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In-depth description: Discovery in the early universe poses black hole growth puzzle

Galaxies contain millions, billions, or hundreds of billions of stars. They are surrounded, and suffused, by a halo made of dark matter. And in the center of almost each galaxy, there is a supermassive black hole: a region with matter packed into such a compact space, not even light can escape its gravitational pull. The masses of these supermassive black holes range from a hundred thousand times the mass of the sun to billions of solar masses. The supermassive black hole in our own galaxy has 4 million solar masses.

The origin and growth of supermassive black holes

How did these black holes form? This is still an open question. Current models allow for several scenarios, all involving the formation of black holes, which grow over time from initial seeds. The origin of these initial seeds is still debated: they could be coming from the very first generation of massive stars, collapsing at the end of their lives, when their nuclear fuel is used up. Alternatively, the seeds could have formed from the direct collapse of gigantic clouds of gas in the early universe. Details are rather sketchy at this point, but all scenarios appear to predict black hole seeds in the range between a hundred and tens of thousands of solar masses – still a far cry from the millions or even billions of solar masses of the most massive black holes observed in the center of distant objects known as quasars. Evidently, these early black holes must have grown significantly.

Growth scenarios for these black holes have several components. The Big Bang happened 13,7 billion years ago; during nearly all of that time, as soon as the first galaxies had formed in the first few hundred million years or so, smaller galaxies have merged to form larger galaxies, their central black holes merging to form even larger central black holes. Another part of the growth comes from accretion, that is gas and material falling onto the black hole, thus adding to its mass.

Infalling matter and quasars

Artists' impression of a quasar: black hole (center) surrounded by a hot accretion disk, with two jets consisting of extremely fast particles perpendicularly to the disk. Zoom Image
Artists' impression of a quasar: black hole (center) surrounded by a hot accretion disk, with two jets consisting of extremely fast particles perpendicularly to the disk. [less]

Incidentally, infalling matter is a great boon when it comes to detecting these distant supermassive black holes in the first place. The infalling material transforms these black holes into the central engines of extremely luminous, compact light sources, known as quasars. The black holes pull in matter from their surroundings, which collects in a so-called accretion disk before plunging into the black hole. Matter falling towards the black hole releases exceptionally large amounts of energy, causing the disk to shine as brightly as all the stars in a large galaxy combined. Some of the energy is emitted in the form of jets, ultra-fast particle streams racing away perpendicularly to the accretion disk.

However, a problem regarding the timing and duration of the growth of black holes remains when looking at these earliest quasars. Recall that astronomers always look into the past: We see the Andromeda galaxy not as it is now, but as it was 2.5 million years ago, since Andromeda's light takes 2.5 million years to reach us. Astronomers have been able to observe quasars out to distances of billions of light years, implying that it takes the light from those quasars billions of years to reach us. In consequence, astronomers have been able to observe quasars, and thus indirectly supermassive black holes, as they were billions of years ago – some of them more than twelve billion years ago, from a time when the universe was less than ten percent of its present age.

Three unusual quasars

Some of these quasars observed in the very early universe contain supermassive black holes with a billion solar masses. How did these black holes grow so fast, in such a limited amount of time? Now a team of astronomers, led by Anna-Christina Eilers of the Max Planck Institute for Astronomy in Heidelberg, has discovered three very young quasars, making things even more difficult for those trying to explain the formation of supermassive black holes.

Eilers and her colleagues had analyzed optical and infrared spectra from a sample of 34 well-studied quasars, that is, the rainbow-like decomposition of these quasars light in the visible and infrared ranges. The quasars are so distant that they allow observers a glimpse of an early epoch of cosmic history, when the universe was still very young: less than a billion years old. Distances in this case are determined from the cosmic redshift of those quasars – a distance-dependent shift of their spectra towards longer wavelengths, due to the expansion of our universe.

In order to observe distant and thus very faint objects such as these quasars, astronomers need to employ the largest telescopes available on Earth – in this case, data was provided the two Keck telescopes, each with a mirror measuring ten meters in diameter, which are located on the summit of Mauna Kea in Hawaii. For some of the quasars, Eilers and her colleagues made use of existing Keck data, for others they recorded their own spectra.

A glimpse of cosmic history

The quasars in question are at high redshifts, corresponding to vast distances and long travel times of the emitted light, and providing insights into a time when the universe was still in its infancy. As such, they are a perfect laboratory for astronomers to study the early phases of galaxy formation and the growth of the first black holes. In fact, the main goal of Eilers and her colleagues had been to learn about the diffuse gas in the intergalactic space at very early times, and to learn about an early era of the universe known as the epoch of reionization. But during their analysis, they found three quasars that would raise a fundamental question about the formation of supermassive black holes in the early universe. The problem is one of timing.

We have seen how the infall of matter can turn black holes into super-luminous quasars. But this phase doesn't last forever. Quasars "switch on" when a sufficiently large supply of matter comes close to the black hole to fall onto the accretion disk – which is rare enough. Once the supply of infalling matter is exhausted, the black hole relapses into obscurity, and the quasar episode is over. For three of their quasars, Eilers and her colleagues found indications that the quasars had switched on only very recently. Their clues are drawn from the immediate environment of the quasars that has been influenced by the quasars’ radiation.

Quasar proximity zones

Figure 1: Basic set-up of the quasar observations: Light from a quasar (right) is absorbed by gas. Absorption is much less in the quasar's proximity zone, which is shown in green for an older quasar, in yellow for a younger quasar. The extent of the proximity zone can be read off the spectrum (bottom). The quasar itself is a central black hole, surrounded by a disk of swirling matter, and possibly sending out particles in two tightly focussed jets (inset, top right). Zoom Image
Figure 1: Basic set-up of the quasar observations: Light from a quasar (right) is absorbed by gas. Absorption is much less in the quasar's proximity zone, which is shown in green for an older quasar, in yellow for a younger quasar. The extent of the proximity zone can be read off the spectrum (bottom). The quasar itself is a central black hole, surrounded by a disk of swirling matter, and possibly sending out particles in two tightly focussed jets (inset, top right). [less]

The light that these very distant quasars emit is mostly absorbed due to neutral gas in the intergalactic space between the Earth and the quasars. In fact, only a relatively small fraction of the light reaches Earth, less than 1% of the originally emitted light, making observations hugely challenging. Close to the quasar, though, radiation intensities are extremely high; sufficiently high, in fact, to ionize the intergalactic gas surrounding the quasar, separating its electrons from their nuclei. The main absorption mechanism in this case is directly linked to spectral lines – associated with the state transitions of atoms (more precisely, with the so-called Lyman-α line and various redshifted versions thereof).

Once the gas is ionized, this kind of absorption is disabled. In a characteristic wavelength range, the ionized gas acts as a window, letting significantly more light escape towards Earth. The region of ionized gas surrounding a quasar is called the proximity zone of the quasar.

After a quasar has switched on and begun to shine, it takes some time for it to ionize the surrounding gas. As the quasar continues to emit extreme radiation, the amount of ionized gas increases, and so does the extent of its proximity zone. At least for some time after the quasar has become active, the extent of the proximity zone can be used as a measure for how long this quasar has already been shining – in short: it provides a measurement of the age of this particular quasar. The size of the ionized region around each quasar can be inferred from the shape of the quasar spectrum, which gives an indication of the amount of light in each wavelength range that was able to escape and reach distant observers here on Earth.

Simulations and age estimates

In order to link the quasar spectra, and the extent and properties of quasar proximity zones, to the quasar age (how long since switch-on?), the astronomers turned to simulations run by Frederick B. Davies, a postdoctoral researcher at the Max Planck Institute for Astronomy who is an expert in the interaction between quasar light and intergalactic gas. The backbone of large-scale structure evolution is the evolution of dark matter in the universe – which is comparatively homogeneous in the big bang phase and has been clumping together for the past 13.7 billion years, forming a giant network of filaments in the process, which is known as the cosmic web.

Based on previous simulations of the cosmic web, Davies simulated the evolution of gas in this changing environment, showing how the gas collects along the filaments, how quasars form, and how surrounding gas is ionized by the quasars' intense light. The simulations also allow Davies to predict how the resulting quasar spectra, modified by gas absorption, would look to an observer on Earth. By comparing the appearance of spectra for quasars of different ages, the astronomers gained an understanding of the link between a quasar's age, the size of its proximity zone, and the telltale fingerprint of such a proximity zone in a quasar spectrum.

Surprisingly young quasars

The surprising result was that three of the quasars had only very small ionized regions around them. These three quasars had apparently switched on no more than about 100,000 years ago, yet had masses of billions of solar masses already. Could this last quasar episode have been preceded by another quasar phase? No, because the ionized gas from this first episode would not have sufficient time to cool down. Recall that we are observing this quasar as it was less than a billion years after the Big Bang; that time would not suffice for the proximity zone to fade away. Traces of earlier activity would still be visible in the quasar spectrum.

Thus, we are left with quasars that have been collecting matter for less than 100,000 years in total, yet have masses of billions of solar masses. That is unusual, to say the least. After all, infalling matter is what makes a black hole grow, makes its mass increase. (Collisions and merges of galaxies, and of their respective central black holes, will not make a marked difference on those comparatively short timescales.) This is where timing becomes a problem: somehow, these quasar's black holes must have managed to grow very rapidly within the limited amount of time.

The limits to black hole growth

This growth is difficult to explain. The fundamental laws of physics limit the amount of matter – in practice: gas – that can fall into a black hole in a given amount of time. After all, infalling matter is what makes a quasar luminous – and above a certain luminosity, the pressure exerted by the quasar's radiation will be sufficient to prevent any additional matter from falling in. This leads to a maximum accretion rate called the Eddington accretion rate, and to a mass-dependent maximum luminosity known as Eddington luminosity.

Given these fundamental limitations, the black holes in question could just about have managed to reach their observed, gigantic mass if they had swallowed matter non-stop over at least a hundred million years. But the measurements indicate that these quasars have been active, in total, for less than 100,000 years. There simply isn't enough time. And since Eilers and her colleagues found three of these problematic quasars, in a sample of only 34, chances are that a sizeable fraction of supermassive black holes are likely to have that particular problem.

Open questions

Forming supermassive black holes so quickly is beyond the capacity of current models of black hole growth. How could such massive black holes grow so quickly? Can they accrete matter a lot faster than current theories predict? Or are the initial seeds from which the black holes start growing already a lot more massive than assumed? But if this is the case, what is the origin of those seeds? At this point, these questions are completely open.

The discovery of these young objects challenges the existing theories of black hole formation and will require new models. These models will hopefully lead to a new understanding of how black holes and galaxies formed – and are likely to change not only our understanding of early cosmic history, but also of black hole and galaxy formation in the present universe. After all, each of the present-day billion-solar-mass supermassive black holes in the centers of galaxies has a history of how it gained its mass – and any new type of growth scenario could play a role in those histories, as well.

The next step

For Eilers and her colleagues, the next step will be to try to detect more of these very young, yet massive quasars. To this end, the astronomers have applied for telescope time to conduct observations using some of the largest telescopes on Earth. It is still possible that the three detected young quasars are just extreme outliers in the overall population of quasars.

But if, on the other hand, observations will reveal the presence of more of these unusual quasars, then a significant fraction of the known quasar population would be a lot younger than previously expected. An estimate of how rare, or how common, such quasars really are should go a long way towards constraining new theoretical models for the formation of the first black holes in the universe.

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