Klaas S. de Boer
    Argelander Institut für Astronomie (Abt. Sternwarte), Univ. Bonn
 

"Black hole" is the name of an object that has an exceedingly strong gavitational pull on its surface, so that even light cannot escape from that object. That objects with the "no-escape effect" exist was predicted in 1915. Schwarzschild had formulated that year a consequence of the relativity theory of Einstein for exceedingly massive stars. In the name "black hole", a name coined much later for such objects, the black refers to the "no-escape effect" (not even light),

Geometry of a black hole. A very compact and high mass object produces a very deep potentiall well. The red circle marks the Schwarzschild limit (also called the event horizon), the location from within which nothing, not even light, can escape. If the well is infinitely deep, the deepest point would be a mathematical singularity. Since a black hole is produced by a very compact object consisting of normal matter, the well has normally a finite depth.

the hole refers to the graphic representation of its gravitational field, most often sketched as a very deep potential well, reminiscent of a hole. (Basics on stellar evolution can be found in the textbook by de Boer & Seggewiss, 2008; hereafter dBS.)

During the following decades one could only speculate if black holes were a reality. From the 1970s onward, X-ray observations uncovered the existence of very small and X-ray-bright celestial objects. Then a hot star was found at the position of such an X-ray source. Astonishingly, it appeared to be a star with a wobble, i.e., the star was orbiting around another object (as recognised from its rhythmically varying velocity). However, its companion could not be detected. With the well-established experience of binary star research it became clear that the companion object should have a mass of at least five times that of the Sun (dBS Ch.19). Stars with so much mass ought to shine brightly. But since nothing was seen, and with no doubt about the binary nature, it was concluded that the companion was a black hole indeed! This discovery was part of the reason to bestow one of the prime movers of X-Ray astronomy, Ricardo Giacconi, with the honour of the Nobel Prize 2002 (text in german).

Such black holes come into existence when a very mass-rich star approaches the end of its life (see evolution). That star has had multiple phases of nuclear burning (first hydrogen to helium, then helium to carbon, and so on; dBS Ch.5) until its central part consists mostly of the most compact element: iron. Iron cannot normally come to fusion to form heavier elements. With ever more weighty outer layers (becoming more compact due to the heavier elements formed there through fusion) the stellar core can no longer carry that weight and the free electrons in the core are forced into the nuclei of iron leading to neutronization. The stellar core collapses (dBS Ch.18), the outer layers rush away in an explosion (that makes the event to supernova Type II) and the massive neutronized core shrinks to within the so-called Schwarzschild limit (formulated in 1915), preventing any light to escape from the surface. Perhaps astonishing: A black hole is born.... to be invisble!

The first quasars ("quasi stellar radiosources") were discovered in the late 1950s in all-sky radio surveys (see Wikipedia). And "quasi stellar objects" were found in the optical. Maarten Schmidt obtained the first optical spectrum of a QSO in 1967. Soon it became clear that these types of object are very similar and then generally were named quasar. Quasars emit gigantic amounts of radiation, sometimes more than all radiation from one large galaxy. This makes clear, that quasars can be seen even in the far reaches of the universe. With infrared telescopes and the Hubble Space Telescope, the "host galaxies" surrounding the quasars have been detected in some cases. These galaxies are normally too dim to be seen against the glare of the quasar, except with special techniques (Wikipedia). X-Ray astronomical measurements revealed also that the centres of some galaxies were bright in X-Ray radiation. In addition, such centres showed numerous spectral structures in the visual spectra signifying the emission of light from highly excited atoms and ions.

Artist's impression of a quasar. A supermassive black hole is surrounded by a disk of material being drawn in. Two polar jets spew highly energized material outward. (Image: Carnegie Institution for Science)

This combination suggested that the centres of large galaxies sometimes have properties like quasars. In the mean time, so-called Active Galactic Nuclei (AGNs) had been found.

The radiation from a quasar comes mostly as a smooth continuum over all wavelengths, signifying it emerges in a very hot plasma. The accepted model of a quasar is an exceedingly heavy black hole surrounded by a disk of material, falling into the black hole's potential well. This falling in means also rotation, so a disk is formed and the inner side of this disk rotates extremely fast, leading to an accretion disk (image). Due to the twisting of the magnetic fields, some of that very hot material is ejected perpendicular to the disk, in polar jets.  [ Remark: the formation of young stars also requires an accretion disk; in it planets can form (dBS Ch.7, and Orion).] With this model and the data on quasars and active galaxies it became clear that many a galaxy has a supermassive black hole in its centre.

The centre of the Milky Way had been investigated for decades but dust in the intervening gas had obscured our view. After devices measuring at near-infrared wavelengths had been developed (IR-radiation is hardly affected by dust), the centre of the Milky Way could be seen. Soon one found stars zipping around that very centre. Studying their positions over longer periods of time showed that they must be under the spell of a very strong gravitational force, something only imaginable if there was a very massive black hole in the centre, an object with a mass of well over 10 million Suns. So our Milky Way is, also in this sense, not an exceptional galaxy.

Really measuring distances in the Universe is neigh to impossible. Astronomers have found ways around this problem. They have devised appropriate "yardsticks" to that end. Details can be found in determining distances (in German). Briefly:

1. One starts with stars. Beginning with the parallactic method, geometric distances to nearby stars could be determined. It led to the definition of a "parsec" (pc), the distance of a star showing one second of arc in parallax.  (This distance equals 3.1 years of travel time for light, hence the derived distance unit of a "light-year". Note: 1000 pc =1 kiloparsec; 1000 kpc =1 Megaparsec.)  With these nearby stars one could calibrate the intrinsic amount of light emitted by these stars. Together with spectroscopic information, this classified the stars rather precisely.
2. Understanding stars, one trusted the spectroscopic information to be sufficient for classification. Then, noting of a star the actual small amount of light received in comparison with what such a star would radiate, one could calculate its distance.
3. As soon as one knew enough stars, one recognised that some variable stars (stars rhythmically varying the amount of emitted light) always had an intrinsic brightness within some limits. Seeing little light of such a star meant it must be far away, yet its distance could be calculated. For this technique the intrinsically bright Cepheid variable stars could be used to even reach neighbouring galaxies.

Edwin Hubble, extending earlier observations, showed in 1928 (with the help of Cepheid stars) that the farther away a galaxy is, the faster it receedes from us. This receeding is found from the spectrum of galaxies: absorption structures in their spectra are shifted toward the red, the so-called red shift. This led to the notion of an expanding universe. The measurements of Cepheid stars in galaxies with the Hubble Space Telescope calibrated this expansion in the nearby part of the universe, in that part, in which the HST could spatially recognise those Cepheids. The expansion rate of the universe amounts to about 71 km/s per Megaparsec distance.

4. A new yardstick emerged once the supernovae of Type Ia were understood. These supernovae are the explosion of a white dwarf star in a binary star system, an explosion occurring once the star (accreting matter from its neighboor) reaches a mass of 1.4 times that of the Sun. This through physics well defined mass suggested all these supernovae are the same, thus becoming equally bright, thereby providing a yardstick reaching to much larger distances than the Cepheid stars. The collection of SN Type Ia supported the expansion rate of the universe of 71 km/s per Megaparsec distance. *)

Finally, if one trusts the relation for the expansion of the universe, one can use the red-shift observed in the spectrum of a galaxy to calculate its distance. Thus, in fact, the yardstick for the far away reaches of the universe is the red-shift. Note the sequence of assumptions leading to this yardstick: calibrating stars, calibrationg Cepheids, trusting the equalness of SN Ia, determining the expansion rate of the universe from galaxies with known Cepheid or SN Ia distances, and trusting that this calibrated expansion rate applies to the entire universe. Astronomers accept this all; deep down they know not all need be perfect, but there is nothing better.

The distance of a far away quasar is derived from the observed red-shift of features in its spectrum. Note that, if a quasar is very far away, its spectrum is considerably red-shifted. This means that features normally appearing at visual wavelegths get stretched by the expansion of the universe into the infrared.

The spectrum of J1342+0928 with the redshifted [C II] emission and the underlying continuum observed with NOEMA. The spectrum shows that the host galaxy has plenty of carbon, so it is chemically developed. (From Venemans+ 2017.)

Similarly, radiation emitted by a quasar at short radio wavelegths will arrive here at long radio wavelengths. Astronomers have acquired a good amount of experience in recognising the actual red-shift in these spectra.

The host galaxy of the just discovered quasar J1342+0928 has a red-shift of 7.54 (as measured with the millimeter interferometer NOEMA, operated by IRAM, in the French Alps and the VLA radio telescope array in Socorro, New Mexico). One looked for the radio emission feature of [C II] at 158 μm, and detected it at 1.349 mm (Venemans+ 2017). This emission all by itself shows that many generations of stars have developed, each contributing to the abundance of carbon in that galaxy.

The red-shift of 7.54 is extremely large. It signifies an extremely large distance from us, at the same time looking back into the past to 13.1 billion years (13.1 Gyr; Gigayears) ago. The previous recordholder was the quasar J1120+0641 with a redshift z=7.09, which (in the "Standard" model of cosmology) equals about 12.5 Gyr.

Stellar black holes form as remnants of massive stars having exploded as supernova (Type II). A binary system with massive stars may lead to a pair of black holes that ultimately will merge, producing gravitational waves. Once a massive black hole is formed, it will draw to its deep potential well any material in the vicinity. Such material will swirl in, forming a so-called accretion disk (see image above). These gases emit light, showing the highly excited state of the gas.

The spectra of quasars show a continuum with numerous emission features superimposed, features going back to light from well known elements in ionized states such as C II at 133 nm, Si IV at 140 nm, C IV at 155 nm, Mg II at 280 nm, and others. The spectrum of J1342+0928 shows several of these, shifted by z=7.54, far to the red from their original wavelengths.

Stars forming in the pristine gas of the early universe must have been exceedingly massive (dBS Ch.15). They may have had a mass of 100 to 1000 times that of the Sun, and they are thought to live about a million years until they explode and leave a heavy black hole behind. During this life span, they produce, through nuclear fusion, copious amounts of elements heavier than hydrogen (dBS Ch.5) that are shed in the explosion (SN Type II), allowing second generation stars to form more easily.  [ Gas containing heavier elements can more easily become dense. ]  Again, more often that not these early heavy stars will be in a binary system, forming a binary black hole, that will merge, the heavy black hole then grabbing material from the vicinity into an accretion disk, making itself ever heavier.

In this way, especially a black hole in the densest part of the galaxy, i.e., in the centre, will quickly become very massive and exhibit features that are known as those of a quasar. It is unclear how long it has taken J1342+0928 to have amassed its gigantic amount of material of perhaps a billion times that of the Sun.

The radiation of the quasar J1342+0928 is the manifestation of the accretion disk around a supemassive black hole of a billion times that of the Sun. The [C II] emission represents some 5 million Solar mass of ionized carbon. From the velocity width of that emission one crudely can derive a total mass of the host galaxy, it turns out to be similar to that of the Milky Way (Venemans+ 2017). These values indicate that the host galaxy is well developed and likely does not differ much from the galaxies in the Milky Way neighbourhood.

The distance of J1342+0928, as derived from the red-shift, indicates it is at 13.1 Gyr in light travel time. Current models for the behaviour of the universe have it to be 13.8 Gyr old, the "Big Bang" supposedly took place 13.8 Gyr ago. Right after the big bang, the universe was extremely hot, and matter separated out after about half a million years. Then the universe cooled further and became dark. Only after the first stars had formed could their bright radiation make the gas transparent again. Was there enough time between 13.8 and 13.1 Gyr ago to form galaxies, to form stars, to form black holes and to come to this very massive black hole with quasar J1342+0928 in the centre of its galaxy?

Astrophysicists now see themselves with the task of solving these problems. Three questions are relevant:
  a. Can one envision the formation of full-fledged galaxies in a time-span of about 600 million years? (Remember, the dynosaurs roamed on Earth until 60 million years ago.)
  b. Can one envision to make a black hole of a billion times the mass of the Sun well within 600 million years?
  c. If a) and b) cannot be answered positively, is one then forced to adjust the "Standard" model for the universe to make the universe quite older than 13.8 Gyr? Contradicting current wisdom?

These questions have to be answered independent of each other, or pehaps in combination. Exactly these questions make the discovery of J1342+0928 so interesting.

*) In this text the problem of a possibly accelerating universe as derived from some SN Ia is, for simplicty, not addressed.
 
Sources:
Bañados, E., Venemans, B.P., Muzzucchelli, C., et al., 2017, Nature, 6 December
de Boer, K.S., Seggewiss, W., 2008, "Stars and Stellar Evolution", EDPSciences; ISBN 978-2-7598-0356-9
MPIA quasar: http://www.mpia.de/news/science/2017-14-distant-quasar
Venemans, B., Walter, F., Decarli, R., et al., 2017, Astrophysical Journal Letters 851, L8
Quasar: quasar at Wikipedia

(2017.12.23)   quasarJ1342.html   originally 2017.12.18