Galaxies

Black Holes, 2025 Update, 2025+ Update

		The concept of black hole has its origin in a solution of Einstein's General Relativity for a spherical object with mass M and radius R. If the mass collapses to a radius less than R = 2GM/c2, where G is the gravitational constant and c is the speed of light, then nothing (including light) can escape from inside this radius. It is called the event horizon or the Schwarzschild radius (named after the astrophysicist who solved the equation). Figure 05-02a shows a schematic diagram of the Schwarzschild geometry.
Figure 05-02a Black Hole [view large image]	Figure 05-02b Worm Hole [view large image]

It is believed that every quasar, active galactic nucleus, and even normal galactic nucleus contains a black hole with a mass of between ten million to several billion solar masses at its core. The difference in appearance is related to the intensity of the activity. Since galaxies rotate, matter falling toward the central black hole will form a rapidly spinning disk of gas - an accretion disk - rather than falling directly into the hole. Kinetic energy released by in-falling matter, and frictional effects within the accretion disk, raise the temperature of the nner parts of the disk to enormous values and provide plenty of energy to power AGN's on all scales from Seyferts to quasars. By a process that is still not fully understood but seems to be related to rotating black hole, the central engine accelerates streams of charged particles to very high speeds. The inner rim of the accretion disk, together with surrounding gas and magnetic fields, forms a nozzle that confines the outward flow of energetic particles into narrow streams that shoot out perpendicularly to the plane of the disk. Figure 05-02c shows a model of the black hole. See Black Hole Visualization, 2024

Figure 05-02c AGN Model [view large image]

		Magnetohydrodynamics is the branch of physics dealing with the behavior of the combined system of magnet field and ionized gas at low density. A popular theoretical scenario applies such formulation to explain the relativistic jet(s) powered by the black hole. The study found that magnetic field lines are twisted into a helix (as shown in Figure 05-02d) by the differential rotation of matter in the accretion disk. Such field provide a force to expel the ionized particles in a collimated jet from the inner circumference of the accretion disk. The energy to maintain the process is derived from the braking action on the rotation of the disk. In this way, matter can slow down and eventually flow inward into the black hole. Figure 05-02e illustrates a mechanism to extract the spinning energy from a rotating black hole, wherein the surrounding ionized gas and the magnetic field act together to set up a voltage difference between the poles. This is similar to a battery driving electric current around to provide another source of energy for the outflowing jet(s).
Figure 05-02d Magnetic Field and Jet [view large image]	Figure 05-02e Rotating BH	See more detail in "Magnetohydrodynamics (MHD) and the Formation of Jet(s)"

Figure 05-02f is a HST (Hubble Space Telescope) image of NGC4261, which is a radio galaxy. The image strongly suggests that it is a black hole fitting the description of the theoretical model. Infrared observation of

		along the source axis. Its finite width and the dashed circle indicate the currently undetermined extent. The much larger warm component (T=320K, red) is well resolved. The arrows indicate the projected orientation of the two interferometer baselines and the angular resolution L/2B, where L is the wavelength and B is the projected baseline. The image shows that the active galactic nuclei are arranged like a thick doughnut. This model requires a continuous injection of kinetic energy to maintain such cloud system. The mechanism is currently unknown; thus a better understanding of the physics of these spectacular objects is needed.
Figure 05-02f NGC4261 [view large image]	Figure 05-02g NGC1068 [view large image]

In a November, 2004 announcement by NASA, a black hole catalogued as SDSSp J1306 appears to be about one billion times as massive as the sun. It is 12.7 billion light-years away. A similarly massive and distant black hole was studied in the same year with the European Space Agency's XMM-Newton X-ray satellite. The object, SDSSp J1030, is 12.8 billion light-years away. These two results seem to indicate that the way supermassive black holes produce X-rays has remained essentially the same from a very early date in the universe. How such massive and energetic structures formed so quickly (only after one billion years of the big bang) remains a major puzzle for scientists. Mergers of smaller galaxies and their black holes may have played a role. Researchers suspect that black hole formation and galaxy development go largely hand-in-hand, but they cannot say which comes first. Figure 05-02h is an artist's conception of a supermassive black hole with matter swirling into it.

Figure 05-02h BH, Supermassive

In 2009, radio observations of 4 early galaxies (1 to 2 billion years after the Big Bang) including J1148+5251 shows that the mass ratio of central bulge to black hole is about 10 times smaller than the more recent data indicating the black hole probably came first.

A 2012 update on the supermassive black hole J1148+5251 reveals that while matter swirls inward from the accretion disk toward the black hole, intense radiation creates outflow of gas at the same time. The process is shown schematically in Figure 05-02i. The orange wavy lines represent photons emitted during the accretion process. Most of them escape the galaxy, but some imping on clouds of gas (blue) and the radiation pressure drives the gas out. The stars and dark are too heavy to be affected by the radiation. It is suggested the elliptical galaxies may lost their gas component in this way. The Milky Way has a mild imitation of such process in the form of the 30-kpc arms.

Figure 05-02i BH Gas Outflow

On 28 March 2011, the Swift satellite designed to look for gamma-ray bursts (GRBs) detected a burst with unusually long duration of more than a month. The new source, called Swift J164449.3+573451, is now believed to be generated by the tidal disruption of a star moving too close to a black hole. The left portion in Figure 05-02j shows the level of X-ray flux recoded by various detectors over the past 20 years with observational duration indicated by horizontal bars. The data indicate the gradual buildup of intensity until it suddenly flared up more than 200 times in few hours. The right portion in the same diagram is the follow-up to show the flux variation for the first 7 weeks of the most recent observation. Time interval of the variation (in luminosity) can be used to calculate the size of the

Figure 05-02j Black Hole Cannibalism [view large image]

source since each part of the source would have to be in contact with others on such a time scale t to coordinate the variations; thus the size r c t. For the most rapid variability of t 100 s, r 3x1012 cm, which is about 50 time bigger than the Sun and about 1/2 the size of Mercury's orbit.

In another 2011 report, the jet from M87 has been observed in great detail with the Very Long Baseline Array (VLBA), which has resolution 400 times finer than the HST. Since black hole is not a clean eater, about half of the food (the in falling star) is left behind in the accretion disk close to the black hole. Interaction of the rotating "crumbs" and magnetic field produces the jet. The observation finds a jet of high-energy particles starts as a broad flow at a distance about 14 to 23 times the

Figure 05-02k M87 Jet [view large image]

Schwarzschild radius of the black hole. The jet becomes more cylindrical and faster with distance from the black hole. Figure 05-02k (a) is a schematic diagram showing different sections of the jet that have different particle densities observed at different frequencies, from about 200 GHz

		M87 used to hold the title of harbouring the most massive black hole with a mass of 6.3x109 Msun. The record was broken in 2011 with the detection of 2 super massive black holes in NGC 3842 (within the Abell 1367 cluster) and NGC 4889 (within the Coma cluster) both with a mass of about 1010 Msun. These giant galaxies all sit at the bottom of the cluster's gravitational potential well collecting a lot of the material falling inward. Estimate of black hole mass depend on
Figure 05-02l Black Hole Correlations	Figure 05-02m Black Hole Formation [view large image]	either the measurement of stellar (or gas, or maser emission from molecular clouds etc.) velocity dispersion or V-band galactic bulge luminosity Lv. The data points usually fall on (or near) a straight

presented astronomers with a problem to explain how could it be created so soon and so big? It is known that the acquisition of material by the black hole is always balanced by ejection of some of the in-falling matter. It has been calculated that a black hole sucking in matter continuously at its maximal rate would double its mass Mbh every 50x106 years, i.e., Mbh=M0x277/5~ 4x104M0, where M0 is the initial mass. This is too slow for a seed black hole of stellar mass to grow into billion-sun in 770x106 years (Figure 05-02o,a).

Figure 05-02n Supermassive Black Hole [view large image]

Two different theories has been proposed to resolve the problem. They all try to make the basic component from solar-mass black hole to middleweight one with mass of 105 to 2x106 Msun, i.e., to bump up M0. One theory suggests that black hole at the center of a star cluster could grow quickly enough to middleweight seed hole (Figure 05-02o,b). The other one involves collapse of primordial gas cloud (Figure 05-02o,c). The former scenario predicts more leftover middleweight seed holes to survive into the present age. Since only a few hundred such holes among 2x105 galaxies have been found so far, it seems that the odd is against the bottom-up version at least for now (see also related observation in Baby Galaxy).

Figure 05-02o Supermassive Black Hole Formation

ChatGPT said:



Not quite. In general relativity the "gravitational charge" isn't rest-mass, it's the whole stress-energy tensor-anything that carries energy, momentum or pressure curves space-time. A beam of light has no rest-mass, yet it bends starlight and is itself deflected by gravity.
The Einstein field equations

contains the energy density of mass-less radiation just as readily as the mass density of matter. So even in an era when every particle was massless (before the electroweak/Higgs symmetry broke), enormous concentrations of radiation could, in principle, collapse gravitationally.
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Massless era black holes: two examples
1. Primordial black-hole (PBH) collapse : Any time the universe was radiation-dominated (including before the Higgs field got its VEV)
If a region that's still inside the causal horizon has an overdensity d/ ~ 0.3

, radiation pressure cannot halt collapse and an event horizon forms
Widely studied; PBHs are still considered as dark-matter or seed-BH candidates.
,
2. Kugelblitz (black hole of pure light)


In principle at any epoch; when radiation focus 10⁴⁶J into a region 10^-16 m, it is so intense that its own energy creates an event horizon. Classical GR allows it, but a 2024 quantum study finds vacuum-polarisation prevents the focus ever getting dense enough.

Both rely only on energy density, not on particles having rest-mass.
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Why "no Higgs = no black holes" is a misconception
1. Energy 0 before SSB
The early universe was a seething plasma with energy > 100 GeV; every photon or gluon carried that energy gravitated.
2. Radiation gravitates strongly
For a perfect radiation fluid, the pressure term in the stress-energy tensor actually strengthens the tendency to curve space-time.
3. Collapse condition cares about total energy inside a radius. The Schwarzschild criterion
R_s = 2Gm/c²
can replaces m with energy E = mc².
4. Cosmic expansion makes PBH formation hard, but not impossible The rapid Hubble expansion opposes collapse; nonetheless, overdensities seeded by inflation can be big enough. Such PBHs could have formed from t ~ 10^-35 sec onward-even long before electroweak breaking-and would span a huge mass range, from Planck-scale to thousands of solar masses.
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What the Higgs field actually changed
" Introduced rest-mass. After SSB, quarks, leptons, W, and Z could slow to vc, enabling new structures: baryons, atoms, stars.
" Gave new collapse channels. Massive stars could now run out of fusion fuel and implode, creating the stellar-collapse black holes we observe today.
But the capacity for gravity-driven collapse-and hence black-hole formation-was present from the first instant the Einstein equations were relevant. The Higgs mechanism merely added another, later route to getting extremely dense objects.
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Bottom line
Black holes do not require particles to have Higgs-generated rest-mass; they require enough energy (of any form) packed into a small enough region.
Even in an all-massless, pre-Higgs universe, sufficiently large radiation over densities could-and in many models did-collapse into primordial black holes.

.
Let's break it down by case:
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?? 1. Black Holes (and other compact objects like neutron stars):
" Jets emerge along the rotational axis of the accretion disk surrounding the black hole.
" The accretion disk is a rotating, magnetized structure of infalling gas and dust. Due to strong magnetic fields and conservation of angular momentum, material cannot fall straight into the black hole and instead spirals in.
" The jets are launched from regions very close to the black hole's event horizon, not from any orbital distance, through a process like:
o Magnetohydrodynamic (MHD) acceleration: Magnetic fields twist and funnel material into narrow jets.
o Blandford-Znajek mechanism: Taps into the spin energy of the black hole via magnetic fields.
? Origin: Inner part of accretion disk, within a few Schwarzschild radius (very near the event horizon).
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?? 2. Planetary System Formation (e.g., Solar System):
" When a protostar forms, it's surrounded by a protoplanetary disk-a rotating disk of gas and dust.
" As gas spirals inward, again, magnetic fields and angular momentum lead to bipolar jets being ejected along the rotational axis.
" These jets help remove angular momentum so that material can fall onto the forming star.
" The jets are not coming from the orbit of Mercury or any specific planet, but rather from the inner regions of the disk, typically within a few AU (astronomical units) of the forming star (much closer than Mercury's orbit).
? Origin: Inner accretion disk around the protostar, again near the center.
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?? Why Paired (Bipolar) Jets?
Because the disk is symmetric around a rotation axis, and magnetic fields are generally aligned with this axis, the material is funneled both up and down, creating two opposite jets.
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If you're curious, I can generate a clean diagram to illustrate this visually.

ChatGPT said:
Exactly - the material falling into a black hole or a protostar does not plunge straight in, as often shown in dramatic artist renderings. Instead, it follows a spiral or helical path within an accretion disk.
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?? Why the Spiral or Helical Path?
1. Angular Momentum:
o Any material in space usually has some angular momentum.
o Conservation of angular momentum prevents it from falling straight in.
o Instead, it spirals inward - much like water going down a drain - forming an accretion disk.
2. Friction and Viscosity in the disk:
o Causes energy to be dissipated as heat and transports angular momentum outward.
o Allows inner material to spiral closer to the black hole over time.
3. Magnetic Fields:
o In the inner regions of the disk, strong magnetic fields can twist and funnel some of this spiraling material into collimated jets.
o These jets follow helical magnetic field lines, giving the jet itself a helical structure, especially when seen in radio or X-ray images.
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?? So, What Happens Close to the Black Hole?
" Innermost Stable Circular Orbit (ISCO):
Material in the disk orbits the black hole until it reaches this point. For a non-rotating black hole, it's at 3 times the Schwarzschild radius (3?Rs). Beyond this, the orbit becomes unstable and the material quickly plunges in - but only after spiraling inward all the way.
" Inside the ISCO:
o Material does fall inward more directly - but even then, not in a perfectly straight line. The motion remains curved due to spacetime curvature and rotation.
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, ,

see video (frequency dependence)

ChatGPT said:
Good observation - but no, the absence of visible jets from the Milky Way's center does not mean there is no accretion disk or inward spiraling material.
Let's clarify:
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?? Our Galactic Center: Sagittarius A*
" The central black hole of the Milky Way, Sagittarius A* (Sgr A*), is about 4 million solar masses.
" It does have an accretion disk, but it is:
o Very faint
o Low-luminosity
o Poorly fed (i.e., not much material is falling in)
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?? Why No Jets (Currently)?
1. Low Accretion Rate:
o Sgr A* is currently starving - not much gas is being pulled in.
o Without enough infalling material, the disk remains thin and dim, and jets don't form or are extremely weak.
2. Jets Require Specific Conditions:
o You need strong magnetic fields, rapid spin, and sufficient gas flow.
o Even if Sgr A* has spin and magnetic fields, the trickle of gas is not enough to power visible jets.
3. Past Activity:
o Evidence from Fermi bubbles - giant gamma-ray lobes above and below the galaxy - suggests that Sgr A* was active in the past.

So jets may have existed millions of years ago.
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? Summary
Feature Status in Sgr A*
Accretion disk Present, but faint
Inward spiraling gas Yes, but minimal
Relativistic jets Not currently visible
Past outbursts Likely (Fermi bubbles)
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So the Milky Way does have an accretion disk, but it is not active enough to produce large visible jets right now. If more material fell in (like a gas cloud or a disrupted star), it might flare up and launch jets again.

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