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Where did the powerful early quasars come from?
Modern cosmology as a whole successfully explains the origin and evolution of the universe - from the insignificant shares of its existence to the present. Nevertheless, there are not so white spots, but rather “sharp corners” - controversial issues, questions about which there is no scientific consensus. Recently, quasars with a very large red displacement were found, containing black holes weighing over a billion sunny. We observe them from the young universe, when its age was hundreds of millions of years. How did these black holes be absorbed by that time to absorb billions of solar masses? How were the embryos of super -massive black holes formed? How massive were these embryos originally?
Observations
In the last decade, a massive “capture” of quasarov with a large red displacement was conducted, that is, those that shine for us from the early universe, from the first billion years of her life. They are not in vain called the "lighthouses of the universe." Firstly, they shine through the cosmic environment, revealing the secondary ionization of the universe (which they themselves produce). Secondly, the early Quasars are interesting in themselves, like everything connected with the young universe.
The search for distant quasars is mainly on terrestrial telescopes in the near infrared range-it is there that the ultraviolet line of hydrogen Laiman-alpha moves. The catch to date is several dozen quasars with a red shift Z> 6.5 (the age of the universe is less than 800 million years), of which several with Z> 7. Recently, a record quasar with the “telephone number” J0313–1806 was found, which has Z = 7.64 (the age of the universe is 650 million years). Moreover, this is a rather bright quasar: its absolute luminosity is 1.4 × 1047 ERG/s, which is three orders of magnitude brighter than our galaxy with its hundreds of billions of stars. This is not a record luminosity for quasars, but it is not much inferior to the record. This means that the mass of the black hole - the “central machine” of this quasar - should be at least a billion of the solar masses, otherwise it is difficult to explain such a luminosity. Indeed, the assessment of the mass by the speed of gas movement in Quasar (Doppler bruise of one of the spectral lines of magnesium) gives a value of 1.6 ± 0.4 × 109 of the solar mass (Mʘ). Other discovered quasars at Z> 7 are only slightly inferior in weight of the central black hole. So, there is a fact: in the first 600, with a little millions of years, black holes have appeared in the universe a lot of order of a billion mass of the Sun. It turns out that this fact is not easy to explain. It would seem that there should not be enough time for the growth of such black holes. Why?
Growth limits
If the star or any other object shines too brightly, the pressure of radiation on the surrounding gas or its own outer layers exceeds the gravity of the object. If this is a star, then it will begin to dump the outer layers. Such stars exist: for example, this keel; They did not have long to live. If it is a black hole that tightens the substance (accreting), then the fall of the substance will stop. This borderline luminosity is called "Eddington Light", its excess is possible, but it requires some special explanations. Eddington lumininity, of course, is proportional to the mass of the object: for the sun it is 1.4 × 1038 ERG/s - almost five orders of magnitude are not enough before it. And for a black hole of a billion of the solar masses, it is, respectively, equal to 1.4 × 1047 ERG/s - and it turns out that the luminosity of the quasar J0313–1806 is close to the Eddington, or rather L ~ 0.6 LEDD.
It is Eddington Lighting that limits the “regular” growth rate of black holes.
The fact is that there is a typical effectiveness of accretion of black holes. If the mass M fell on the black hole, then with the most effective accretion mode, the energy of about 0.1 ms2 is released. This is a common mode: a geometrically thin, optically thick accretion disc of Shakura - Sunyaeva. If the rate of the fall of the substance exceeds 10 LEDD/C2, then the luminosity will exceed LEDD, the disk will begin to swell and dissipate, the flow of the substance will fall into the black hole.
“Critical”, or Eddingonon, the pace of accretion, of course, is proportional to the mass of the black hole. This means that the “regular” growth of the black hole is exponentially - a height of 10 to 100,000 mʘ and from 100,000 to a billion solar masses takes the same time.
The paradox is that the growth was supposed to begin with germinal black holes weighing about ten thousand mass of the sun (for a record quasar - at least 20,000 mʘ), otherwise they do not have time to grow to Z ~ 7 to the observed values. It is clear where the embryo of the masses of 100 or even several hundred ms can come from - from the collapse of giant stars of the first generation (population III). But here you need either embryos a hundred times harder, or a “supercritical” growth rate. Both are not excluded, but the explanation requires a fair voltage.
Let's start with supercritical growth.
Anomally rapid growth
The Eddington limit of luminosity is far from absolute and completely overcome, especially for a while. Strictly speaking, it only applies to optically thin plasma. If, for example, a star falls on the gravitating center, then the luminosity can bounce to any size. In addition, the rapid growth rate of a black hole is not necessarily associated with overcoming the Eddington limit. There is another option - the low efficiency of highlighting, that is, the substances falls a lot, and there are few radiation from it - and no problems.
There is an accretion option called Adaf (AdVction Dominated Accretion Flow) - a geometrically thick but optically thin disk. The temperature balance does not have time to establish in it: the ions are hot, but they do not shine, but the electrons that should shine are cold. All the energy of ions is carried into a black hole. This, apparently, is in the center of our Galaxy and the famous Galaxy M87. True, this option works only at a relatively small rate of accretion and is unlikely to be suitable for a supercritical regime.
A more suitable option is the so -called slender (SLIM) Accretion disk. Actually, it is into it that the canonical subtle disc of Shakura - Sunyaev at the plain pace of accretion should turn into it. The released heat does not have time to emit out and carry it into a black hole. The disk swells, but moderately. Apparently, therefore, Marek Abramovich (one of the main classics according to the accretion modes) called him “Slim Disk”. In principle, such accretion can stably go in supercritical mode and could solve the problem of early quasars, if not for one “but”. The fact is that the radiation efficiency in this mode greatly depends on the rotation of the black hole. If the rotation is weak, the disk radiates little and a lot of substance with moderate luminosity can fall on a black hole. If the moment of rotation of the black hole is close to the maximum (which is quite likely), then the inside of the accretion disc near the last stable orbit highlights most of the energy released in the disk - the effectiveness is the same as in the case of a thin disk. Therefore, such a regime is not a panacea. He can accelerate the growth of a black hole at some stage, but is unlikely to solve the problem of early quasars.
There is another aspect - Feedback, feedback, the influence of a bright source on the environment. Suppose that something optically thick falls on a black hole - stars, dense clouds of gas, etc. Lighting is huge, and this is just the case when the Eddington limit does not work. But another ambush appears: Quasar ionizes and warms up the environment around itself so much that the formation of stars stops, and the grew grown pressure of hot gas is much superior to the burden of the black hole. As modeling shows, the rapid growth of the black hole with such “hypereuddington” accretion ceases at a level of only 108 mʘ.
So, it seems that it is very difficult to overcome the slope of the curves shown in Fig. 1, and grow a quasar with a black hole of 109 m for 600 million years, starting from a black hole of star origin.
So far, these were reasoning at a qualitative level. It is worth saying a few words about how the people are trying to explore the problem in numerically.
Young universe in a supercomputer
The numerical modeling of the evolution of the early universe is far from a new occupation. The Millennium project is most famous, which issued a spectacular picture of a large -scale structure. Since then (the beginning of the 2000s), some (albeit not radical) progress has occurred both in computing technology and in modeling methods. The task is initially difficult, since it includes the gravity and hydrodynamics of the cosmic environment with different components (dark matter, a two -phase baryon environment (hot ionized and cold neutral gas), stars).
Relatively recently (December 2020), a preprint [ 2 ] has been published with the results of a very impressive account similar to the Millennium, but with a significantly better resolution. Firstly, a hybrid method (soft particles + mesh) was used, which reduces numerical noise and various artifacts such as numerical viscosity. Anyway, the possibilities of a numerical account are far from honestly trace everything that happens on the entire scale of the scale, covering many orders of magnitude. Therefore, the authors had to resort to a number of tricks: an invoice in two doses, first a rough estimate of evolution in a cubic gigaparsek, then the choice of the most heavy cloud of 1013 mʘ, formed in this gigaperseck, and then further work with it alone. Since it is impossible to simultaneously monitor large and small scales, the formation of individual stars and black holes was model: where there were suitable conditions, the star population III automatically appeared, part of which turned into black holes weighing 10-100 mʘ. These black holes played the role of "light embryos." Data on heavier black -raised embryos (103–106 m) were entered manually in the assumption that they appear as a result of some processes that cannot be reproduced by direct modeling (see below). For accretion on a black hole (the main material of accretion is interstellar gas), models in different versions were also used, the reverse influence of the growing quasar on the environment was modeling more correctly.
Heavy embryos
Naturally, primary black holes are attracted as one of the solutions to the problems of the early quasars. Then everything is explained simply: embryos weighing 105 mʘ formed along with the Universe, in its first moments, and then grew to the observed values. The problem is that primary black holes, especially such a mass, are poorly combined with the theory of cosmological inflation; More precisely, their explanation requires special theoretical efforts. Yes, they could form as a result of the fluctuations of the metric with the density of the universe comparable to Plankovskaya, anything could form there - space strings, domain walls, magnetic monopolis. However, the swelling of the space, which went with a density of several orders of magnitude lower than Plankovskaya, spreads all this exotic into colossal distances, so it is extremely unlikely to detect something similar within the horizon of the Universe. There are rather tricky models in which primary black holes of large masses are obtained at the end of inflation or even after it. But still, this is a kind of extraordinary: to develop such models is interesting and useful, but in order to take them for the truth, emergency certificates are required. They are not yet.
Can a heavy embryo form in the first 100-200 million years of the life of the Universe? These are the so-called dark centuries, which we know almost nothing, something is very difficult to observe there because of a huge red shift, and there are almost no bright sources. So far, you can only theoretically or numerically try to reproduce what is happening there. A lot of interesting things loom.
Firstly, the hierarchical fusion of objects-stars and black holes. Recently there was a fusion of two record black holes, one of which, in all likelihood, was already the result of a merger [ 3 ]. Stars can also merge - with each other (then collapse into black holes) and with black holes. This can happen in a tight cluster, where heavy objects due to repeated interactions lose the moment of rotation, passing it to light objects, and sit in the center of the cluster, where they merge. There are work where the dynamics of stars can be traced in a tight cluster with the formation of black holes of 1000 mʘ. This is probably far from the limit. Note that during the merger of stars and black holes, the Eddingonian limit does not affect at all, and when the black holes merge, the efficiency of highlighting in an electromagnetic spectrum is generally close to zero.
Secondly, there may be a direct collapse mechanism (bypassing the stage of stars) of massive gas clouds weighing about 106 m in a black hole. Such an opportunity is discussed in the work [ 2 ], the corresponding links are also given. A similar process is quite difficult to imagine, since it requires effective gas cooling mechanisms (the option of cooling through the radiation is considered) and the reset of the rotation moment. Nevertheless, some reasonable options for such a collapse exist. Their discussion deserves a separate article.
It is worth saying about one simplifying circumstance: Quasarov with a mass of ~ 109 m at a red shift Z ~ 7, it seems very little. They should be well visible: the gas density in that era is very high, and the rate of accretion should be close to critical. Nevertheless, only a few pieces were found in the entire huge universe. This means that for their explanation, you can attract rare events, for example, an abnormally dense star cluster, where an abnormally massive germ of the future Quasar was formed, etc.
In general, it seems that the problem is solved without any extreme, albeit with some tension. The most likely key to the solution is the very first hundreds of millions of years where you can count on supercritical (even “hypercritic”) growth of the embryo of the black hole to 105–106 mʘ. The further supercritical growth of the ultra -massive black hole seems less likely, but it is not needed if a heavy embryo could form. For clarification, further research is necessary, so far - in numerical methods.
As for observations, it is worth once again to put hope on the upcoming telescope James Webb, which will allow you to look deeper into the “dark centuries” of the Universe.
Boris Stern
The author is grateful to Konstantin Zveznov for valuable comments