VOLONTERI – is research director at the Institut d’Astrophysique de Paris. A theorist, she is the principal investigator of the BLACK project, which investigates how supermassive black holes formed and influenced their host galaxies, especially as quasars, in the early universe.
The following edited transcript of their roundtable discussion is provided courtesy of The Kavli Foundation. The participants have been provided the opportunity to amend or edit their remarks.
THE KAVLI FOUNDATION: Before we talk about the new discovery, what is a quasar and why do you find them so fascinating?
ROBERTO MAIOLINO: Quasars are the cores of galaxies powered by supermassive black holes gobbling up matter at a high rate. These black holes have masses typically exceeding one million times that of our Sun. The process of consuming matter radiates a lot of energy as light. In fact, most quasars are so bright, they outshine their host galaxy by a large factor. Most quasars are also very far away, and the ones we are particularly interested in are those that developed when the universe was young—less than a billion years old.
You can think of quasars as lighthouses in the dark of the early universe. Just as a lighthouse’s beam might shine on nearby land forms, making them visible from far away, quasars enable us to investigate the very distant universe and understand the physics of primordial galaxies. We think that quasars indicate special regions in the early universe where matter is particularly dense. As the cosmos developed, these so-called overdense regions probably ended up being populated by a large number of galaxies. So quasars help us to learn about these sites of galaxy formation. We also believe that quasars are tightly connected with the evolution of their young, host galaxies.
MARTA VOLONTERI: I’m interested in whether quasars can illuminate the origins of supermassive black holes, which can possess less than a million to several billions of times the mass of the Sun. Black holes exist in the center of most galaxies, including the Milky Way, but we don’t know how they got there.
LINHUA JIANG: What makes distant quasars so interesting to me, as an observational astronomer, is that they are very difficult to find.
TKF: And we now have twice as many of these lighthouses in deep space to observe. Why is that important?
MAIOLINO: Until now, we have only had a chance to study a few ultra-distant quasars. What those can teach us about the nature of quasars, and more broadly about the general state of the cosmos long ago, is highly limited. With the newly discovered quasars, we will be able to gauge the variety of these monstrously powerful objects in the universe and how they affect their host galaxies.
JIANG: Echoing what Roberto just said, now that we have a much larger sample of quasars than ever before, roughly 200, we can study them to learn about their individual variation and how they collectively influenced the primordial cosmos.
TKF: The more quasars, the merrier, right?
TKF: Quasars were identified in the early 1960s, and yet the tally remains pretty small compared to the hundreds of billions of galaxies known to exist in our universe. Why are quasars so difficult to find?
MAIOLINO: Quasars are typically so far away, we generally only see them as point-like sources of light through our telescopes—the same as we see stars. That’s how these objects got their name—“quasars,” for “quasi-stars.” We didn’t know these objects were inside other galaxies, and not just stars, until we measured the light coming from them, which showed they were very far away. The identification of quasars, especially the very distant ones, generally require extensive observing campaigns with large telescopes. Luminous distant quasars are also very rare, hence finding them among the plethora of other celestial objects is often a difficult process.
TKF: So finding quasars depends heavily on building increasingly powerful and sensitive telescopes?
JIANG: Yes. To find the most distant quasars, which are not as bright as closer quasars, you really need telescope surveys that take images across a very large part of the sky. My colleagues and I used both the Sloan Digital Sky Survey and the Pan-STARRS survey to find the quasars that we recently reported. Before those surveys began, we really knew very little about distant quasars.
TKF: And while quasars have been hard to find in the past, do you expect this to change?
JIANG: Yes. With the next generation of telescopes, we should find many more quasars.
VOLONTERI: We are probably seeing just the tip of the iceberg. We know that small objects are more common in the universe than big things. We see this when it comes galaxies, stars, planets . . . really everything else! We would therefore expect there to be a lot more quasars out there that are smaller and fainter. Also, the luminosity of the quasars we’ve detected is extremely high, so we are probably only seeing the brightest outliers. That means we are studying quasars with a very limited range of properties.
TKF: Roberto, you mentioned earlier that quasars outshine their host galaxies. How does all this energy affect their host galaxies?
MAIOLINO: Quasars can “kill” themselves and their galaxies by completely cleaning out a galaxy’s gas content. This happens because they drive some of the most powerful outflows of gas in the universe that we’ve ever seen, and when they do, they remove the fuel available for star formation.
VOLONTERI: Right. A quasar dumps so much energy into a host galaxy that it can influence how often stars form.
TKF: As for black holes, what do quasars reveal about them, and why is this important?
VOLONTERI: Knowing more about the black holes powering quasars will allow us to know more about how galaxies develop, and knowing about the evolution of galaxies allows us to trace the universe’s history overall. That’s why finding more quasars to study is so fundamental.
MAIOLINO: Observations have shown us that a significant fraction of these primordial black holes is extremely massive. In the local universe, black holes typically have masses of only one-thousandth of their host galaxy. But in the distant, early universe, we now know some black holes can reach masses close to 10 percent of that of their host galaxy. That’s amazing and this tells us that in the early universe, black holes overtake galaxies in terms of forming and growing. Only later in the universe’s history do the galaxies catch up. So observations are already giving us some indications about the early evolutionary path of our universe.
JIANG: A mystery, though, is that there does not seem to have been enough time for the universe to have grown these supermassive black holes, given how early in cosmic history we begin to see them as quasars. So for a supermassive black hole formation scenario to be right, it has to account for that rapid growth.
TKF: Shifting gears here, let’s talk about a period in the history of the universe when it literally went from dark to light. Linhua, what role do we think the earliest quasars had in this transformation?
JIANG: The idea is still controversial, but quasars may have provided the energy that fueled a change in the gas between the galaxies, allowing light to pass through it. That turning point, when the universe was roughly a billion years old, is known as the "epoch of reionization." It happened when neutral atoms of hydrogen gas became ionized, which is how they had originally been when the universe began in a hot, dense state. The question is, how and why did this happen? Ionization takes a lot of energy. What were the cosmic sources of the high-energy light that drew the universe out of the so-called dark ages, the era before the first stars and galaxies formed? Could quasars be the answer? At the moment, that seems unlikely because there are so few quasars known. But, as Marta said earlier, we are probably seeing only the tip of the quasar iceberg. There could be a lot more that we haven’t seen yet.
VOLONTERI: We have recently made a theoretical breakthrough that will help us figure out how much of a role quasars played in the epoch of reionization. We can now accurately monitor radiation inside of our computer simulations as galaxies evolve. We should soon be able to count how many light particles can leave a galaxy and start ionizing extragalactic gas, which I think is really awesome.
TKF: Looking ahead, what are some of the projects and missions that could help us find even more quasars and better characterize them?
MAIOLINO: I expect that the Large Synoptic Survey Telescope, or LSST, will greatly expand our numbers of distant quasars using visible light, when it opens in 2022. If we want to look even further back on time, before the epoch of reionization, then we need to use infrared light. The prime surveys for doing that will both be space-based. One is called EUCLID, launching in 2020, and the other is WFIRST, launching in the mid 2020s. I’d expect these missions to deliver very distant galaxies and quasars and to help detect quasars hidden by cosmic dust.
JIANG: Once we find new candidates, we have to confirm them as quasars by looking for chemical signatures in the light observations using a method called spectroscopy. It is very costly to allocate the time on telescopes to take the long observations we need to do spectroscopy. But we will do it, because it allows us to learn a lot about the properties of quasars.
MAIOLINO: Right. We will want to investigate the physical properties of distant quasars even better than we can do now. The James Webb Space Telescope, the successor to the Hubble Space Telescope, and a few other next-generation facilities, like the Thirty Meter Telescope, the Giant Magellan Telescope, and the European Extremely Large Telescope will enable us to scrutinize what’s happening in the quasars’ host galaxies and with their supermassive black holes.
TKF: What mysteries about quasars do you still hope to answer?
MAIOLINO: Observations of the earliest quasars show that their host galaxies are already enriched with huge amounts of heavy elements, such as iron, as well as cosmic dust, small particles that are ejected into space when the stars die. This enrichment process takes time—many hundreds of millions of years.
Yet, we see these distant galaxies, illuminated by quasars, when the age of the universe was less than one billion years. That suggests that everything in these early galaxies with quasars seems to be going on at a much faster rate than any other galaxies that we know of in the universe, and we don’t know why.
I’m confident that upcoming observations will shed a lot of light on these amazing objects.
JIANG: Studying distant quasars will help us gauge the “clumpiness” of gas in the spaces between the galaxies. We’ll learn more about the early history of galaxies and how the cosmos got its shape, so to speak.
VOLONTERI: As we’ve said, 200 distant quasars is only the tip of the iceberg. We still don’t know about the broader population of quasars and how they can explain the growth of black holes in galaxies, so we need more data.