In-depth description: The most distant black hole in the cosmos: quasar at a distance of 13 billion light-years discovered
Researchers have discovered the most distant active black hole yet known: a quasar so far away that its light has taken 13 billion years to reach us. Light from that quasar tells us about the properties of the universe a mere 690 million (0.69 billion) years after the Big Bang.
Quasars: extremely bright and incredibly distant
Quasars are exceedingly bright astronomical objects. They are the active nuclei of distant galaxies, and their light is produced when matter (such as gas, or even whole stars) spirals into a distant galaxy's central, supermassive black hole. Such matter collects in a so-called accretion disk around the black hole, reaching temperatures of up to a few hundred thousand degrees Celsius before finally falling into the black hole itself. The formation of quasars and their interactions with their host galaxies is an active area of study.
Typical quasars are as bright as a few trillion suns, and thus about ten times brighter than all the stars in our own galaxy combined. With such extreme luminosities, quasars are visible over large distances and are among the most distant astronomical objects we can observe.
For all these distant galaxies and quasars, their distance is typically determined making use of a systematic relation between distances and redshifts that follows directly from the models of cosmology. The redshift, concretely: how strongly the wavelengths of an object's light are shifted towards longer wavelengths by the expansion of the universe, can be determined from the spectra of galaxies and quasars. Using the standard model of cosmology, those redshifts can be converted to distance values.
Because of this connection between the redshift z and the distance, very distant objects are also referred to as "high-z". For the newly discovered quasar, the distance value corresponds to a redshift of z=7.5, meaning that its light reaches us at 7.5+1 = 8.5 times the wavelength at which it was originally emitted.
Probing the early universe with quasars
Distant quasars are not just a matter of humans' fondness for records and extremes – they carry key information about the properties and evolution of the universe! For one, quasar light can be used to "x-ray" the cosmos: hydrogen atoms between the distant quasar and the observer will absorb some of the light, and leave tell-tale signs of their presence in the spectrum of the quasar (that is, the rainbow-like composition of the quasar light into different wavelengths, or colors). In this way, quasars can be used to study the large-scale distribution of atomic intergalactic matter in the cosmos.
Such quasar-based studies of the distant large-scale universe promise answers to some very fundamental questions. How, for instance, has the fraction of neutral hydrogen (as opposed to ionized hydrogen) changed in the early universe? In the current models, the bright first stars reionized the gas filling our universe between 12.5 and 13.5 billion years ago, comparatively shortly after the Big Bang, stripping the electrons from most of the hydrogen atoms filling the cosmos back then. This cosmic reionization was a fundamental transition in the early universe.
There are currently several competing models for how this transition happened, some favoring an earlier, some a late onset of the reionization. With distant quasars, one can hope to pinpoint this transition: By measuring the amount of neutral hydrogen atoms in the distant intergalactic medium, one can constrain the fraction of neutral and of ionized matter, and rule out at least some of these models.
Witnessing galaxy evolution
Distant quasars are also interesting in and of themselves. After all, the long light travel times mean that we see the most distant quasars as they were when the universe was less than a billion years old. Research into the evolution of galaxies, of the central black holes of these galaxies and of the active phases when such a black hole becomes a quasar, is a highly active sub-field of cosmology. The different evolution models currently under discussion make different predictions for the rate of black hole growth and of galaxy growth.
The more distant the quasar, the deeper we are peering into the past. Different models of how galaxies and their central black holes have grown over time make different prediction for the maximal possible masses of both galaxies and their central black holes for different times in cosmic history. By observing the most distant quasars, which we see as they were billions of years ago, we can put those models to the test. After all, we see each quasar as it was in some bygone cosmic era – if light from a certain quasar takes 13 billion years to reach us, we will see that quasar as it was 13 billion years ago. If the quasar's black hole mass back then was larger than the maximal mass predicted by a specific model, that would count as strong evidence against the model.
For these reasons, finding very distant quasars has been an important goal of observational astronomy for decades. Of particular interest are quasars that we see as they were during the first billion years of cosmic history (redshift z > 6). At least for the era between about 850 million and one billion years after the Big Bang, astronomers had found a few dozen of these quasars between 2000 and 2010: the Sloan Digital Sky Survey (SDSS), a systematic survey covering the Northern hemisphere identified 20 quasars from that early period (redshift 6 < z < 6.5), and the Canada-France High-z Quasar Survey found another 15, some of those in the Southern hemisphere.
Systematic search for distant quasars
More distant quasars were harder to come by. In 2010 the group of Fabian Walter and Bram Venemans at the Max Planck Institute for Astronomy (MPIA) set out for a systematic search. The astronomers made use of large surveys, most notably the PanSTARRS1 survey to find the most distant quasars in the Universe. The Pan-STARRS1 survey (short for "Panoramic Survey Telescope & Rapid Response System 1") utilised a 1.8-meter telescope at the summit of Haleakalā, on Maui, to digitally map three quarters of the sky in visible and near infrared light. The survey took approximately four years to complete, and scanned the sky 12 times in five filters. Candidate objects that might be distant quasars were selected from the surveys, and then observed more closely using various telescopes accessible to MPIA researchers through special agreements. Finding and characterizing the most distant quasars became the PhD thesis of Eduardo Bañados, then a graduate student at MPIA.
This search nearly doubled the number of known quasars with redshifts higher than z=6, from dozens to about a hundred, with new finds in particular in the Southern hemisphere. An article describing the discovery and physical characterization of a sample of the most distant quasars was recently published by a current MPIA PhD student, Chiara Mazzucchelli.
A closer look at distant host galaxies
In parallel, members of Walter's group began to look at the newly discovered quasars in more detail. The stellar light of the host galaxies of these distant active galactic nuclei, radiating mostly in ultraviolet, visible light, and near infrared, are overshadowed by the powerful radiation of the quasar itself. On the other hand, at far infrared, submillimeter and millimeter radiation, and thus at much longer wavelengths, the host galaxy dominates the emission – so observations at these wavelengths are the method of choice when searching for host galaxies.
Via the Max Planck Society, MPIA has access to the NOEMA interferometer on Plateau de Bure in the French Alps, which combines several 15-m-antennas for millimeter radiation. For the quasars visible from the Southern hemisphere, the researchers used ALMA, a submillimeter/millimeter observatory in the Chilean Atacama desert. Using these telescopes, the astronomers were able to detect dust and gas emissions from the host galaxies of all of the quasars their systematic search had found.
These findings are important indicators of chemical evolution in the universe. Right after the Big Bang, the only elements in the universe were hydrogen (75%, by mass) and helium (25%). Pretty much all of elements heavier than helium we find the present-day universe were produced in stars, over the course of the billions of years following the Big Bang. The host galaxy studies indicated, in line with earlier results, that there was already a substantial amount of these metals (as elements heavier than helium are called in astronomy) in galaxies about a billion years after the Big Bang.
Going to even greater distances
In all those areas, data from an even earlier phase of cosmic evolution promises additional interesting information – on galaxy evolution as well as on chemical evolution. That is why the scientists decided to push even further, and set their sight on quasars more than 12.9 billion light years away (redshift z ≥ 7). At the start of their search, only one quasar in this distance range was known. It was 12.96 billion light years away; Bram Venemans at MPIA was the first astronomer to detect its host galaxy, using the IRAM Plateau de Bure interferometer.
The new search profited from the international element characteristic for successful scientific careers: when Bañados finished his PhD in 2015, he became a postdoctoral researcher at the Carnegie Institution for Science in the US, as a Carnegie-Princeton Fellow. Through his new institute, Bañados gained access to the Carnegie Institution's Magellan telescopes, two 6.5 meter telescopes at Las Campanas Observatory in Chile, significantly strengthening the observation powers of the quasar search.
The astronomers started this new stage of their search by looking at large-scale infrared surveys: the ALLWISE survey by NASA's WISE infrared space telescope, a large area survey by the United Kingdom Infrared Telescope (UKIRT) on Hawaii, and a survey by the Dark Energy Camera (DECam) at Cerro Tololo Inter-American Observatory in Chile. From the hundreds of millions of sources documented in these surveys' extensive catalogues, the astronomers selected several hundreds of quasar candidates. Those candidates were then observed more closely with numerous telescopes, including the Magellan telescopes.
Breaking the distance record
It was at this stage of the search that Bañados discovered the quasar J1342+ 0928 (whose designation, as is customary, is composed of coordinate values giving its position in the sky) using one of the Magellan telescopes. Astronomers had long been looking for a quasar as distant as this one. J1342+ 0928 broke all previous distance records for quasars in the early universe. The discovery observations unambiguously showed that this quasar is at a redshift z=7.5. This corresponds to a distance of 13.01 billion light years – light from that distant quasar took 13.01 billion years to reach us. The astronomers were seeing that quasar as it was a mere 690 million years after the Big Bang.
Further analysis showed that this was a comparatively bright quasar, emitting 40 trillion times as much energy per second as the sun. From estimates of the total quasar population, there should only between 10 and 100 quasars in total that are at least as distant and at least as bright as this one. A rare probe of the early universe indeed!
From the properties of another spectral line, that of ionized magnesium (MgII), the astronomers derived a value of 800 million solar masses for the mass of the quasar's central black hole. This large mass poses a challenge to models of supermassive black hole formation in the early universe. Such models would either need to show how there could have been "seed black holes" with masses of about 10,000 solar masses a mere 65 million years after the Big Bang, or they would need to demonstrate how the earliest black holes could grow more quickly than is commonly assumed (faster than the so-called Eddington limit).
Reionization happened rather late
The absorption features in the quasar light – traces of intergalactic material between the quasar and us – have another interesting consequence. Evidently, near the quasar, between 38 and 77 percent of intergalactic hydrogen were still in the form of atoms, and not yet ionized. The quasar observation give a glimpse of the reionization phase – and it provides evidence for those models where reionization sets in comparatively late in cosmic history.
When Bañados reported his exciting finding to his collaborators at MPIA, the astronomers acted quickly. Ordinarily, astronomers need to apply for time at large telescopes, a process that usually takes a few months. But for urgent observation requests, and to follow up quickly on new discoveries, most observatories have what is known as "Director's discretionary time" (DDT), which allows for a quick decision by an observatory director (or their representatives) to allocate observation time. The group at MPIA submitted DDT proposals both for the NOEMA interferometer and for the Extended VLA (EVLA) antenna field of the National Radio Astronomy Observatory in New Mexico.
The first NOEMA observations, with 8 antennas observing in unison, were undertaken mere days after Bañados had first discovered the quasar, analyzed at MPIA, and they showed clear traces of the quasar's host galaxy. Using the spectral lines of ionized carbon, commonly designated [CII], and the dust continuum, this analysis showed that the newly discovered quasar was very special indeed. The [CII] line also confirmed the quasar redshift of z=7.5 (corresponding to all wavelengths of its light stretched by a factor 8.5 since they were emitted, as a direct consequence of cosmic expansion).
A precocious host galaxy
The quasar host galaxy itself is highly active. The observations indicate that it is forming between 90 and 600 solar masses worth of stars per year (compared with about a single solar
mass per year in our home galaxy, the Milky Way). Equally important, the galaxy already contains copious amounts of metals (elements heavier than helium) and dust. The observations indicated the presence of about 100 million solar masses' worth of dust, and at least five million solar masses of carbon in the galaxy's interstellar medium.
All of these metals need to have been produced in massive stars, and spread throughout the interstellar medium by the supernova explosions that mark the end of such massive stars. The extreme brightness of the central accreting black hole makes it nearly impossible to directly detect the stellar light from the host galaxy. However, from the amount of dust and ionized carbon detected with the millimeter observations, the astronomers were able to estimate that quasar host galaxy could contain stars with a total mass of 20 billion solar masses – quite a lot, compared with the total stellar mass of between 40 and 60 billion solar masses of the stars our own Milky Way galaxy at the present time.
In other words: In only 690 million years, the host galaxy of the newly discovered quasar had already formed about half of the stars that the Milky Way formed within several billion years! It should be mentioned, though, that there is considerable uncertainty attached to these calculations, and it is possible that the number of stars formed in the quasar host is considerably smaller. In that case, in order to get the considerable amount of metals revealed by the observations, most of the stars would need to have been very massive.
Not only was the quasar host galaxy producing copious amount of dust 690 million years after the Big Bang, at the stage shown by the observations – it must have been producing numerous stars, and the associated heavier elements, in the millions of years before that! This puts strong constraints on models of galaxy evolution.
The new quasar will be an object of study for many years to come. Follow-up observations at millimeter wavelengths with ALMA have already been approved. These observations will shed light on the physical conditions in the quasar host galaxy, such as the temperature and the metal content of the gas that is forming stars. The quasar will also be targeted at various other wavelengths, painting a complete picture: The astronomers have already been given time on the Hubble Space Telescope for near-infrared observations, NASA's Chandra Space Telescope for observations in X-Rays, and infrared observations with NASA's Spitzer Space Telescope.
Furthermore, the quasar will be a prime target for the successor of the Hubble Space Telescope, the James Webb Space Telescope. With this facility, to be launched in 2019, astronomers will be able to disentangle the optical and near-infrared light of the stars in the host galaxy from that of the accreting black hole, and will thus finally be able to detect the stars in that distant galaxy directly.
Finally, the success of locating a quasar at such a large distance will fuel additional searches for these rare objects. Currently, several new facilities are being built that should allow astronomers to discover many more of these quasars in the early universe – notably ESA's Euclid space telescope, slated for launch in 2020.
Exciting times for reconstructions of some of the earliest phases of cosmic history –and distant quasars will play an important role!