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BASIC EXTRAGALACTIC ASTRONOMY Part 9: Supermassive Black Hole and Host Galaxy Coevolution
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BASIC EXTRAGALACTIC ASTRONOMY
Part 9: Supermassive Black Hole and Host Galaxy Coevolution
Rudy E. Kokich, Alexandra J. Kokich, Andrea I. Hudson
7 July, 2021
(38) MECHANISMS OF GALAXY AND SMBH FORMATION AND GROWTH
Exceptions and variances aside, the general relation between the central supermassive black hole (SMBH) mass and the baryonic mass of the host galaxy remains convincing, and seems to suggest a close evolutionary connection (see Part 8). It has been proposed that even closer correlations might exist between the SMBH mass and the mass of the central galactic bulge which surrounds it, or between the SMBH mass and the total mass of the host galaxy, including dark matter.
As previously mentioned (Part 7, Section 33), there exists no comprehensive theory of galaxy formation and evolution which explains the observed variety of structure and composition, or the correlation between a host galaxy and its central SMBH. The wide diversity of galactic morphology suggests that the evolution of galaxy-type objects does not follow a single path, but consists of a series of parallel and intersecting lineages. Nevertheless, a number of "reasonable" hypotheses have been proposed regarding the coevolution of galaxies and their central SMBHs.
Galaxies grow by the process of merging, whereby each smaller member contributes its mass and SMBH to the aftermath. This may temporarily result in a host galaxy with twin or multiple SMBHs, which ultimately themselves merge into a larger one. A few of such SMBH binary systems have been identified in active galaxies by the XMM-Newton X-ray Observatory. Many more might exist within quiescent galaxies, but are impossible to discover with current technology unless illuminated by accretion disks. To date, dozens of stellar-mass black hole mergers and one IMBH merger have been detected by the Laser Interferometry Gravitational-wave Observatory (LIGO). However, LIGO does not have a sufficiently long baseline to detect SMBH mergers which would generate gravitational waves with periods of days to weeks.
In the process of galaxy mergers and close tidal encounters, gravitational interactions between central SMBHs might result in the expulsion of one or both from the host systems. This might explain the surprising absence of a central SMBH in the giant elliptical galaxy A2261-BCG mentioned in Part 8.
Galaxies also grow by a comparatively slow process of consuming the surrounding intergalactic gas and dust. A fraction of that matter contributes to the peripheral mass of the host galaxy, while a fraction eventually gravitates to the center to be accreted by the SMBH, thus resulting in relatively proportional growth of both.
Regarding the question of whether the central SMBH and the host galaxy originate simultaneously, or if one precedes the other, observational evidence suggests that both courses are possible.
Incipient stellar systems without central SMBH can acquire black holes by a couple of mechanisms. The most common is the formation of stellar-mass BHs, with masses between 3.3 and 70 (possibly up to 150) solar, remaining after supernova explosions of massive stars at the end of their brief lifetimes. This process is expected to be more frequent in the densest, star-forming regions of a galaxy and, by recent theoretical and simulation models, in the early stages of globular cluster formation.
A less common, but more productive mechanism for the acquisition of black holes is that of small galaxy mergers. Several Galactic objects, previously regarded as large globular clusters, are now suspected to be remnant cores of minor galaxies which have merged with the Milky Way. M 54 (NGC 6715), 47 Tucanae (NGC 104), Omega Centauri (NGC 5139), NGC 5286, and NGC-224-G1 (in the Andromeda Galaxy) were all observed with velocity dispersion studies to contain intermediate-mass black holes of several thousand solar.
These findings indicate that emerging stellar systems are capable of generating stellar-mass and intermediate-mass black holes over extended periods. However, the assembly of these components into enormous SMBHs of millions and billions of solar masses would require many thousands of small black hole mergers, which is considered highly improbable over the estimated age of the Universe.
The discovery of truly giant SMBHs within compact galaxies (e.g. NGC 1277) has lead to the proposition that central SMBH formation can predate the formation of host galaxies.
Studies of 41,000 distant quasars by Kurt et al. (2007) and Jiang et al. (2007, 2010), showing that SMBHs present in the early universe were approximately 40 times more massive than those formed in more recent epochs, lead to the same conclusion (see Part 5, Section 28). It appears intuitive that larger SMBHs would form within the small volume of the early Universe, when the density of dark matter, baryonic matter, and radiation was much higher.
In Part 7, Section 33 we mentioned another line of evidence that SMBHs with billions of solar masses can indeed precede the formation of organized galaxies. Working with the integral field spectrograph at the ESO’s VLT telescope, Borisova et al. (2016) documented a large nebulous halo displaying starburst activity around each one of the 19 quasars studied at redshifts between 3 and 4. In some cases, these quasar halos extended more than a million light years from the central SMBH – about 20 times the radius of the Milky Way.
Fig. 9-1: Nebulous halos around every quasar studied which formed in the early Universe
The starburst activity model was confirmed by Kurk et al. who studied five quasars with redshifts around 6, and masses ranging between 0.3 and 5.2 billion solar. Using the infrared spectrograph at the VLT, they measured halo metallicities consistent with multiple generations of stellar nucleosynthesis. They conclude that starburst activity and stellar nucleosynthesis around supermassive black holes commenced very early in the history of the Universe. It is not yet clear which type of stellar systems evolve from these objects. While it is modelled that protogalaxies grow by gradual merger of small precursors, and by the accretion of primordial gas, there is convincing evidence that immensely large systems were also forming in the very young Universe under the influence of strong gravitational fields around the earliest supermassive black holes.
In January 2021, an international team led by Feige Wang reported the discovery of the most distant known quasar, QSO J0313-1806, at redshift z = 7.6423. It is composed of a central SMBH of 1.6 billion solar masses within an early host galactic structure exhibiting an accelerated star-formation rate around 200 solar masses per year. Based on the redshift, the object lies at a light travel distance of about 13.1 billion light years, having formed only 670 million years after the Big Bang. The formation of such a massive SMBH by the mechanisms of merging and accretion is considered highly unlikely in so short a time frame.
(39) THE ORIGIN OF THE EARLIEST SUPERMASSIVE BLACK HOLES
The existence of enormous SMBHs so soon after the Big Bang raises inevitable questions regarding their origin. While it is possible that mergers of stellar-mass black holes contribute to the gradual increase in black hole mass and size, the process appears to be far too slow to explain the existence of the earliest SMBHs. Vitral and Mamon (2021) report strong evidence for a subcluster of stellar-mass black holes in the core of globular cluster NGC 6397, which have still not merged into a single black hole although the cluster's age is estimated to be 13.4 billion years.
Gieles et al. (2021) recently reported a population of over 100 stellar mass black holes in the "fluffy" globular cluster Palomar 5, which have still not merged after more than 10 billion years of evolution.
Even in densely populated environments like globular clusters, black hole collisions, like star collisions, seem to be exceptionally rare events.
The availability of matter for accretion is another factor which constrains the growth rate of a black hole. However, even when matter is abundantly available, the accretion rate is limited by the physical requirement for maintaining a hydrostatic equilibrium around the accretion disk between the radiation pressure acting outward and the force of gravitation on the matter falling inward. Too much infalling matter results in higher temperature and greater luminosity in the accretion disk. This causes an increase in outward radiation pressure which, in turn, opposes and restrains the inflow of matter. The self-regulated balance, referred to as the Eddington Limit, imposes a maximum average growth rate on a black hole, and a maximum average sustained luminosity the accretion disk can achieve. Episodes of super-Eddington accretion and luminosity by a factor of 1.1 to 1.5 are not uncommon in the Universe wherever abundant matter rapidly approaches a black hole. However the effect is temporary, and does not sufficiently increase the overall growth rate of a black hole to explain the existence of SMBHs in the early Universe.
According to Feige Wang, the co-discoverer of the most distant quasar, "Black holes created by the very first massive stars could not have grown this large in only a few hundred million years."
Several possible mechanisms have been proposed for the formation and growth of supermassive black holes in the early Universe, independently of mergers and accretion. These can be grouped into two basic models.
1) Direct Gravitational Collapse of Matter
This model dates back to 1966, long before the actual discovery of black holes, especially the central SMBHs, and long before the formulation of the modern Lambda-CDM cosmological framework. At that time Russian theoretical astrophysicists Zeldovich and Novikov speculated about primordial black holes (PBH) created shortly after the Big Bang. They reasoned that black holes of all sizes would form in the early expansion stage of the Universe wherever sufficient concentrations of primordial gaseous matter occurred within a volume of space defined by the corresponding Schwarzschild radius (see Part 5). The idea was later elaborated by English theoretical physicist Stephen Hawking (1971).
Recall that no cataclysmic event is necessary for the creation of large black holes. In Part 5, Section 22, we calculated that the average density of a 16 billion solar mass black hole is about 51 times lower than the density of air at sea level. Counterintuitively, more massive black holes have exponentially lower average densities, and their formation would be favored over the less massive ones whose formation requires significantly higher density gradients. Random variations in surprisingly low densities within the relatively small volume of the very early Universe would lead to regions which spontaneously decouple from cosmological expansion by gravitational collapse into black holes. Baryonic PBH formation by direct gravitational collapse depends on the presence of localized overdensities of ordinary matter, probably within dark matter halos. It would have occurred in the early stages of the matter-dominated epoch in the Universe, approximately 50,000 years after the Big Bang. Over the last two decades, the model has been refined with computer simulations and theoretical work to explain the disposal of angular momentum and the dissipation of heat within infalling matter.
If large black holes can form by the direct collapse of ordinary baryonic matter, it seems even more likely that they can also form by the direct collapse of overdensities of dark matter which is approximately six times more prevalent in the Universe, and six times more dense on the average. Therefore, dark matter PBHs could have appeared much earlier than baryonic PBHs in the matter-dominated epoch, predating host galaxies, and without requiring prior star formation, seed black holes, and unrealistic black hole merger and accretion rates. A recent theoretical study by Arguelles et al. (2021) reports a thermodynamic stability analysis of self-gravitating fermionic dark matter (assuming that dark matter is composed of the same class of fundamental particles as ordinary matter). Mathematical modelling revealed a stable final configuration of dark matter comprised of a small, dense core with an extended, diluted halo. In larger structures, the dense core collapses into a black hole, while in smaller structures it gravitationally mimics a black hole by increasing stellar velocity dispersion in the nucleus. Meanwhile, the diluted dark matter halo continues to explain observed galaxy rotation curves.
Inasmuch as ordinary matter and dark matter are intermixed in most (but not all) regions of the Universe, it seems reasonable to propose that a majority of primordial black holes formed by the direct collapse of matter were actually fermionic PBHs, composed of all forms of matter which interact gravitationally.
2) Radiation Field Collapse
The second group of models regarding the origin of primordial black holes (PBH) involves their formation immediately following the Big Bang, during extreme temperatures and density fluctuations of the radiation-dominated epoch of the Universe. These models rely on hypothetical particles like the gravitinos and tachyons, and on hypothetical mechanisms such as nucleation of false vacuum bubbles, cosmic string collapse, bubble collisions, domain wall collapse, scalar field fragmentation and tachyonic preheating. However, these models have two details in common. The resulting black holes are initially formed by the collapse of radiation fields, long before any type of matter came into existence, and they can be of any size range, from microscopic to supermassive.
Until quite recently, swarms of radiation field primordial black holes in the sub-lunar mass range orbiting within galactic halos have been optimistically considered as viable candidates for dark matter. Numerous studies using a wide range of approaches have so far failed to show evidence for black holes of any size within galactic halos.
In spite of strong circumstantial evidence for their existence, primordial black holes (PBH) by strict definition remain hypothetical objects. They are of great scientific interest because their abundance, distribution, and structure could in principle be used to constrain the conditions in the very early Universe. However, at the present time technological means are lacking to detect their formation after the Big Bang, or their presence within empty space in the early Universe, independently of a surrounding accretion disk or an extensive halo of star-forming gas. Further, not even theoretical methods have as yet been proposed to distinguish between the baryonic PBHs, dark matter PBHs, and radiation field PBHs.
That said, observational evidence strongly suggests that one of the major pathways in galaxy evolution involves gravitational attraction of primordial matter toward pre-existing central SMBH seeds, resulting in subsequent formation of immense starburst stellar systems.
(40) CENTRAL SMBH GRAVITATION AND RADIATION EFFECTS ON THE HOST GALAXY
The discovery of persuasive relationships between the central SMBH, stellar kinematics, and morphological properties and luminosity of the host galaxy has led astronomers to ask how a black hole with a relatively small sphere of gravitational influence affects the evolution of the entire host galaxy. Ideally, the question should be restricted to the early galaxies or the relatively isolated "field" galaxies which have not undergone major deformations resulting from mergers or tidal interactions.
As previously mentioned, nonaccreting SMBHs emit no electromagnetic radiation, and are not directly identifiable. Their presence can be inferred by recording in high resolution proper motions of individual stars orbiting in its vicinity (so far done only within the Milky Way), and by the widening of the spectral lines of luminous matter near the galactic core due to increased velocity dispersion. Meanwhile, actively accreting SMBHs can be highly luminous throughout the electromagnetic spectrum. They serve as energy sources which power active galactic nuclei (AGN) and quasars (very luminous distant AGNs in which only the star-like nucleus is visible in the optical band, but not the faint surrounding host galaxy).
Both types of central SMBHs, nonaccreting and accreting, exert gravitational influence on the host galaxy. Since the gravitational force decreases exponentially with distance from the center, the sphere of "significant influence" is small relative to the size of the galaxy. However, if we regard that sphere of influence as a theoretical thermodynamic system, it is possible to predict a number of interesting phenomena which have been confirmed observationally.
In a densely populated galactic nucleus, stars and gas clouds undergo constant gravitational interaction and exchange of kinetic energy. The law of the conservation of momentum specifies that within a thermodynamic system total momentum (kinetic energy times the mass) of all the components of the system will remain constant. This means that, as some stars tend to fall inward toward the central SMBH, and their momentum (kinetic energy and speed) increases, they will gravitationally transfer the additional momentum to peripheral stars, causing them to increase their orbital speed and/or change their direction. It can be shown that frequent distant interactions are more effective in the exchange of momentum than rare close encounters. If a peripheral star acquires sufficiently high momentum, it becomes "unbound", and leaves the system through a process called evaporation. Generally speaking, and over long periods of time, for every star which spirals inward toward the central SMBH, an equivalent mass of matter will spiral out of the galactic nucleus into the galactic disk or the halo. The process of evaporation can be greatly accelerated by tidal shocks which impart additional kinetic energy to the defined thermodynamic system in the nucleus. Tidal shocks may be caused by galaxy mergers or close encounters, supernova explosions, passage of globular clusters through the galactic bulge, or entry of large molecular clouds. Gravitational perturbations within incoming or outgoing gas and dust clouds may ignite starburst activity within the bulge of the host galaxy. A starburst galaxy may present with or without an active galactic nucleus if its central SMBH is accreting or nonaccreting respectively. Gravitational influence is the less dramatic way in which the central SMBH affects the evolution of the host galaxy.
Actively accreting central SMBHs influence host galaxies by several spectacular mechanisms. One involves narrow, collimated polar jets, or relativistic jets, whereby large quantities of baryonic matter in the form of superheated plasma are ejected at relativistic speeds along the black hole's axis of rotation, up to millions of light years into the halo and intergalactic space (see Part 5, Section 22).
Another mechanism is mediated by energetic ultraviolet radiation emitted by the accretion disk. Ionizing radiation causes the destruction of molecular star-forming clouds by converting them to hot ionized hydrogen (Hii) regions, resulting in inefficient star formation on galactic scales.
The third mechanism, which can be the most influential, involves SMBH outflows. When black holes accrete rapidly, especially when they do so in excess of the Eddington Limit, they turn a significant portion of the gravitational energy and nuclear energy of accreted matter into radiation energy. Outward radiation pressure upon infalling matter causes a fraction of the matter to be ejected far into the galactic halo as outflows of superheated plasma. Unlike the collimated polar jets, these outflows emanate from the hottest, inner regions of the accretion disk in the form of relativistic winds and broad jets. The outflows are expelled at a broad angle perpendicularly to the plane of the accretion disk, and usually perpendicularly to the plane of the host galaxy. The return of energy and momentum from the black hole to the interstellar medium through the outflows and radiation is called SMBH or AGN feedback.
Fig. 9-2: Illustration of SMBH outflows from the inner part of the accretion disk (A), and a CHANDRA X-ray image of outflows from quasar APM 8279 escaping at relativistic (0.2c) velocities (B). The double image of the quasar is due to gravitational lensing by an invisible foreground galaxy.
SMBH feedback drives interstellar gas and dust into thin, high density shells which are accelerated away from the SMBH by radiation pressure and SMBH outflows. The initial effect is an outbreak of starburst and supernova activity within the perturbed, overdense regions of the galaxy. In a 2019 study based on a quasar survey of X-ray, infrared, and optical images, A. Kirkpatrick labeled this brief, early quasar stage as the cold quasar phase. The word "cold" in this case pertains to the presence of relatively cool molecular clouds which serve as a medium for new star formation. In the population of very distant AGN galaxies, such molecular clouds are only detectable at infrared wavelengths.
In the long run, the combined effect of radiation pressure, SMBH winds, and supernova explosions expels the interstellar matter from the galaxy and quenches new star formation. Expulsion of gas and dust from the host galaxy additionally enriches the galactic halo and the intergalactic medium, while depriving the central SMBH of accretion matter, and restraining its further growth. Thus, as the SMBH feedback quenches the host galaxy, the gas depleted galaxy in turn quenches its central SMBH.
Based on the content of galactic (interstellar) gas and dust, high redshift AGN galaxies are found in five overlapping evolutionary stages. The first two or three may be regarded as the cold quasar phase.
1) Completely obscured AGNs, detectable only in the X-ray band,
2) Partially obscured AGNs, detectable in X-ray and far-infrared bands,
3) Dust reddened quasars, in which the accretion disk becomes visible in the optical band, and the starburst regions of the host galaxy are detectable only in the far-infrared band,
4) Luminous blue quasars, unobscured in the optical band, which have expelled most of the gas and dust from the host galaxy, but are still undergoing active accretion, and
5) Elliptical galaxies, which have expelled and depleted virtually all gas and dust, and which display no SMBH accretion, and virtually no new star formation. The presence of a nonaccreting central SMBH is revealed only with spectroscopic velocity dispersion studies of the nuclear region.
(41) SMBH INFLUENCE ON THE INTERGALACTIC MEDIUM (IGM)
As evidenced by quasars, energy levels generated by an accreting supermassive black hole can be orders of magnitude higher than the energies released by the rest of the host galaxy. Consequently, the extent of physical effects an SMBH exerts on the intergalactic medium through SMBH/AGN feedback dwarfs in size the extent of the visible stellar population of the host galaxy. Inspired and co-authored by theoretical astrophysicist Annalisa Pillepich, a TNG50 computer simulation by Dylan Nelson (2021) shows the changes in the density of intergalactic gas caused by the expulsion of galactic gas by feedback from an active central SMBH. The feedback, in the form of a high density shell surrounding a large low density bubble, is oriented mostly perpendicularly to the plane of the galaxy, and extends beyond 100 kpc in either direction.
Fig. 9-3: Simulation of intergalactic gas density around a large galaxy influenced by SMBH (or AGN) feedback. The model predicts lower IGM density in regions perpendicular to the galactic plane.
The existence of high and low gas density regions around active galaxies is not merely theoretical. It has been documented experimentally by observing the properties of satellite galaxies as they orbit through the intergalactic medium (IGM).
On the average, IGM gas density is extremely low - approximately one atom per cubic meter, which is a million times lower than the density of the interstellar medium within galaxies (about one atom per cubic centimeter). Nevertheless, IGM offers substantial headwind resistance to galaxies rapidly moving through it. While the effect on massive stars is undetectable, the effect on the tiny particles of interstellar gas and dust can be quite remarkable. Galaxies moving rapidly through denser regions of the IGM leave behind visible trails composed of gas stripped from the galaxy and thin populations of new stars born within the trail. The process of losing gas while travelling through the IGM is called ram pressure stripping.
Fig. 9-4: HST visible and Chandra X-ray composite image of a galaxy undergoing ram pressure stripping.
While ram pressure stripped galaxies do manifest some new star formation within the turbulent trail, the galaxies themselves, depleted of interstellar matter, become quenched, resulting in very low star formation rates.
A study by Martin-Navarro et al. (2021) reports an analysis of star formation rates within 124,163 satellite galaxies orbiting 29,631 large central galaxies with active galactic nuclei (accreting central SMBHs). The findings revealed that satellite galaxies located perpendicularly to the planes of central galaxies had relatively higher star formation rates. This implies higher gas content due to less ram pressure stripping within satellite galaxies moving through the low density IGM bubbles generated by SMBH outflows. Statistically speaking, the effect is only noticeable in satellite galaxies which have recently joined the central galaxy, and have not been quenched because they happened to arrive within a low density bubble. The effect will not be noticeable in those satellites which have already orbited the central galaxy several times, and have lost their interstellar gas while passing through the denser IGM regions.
The net result of SMBH outflows is therefore threefold. While they quench star formation in the central galaxy by expelling its interstellar gas, they actually enable star formation in many satellite galaxies within the halo. Through the process of merging, this effectively increases the "rain" of new stars upon the central galaxy, and increases the number of stars with random motion within the central galaxy. The radiation effect of an active central SMBH upon its host galaxy extends far into the circumgalactic space by increasing anisotropy in the intergalactic medium. There is compelling evidence that central black holes regulate host galaxy evolution on scales which dwarf in size by several orders of magnitude the visible portions of galaxies.
Events surrounding an actively accreting supermassive black hole are entirely outside of human experience, and words may be inadequate to describe them. The following composite image of high energy X-rays and low energy radio waves depicts the maelstrom of superheated plasma, twisting magnetic fields, and radiation emanating from the relatively minor and relatively inactive central SMBH of the Milky Way. Many galaxies contain black holes which are thousands of times more massive and energetic.
Fig. 9-5: Chandra X-ray and radio data composite image of the Milky Way center (NASA)
The model of SMBH and host galaxy co-evolution described here is valid in general terms, and attractive in its simplicity. However, on the level of fine details, the literature is full of theoretical controversies and observational inconsistencies suggesting that the process is far from completely understood.
See the following links for the previous parts of this article series: