Kurzgesagt – In a Nutshell
Sources – Quasars
Thanks to our experts —
Dr. Tacy Webb
McGill University
Dr. Matt Caplan
Illinois State University
–The universe seems like a vast empty ocean sprinkled with galaxies like rare islands. But this is an illusion. Just about 15% of all atoms are found in galaxies, while the rest is thought to be drifting in between, in the intergalactic medium.
It’s been known for quite some time that, in the present universe, most of the atoms (often called “baryons” by astronomers, to differentiate this kind of matter from the much more abundant “dark matter”) do not reside in galaxies:
#Nicastro, F. (2018): “Observations of the missing baryons in the warm–hot intergalactic medium”, Nature, vol. 558.
https://www.nature.com/articles/s41586-018-0204-1
https://arxiv.org/abs/1806.08395 (open access version)
Here the galaxies’ share of all atoms in the universe is given in the last column of the first three rows. According to these data, galaxies just account for less than 20% of all ordinary matter in the cosmos.
Although the exact figures differ among sources and astronomical surveys, similar data have been found in all studies until today:
#D’Odorico, Valentina D’Odorico (2022): “Portraying the missing baryonic mass at the cosmic noon: the contribution of CUBES”, Experimental Astronomy
https://link.springer.com/article/10.1007/s10686-022-09859-4
Quote: “Cosmological, hydrodynamical simulations (e.g. [7, 8, 14, 27, 53, 57]) predict that, at 𝑧 ~ 0, most of the baryons are in the form of unvirialized, diffuse gas: 40−50% have been shock heated by the processes of structure formation and resides in the warm-hot intergalactic medium (WHIM) characterized by temperatures of 105 K < 𝑇 < 107 K. Another 30−40 % is in the diffuse warm medium at 𝑇 < 105 K. The cold gas and stars in galaxies contribute less than ~20 % and the very hot gas with 𝑇 > 107 K (the normal X-ray emitting gas, predominantly in collapsed and virialized clusters of galaxies) gives a negligible contribution.”
Therefore, in the present-day universe, just a minority of atoms are found to reside in galaxies. The rest of the atoms are thought to be in the “intergalactic medium”, the diffuse gas that (in its various forms) fills the space between galaxies.
It is worth mentioning that some atoms are still missing. The total amount of atoms in the universe is known from studies of the baby universe. However, about 30%-40% of all atoms haven’t been explicitly found yet – an old enigma called the “missing baryons problem” (a problem that, importantly, has nothing to do with dark matter and dark energy):
#Nicastro, F. (2018): “Observations of the missing baryons in the warm–hot intergalactic medium”, Nature, vol. 558.
https://www.nature.com/articles/s41586-018-0204-1
Quote: “It has been known for decades that the observed number of baryons in the local Universe falls about 30–40 per cent short1,2 of the total number of baryons predicted3 by Big Bang nucleosynthesis, as inferred4,5 from density fluctuations of the cosmic microwave background and seen during the first 2–3 billion years of the Universe in the so-called ‘Lyman α forest’6,7 (a dense series of intervening H I Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm–hot intergalactic medium.”
–Like the roots of some massive tree, gas is spread out from each galaxy, gravity funneling fresh mass into them, like in a dense, cosmic forest. Here in the intergalactic medium, are the raw materials of creation: hydrogen and helium, woven into sheets and filaments that flow into galaxies where they eventually create stars.
The large-scale distribution of matter in the universe follows an organic-looking, filamentous pattern known as the “cosmic web”. Galaxy clusters sit mainly at the intersections between filaments, and the intergalactic medium (made mostly of hydrogen and helium, since all other chemical are synthesized in the interior of the stars) flows from galaxy to galaxy along the filaments:
#Springel, Volker et al. (2005): “The Millennium Simulation Project”, Max Planck Institute for Astrophysics
https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/
In the pictures on the left, the scale-bar (“2 Mpc/h”) equals about 10 million light years. The scale-bar on the right (“125 Mpc/h”) equals about 600 million light years. (In the units used here, 1 pc = 3.26 light-years, 1 Mpc is one million parsecs, and h denotes the value of the Hubble constant in units of 100 km/s/Mpc).
Galaxies are constantly exchanging intergalactic gas with their environments along the filaments of the cosmic web. Most of the universe is gaseous, and galaxies are just a small “tip of the iceberg” within this filamentous, gaseous cosmic structure:
#Geach, James E. (2011): “The Lost Galaxies”, Scientific American, Volume 304, Issue 5.
https://www.scientificamerican.com/article/the-lost-galaxies/
https://people.umass.edu/wqd/a191/Scientific_American/scientificamerican0511-46_lost_galaxies.pdf
Quote: “What Is a Galaxy? Perhaps the most exhilarating experience a scientist can have is the feeling of a sea change in one’s perspective on the world. For me, that came when I had to reevaluate what I thought of as “a galaxy.” Traditionally we think of luminous galaxies as isolated and discrete island universes, as German philosopher Immanuel Kant put it. In some sense, that is clearly true. But the bright galactic islands of light are just the visible tips of a much wider, but still elusive, sea of baryonic matter. This material pervades the universe, distributed within and shaped by a vast, underlying dark architecture, continuously evolving through gravity.
[...] What we think of as galaxies formed from this raw material, pulled into dense concentration by gravity. But these structures are not fixed groups of baryons. Material moves among them as part of a vast cycle that has been in operation since the big bang. The competing influences of gravity and feedback cause gas to cool onto, and later get ejected from, galaxies. [...] The baryons that make up your body have participated in this cycle for nearly 14 billion years; the matter within your fingernail could have formed in stars in other galaxies and then spent billions of years exiled in intergalactic space before coming to rest in our solar system. You are just an ephemeral phase, a brief host, to this rare substance we call “normal.”
[...] The big picture you should have in mind is that galaxy evolution is just a small component of the large-scale evolution of the intergalactic medium. The baryonic universe is predominantly gaseous, not galactic. The intergalactic medium is a battleground of forces, and amid this maelstrom, galaxies form. Galaxies are just one processing stage in a cycle that is continuously shifting baryons from one phase to the next, and at any one time, most of the baryons in the universe are not inside galaxies.”
–But if we look closely, we see who is actually in charge. Small as a grain of sand compared to the filaments, the centers of some of these galaxies shine with the power of a trillion stars, blasting out huge jets of matter, completely reshaping the cosmos around them. Quasars, the single most powerful objects in existence, so powerful that they can kill a galaxy.
If most of the atoms in the universe are found in the intergalactic medium rather than in galaxies, that is because there is a mechanism that “spits” the gas out from the galaxies and back into the cosmic environment. In the context of galaxy formation, such a mechanism is known under the generic term of “feedback”, and the supermassive black holes that sit at the center of most galaxies are thought to play a major role in it. As we’ll see below, active supermassive black holes at the center of galaxies are also known as “quasars”:
#Geach, James E. (2011): “The Lost Galaxies”, Scientific American, Volume 304, Issue 5.
https://www.scientificamerican.com/article/the-lost-galaxies/
Quote: “Something must be regulating the cooling of gas and the formation of stars in galaxies. The process has made small galaxies inefficient at forming stars and limited the size of massive galaxies. Theorists began considering a variety of additional physical processes that would provide this regulation. Known collectively as galactic feedback, these processes can counter, or reverse, the gravitational collapse of gas into galaxies and thus limit the number of stars that can form. They include supernovae explosions, stars’ ultraviolet radiation and outflows, and the tremendous energy released during the growth of the supermassive black holes that lurk in the core of all massive galaxies [see “Black Hole Blowback,” by Wallace Tucker, Harvey Tananbaum and Andrew Fabian; Scientific American, March 2007]. In the most massive galaxies, black holes are probably the most dominant feedback mechanism; in lower-mass systems, supernovae and stellar winds are more important.”
Quasars can be extremely luminous. Today, examples of quasars as luminous as 1014 Suns (hundreds of trillions of Suns) are known:
#Xue-Bing Wu et al. (2015): “An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30”, Nature, volume 518.
https://www.nature.com/articles/nature14241
https://arxiv.org/abs/1502.07418 (open access version)
Quote: “Assuming an empirical conversion factor from the luminosity at 3,000 Å to the bolometric luminosity21, this gives Lbol = 5.15 × L3,000 = 1.62 × 1048 ergs s−1 = 4.29 × 1014L⊙ (where L⊙ is the solar luminosity).”
#ESO(2011): “Most distant quasar found” (retrieved 2023)
https://www.eso.org/public/news/eso1122/
–In the 1950s astronomers noticed mysterious loud radio-waves coming from spots all over the sky.
The first quasars appeared in the radio-surveys of the 1950s:
#Shields, Gregory A. (1999): “A Brief History of Active Galactic Nuclei”, Publications of the Astronomical Society of the Pacific, Volume 111, Number 760.
https://iopscience.iop.org/article/10.1086/316378/meta
Quote: “The advances in radio astronomy in the 1950s revealed a new universe of energetic phenomena and inevitably led to the discovery of quasars. [...]
The early 1950s saw progress in radio surveys, position determinations, and optical identifications. A class of sources fairly uniformly distributed over the sky was shown by the survey by Ryle, Smith, & Elsmore (1950) based on observations with the Cambridge interferometer. [...]
Radio sources were categorized as "Class I" sources, associated with the plane of the Milky Way, and "Class II" sources, isotropically distributed and possibly mostly extragalactic (e.g., Hanbury Brown 1959). Some of the latter had very small angular sizes, encouraging the view that many were "radio stars" in our Galaxy.”
–They were named “quasi-stellar radio sources”, or “quasars” because they were dots like stars, but emitted radio waves rather than visible light.
Because of their pointlike appearance in the sky, quasars were initially thought to be a weird kind of stars within the Milky Way:
#Shields, Gregory A. (1999): “A Brief History of Active Galactic Nuclei”, Publications of the Astronomical Society of the Pacific, Volume 111, Number 760.
https://iopscience.iop.org/article/10.1086/316378/meta
Quote: “Several other apparently starlike images coincident with radio sources were found to show strange, broad emission lines. Such objects came to be known as quasi-stellar radio sources (QSRS), quasi-stellar sources (QSS), or quasars. Sandage reported the work on 3C 48 in an unscheduled paper in the 1960 December meeting of the AAS (summarized by the editors of Sky & Telescope [Matthews et al. 1961]). There was a "remote possibility that it may be a distant galaxy of stars" but "general agreement" that it was "a relatively nearby star with most peculiar properties."
–Everything about them was strange. Some flickered, others emitted high energy X-rays in addition to radio waves, but all seemed to be tiny.
Some of these pointlike sources flickered:
#Shields, Gregory A. (1999): “A Brief History of Active Galactic Nuclei”, Publications of the Astronomical Society of the Pacific, Volume 111, Number 760.
https://iopscience.iop.org/article/10.1086/316378/meta
Quote: “Another class of source, exemplified by Cyg X-1, showed no periodic variations but a rapid flickering (Oda et al. 1971) indicating a very small size.”
(Later, Cygnus X-1 would turn out to be a stellar-mass black hole in the Milky Way, becoming the first black hole to be ever identified as such)
And others were also associated to X-ray emission:
#Shields, Gregory A. (1999): “A Brief History of Active Galactic Nuclei”, Publications of the Astronomical Society of the Pacific, Volume 111, Number 760.
https://iopscience.iop.org/article/10.1086/316378/meta
Quote: “On 1962 June 18, an Aerobee sounding rockets blasted skyward from White Sands proving ground in New Mexico. It carried a Geiger counter designed to detect astronomical sources of X-rays. The experiment, carried out by Giacconi et al. (1962), discovered an X-ray background and a "large peak" in a 10° error box near the Galactic center and the constellation Scorpius. A rocket experiment by Bowyer et al. (1964) also found an isotropic background, confirmed the Scorpius source, and detected X-rays from the Crab Nebula. Friedman & Byram (1967) identified X-rays from the active galaxy M87. A rocket carrying collimated proportional counters sensitive in the 1–10 keV energy range, found sources coincident with 3C 273, NGC 5128 (Cen A), and M87 (Bowyer, Lampton, & Mack 1970). The positional error box for 3C 273 was small enough to give a probability of less that 10−3 of a chance coincidence. The X-ray luminosity, quoted as ∼1046 ergs s−1, was comparable with quasar's optical luminosity.”
–They also moved extremely fast, as much as over 30% the speed of light. The only explanation was that they must have been so distant that their apparent speed was actually the expansion of the universe moving them away from us.
The first two quasars to be ever identified as such were 3C 273 and 3C 48, discovered in the 1950s and analyzed in the early 1960s. By studying their properties, astronomers came to the conclusion that those “pointlike sources” were receding from Earth at the astonishing speeds of 47,400 km/sec (16% of the speed of light) and 110,200 km/sec (37% of the speed of light), respectively. Those amazing speeds led astronomers to conclude that the effect had to be due to the “cosmological redshift” (i.e. the expansion of the universe):
#Schmidt, M. (1963): “3C 273 : A Star-Like Object with Large Red-Shift”, Nature, volume 197.
https://www.nature.com/articles/1971040a0
Quote: “The stellar object is the nuclear region of a galaxy with a cosmological red-shift of 0.158, corresponding to an apparent velocity of 47,400 km/sec.”
#Greenstein, J. et al. (1964): “Red-Shift of the Unusual Radio Source 3C 48”, Nature, volume 197.
https://phas.ubc.ca/~heyl/ASTR304_2003W/papers/greenstein.pdf
Quote: “The weighted mean red-shift dλ/λo is 0.3675 ± 0.0003, an apparent velocity of +110,200 km/sec.”
– But this meant that quasars aren’t stars, but the active cores of galaxies billions of lightyears away! And it gets crazier. To appear as bright and loud, given these vast distances, they are thousands of times brighter than the entire Milky Way. Monsters, exploding and screaming into the void with a violence not thought possible before.
Those high speeds also indicated that the sources had to be billions of light years away (due to the expansion of the universe, the recession speed of a source is proportional to its distance). Furthermore, to be visible from such amazing distances, they had to be far brighter than a normal galaxy:
#Schmidt, M. (1963): “3C 273 : A Star-Like Object with Large Red-Shift”, Nature, volume 197.
https://www.nature.com/articles/1971040a0
Quote: “The distance would be around 500 megaparsecs, and the diameter of the nuclear region would have to be less than 1 kiloparsec. This nuclear region would be about 100 times brighter optically than the luminous galaxies which have been identified with radio sources thus far.”
(The parsec is a unit of distance used by astronomers that amounts to 3.26 light-years.)
#Greenstein, J. et al. (1964): “Red-Shift of the Unusual Radio Source 3C 48”, Nature, volume 197.
https://phas.ubc.ca/~heyl/ASTR304_2003W/papers/greenstein.pdf
Quote: “So large a red-shift, second only to that of the intense radio source 3C 295, will have important implications in cosmological speculation.”
Quasars can be several thousand times brighter than a normal galaxy, implying that they are among the brightest objects in the universe. The Milky Way or Andromeda have luminosities of the order of 1010 Suns:
#van den Bergh, Sidney (1999): “The local group of galaxies”, The Astronomy and Astrophysics Review, volume 9.
https://link.springer.com/article/10.1007/s001590050019
https://arxiv.org/abs/astro-ph/9908050 (open access version)
But some quasars have luminosities of the order of 1014 Suns, i.e. thousands of times more than Andromeda or the Milky Way:
#Xue-Bing Wu et al. (2015): “An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30”, Nature, volume 518.
https://www.nature.com/articles/nature14241
https://arxiv.org/abs/1502.07418 (open access version)
Quote: “Assuming an empirical conversion factor from the luminosity at 3,000 Å to the bolometric luminosity21, this gives Lbol = 5.15 × L3,000 = 1.62 × 1048 ergs s−1 = 4.29 × 1014L⊙ (where L⊙ is the solar luminosity).”
–As we mapped the sky, we discovered over a million quasars. And they all seemed to be very far away. Looking into space, far away means very long ago, because their light takes so long to reach us. Quasars were common in the early universe, having peaked in number 10 billion years ago when galaxies, and the universe itself was still very young.
In the last years, the most complete catalogs of quasars have grown from over a hundred thousand sources to over a million:
#Véron-Cetty, M.-P. et al. (2010): “A catalogue of quasars and active nuclei: 13th edition”, Astronomy & Astrophysics, Volume 518.
https://www.aanda.org/articles/aa/abs/2010/10/aa14188-10/aa14188-10.html
Quote: “This catalogue is aimed at presenting a compilation of all known AGN in a compact and convenient form, and we hope that it will be useful to all workers in this field. [...] The present version contains 133 336 quasars, 1 374 BL Lac objects, and 34 231 active galaxies (including 16 517 Seyfert 1s), almost doubling the number listed in the 12th edition.”
#Secrest, Nathan J. (2021): “A Test of the Cosmological Principle with Quasars”, The Astrophysical Journal Letters, Volume 908, Number 2.
https://iopscience.iop.org/article/10.3847/2041-8213/abdd40
Quote: “We study the large-scale anisotropy of the universe by measuring the dipole in the angular distribution of a flux-limited, all-sky sample of 1.36 million quasars observed by the Wide-field Infrared Survey Explorer (WISE).”
And they seem to have been most abundant in the young universe: about 3,000 billion years after the big bang, or around 10,000-11,000 billion years ago:
#Barbara Ryden (2003): “Quasars”, Astronomy 162, Ohio State University (retrieved 2023).
https://www.astronomy.ohio-state.edu/ryden.1/ast162_8/notes36.html
Quote: “The Age of Quasars, it seems, is over. Surveys of the sky, searching for quasars, reveal that the highest density of quasars is at a distance of roughly 11 billion light-years, indicating that quasars were most prevalent in the universe 11 billion years ago. Between 9 and 13 billion years ago, there were plenty of quasars in the universe. More recently, however, there have been very few.”
Although some studies have raised the possibility of a “quasar plateau” ending at 3 By ago rather than a “quasar peak” at 3 By ago:
#Mortlock, Daniel (2014): “The age of the quasars”, Nature, volume 514.
–There’s only one way to generate the vast amounts of energy a quasar shines with: feeding supermassive black holes. We still don’t know how exactly they formed, but it seems that every galaxy has one in their center. But how can the brightest things in the universe be black holes, which trap anything and everything that crosses their event horizon? Well the light of a quasar is not coming from inside these black holes. Rather, it comes from the space around them, a massive orbiting disk of gas called an ‘accretion disk.’
Today we know that quasars are extremely hot “storms” caused by a supermassive black hole that is actively devouring matter at the center of a galaxy, i.e. an “active galactic nucleus” (AGN). As the infalling matter spirals onto the black hole, it’s accelerated to almost the speed of light and heated at extremely high temperatures, making quasars the brightest objects of the universe:
#NASA (2016): “Active Galaxies”, Imagine the Universe (retrieved 2023)
https://imagine.gsfc.nasa.gov/science/objects/active_galaxies1.html
Quote: “Active galaxies are galaxies that have a small core of emission embedded at the center of an otherwise typical galaxy. This core is typically highly variable and very bright compared to the rest of the galaxy.
For normal galaxies, we think of the total energy they emit as the sum of the emission from each of the stars found in the galaxy, but in active galaxies, this is not true. There is a great deal more emitted energy in active galaxies than there should be and this excess energy is found in the infrared, radio, UV, and X-ray regions of the electromagnetic spectrum. The energy emitted by an active galaxy, AGN for short, is anything but normal. So what is happening in these galaxies to produce such an energetic output?
Most, if not all, normal galaxies have a supermassive black hole at their center. In an active galaxy, its supermassive black hole is accreting material from the galaxy's dense central region. As the material falls in toward the black hole, angular momentum will cause it to spiral in and form into a disk. This disk, called an accretion disk, heats up due to the gravitational and frictional forces at work.”
The supermassive black holes powering quasars are several billions of times more massive than the Sun:
#ESO Supernova: “How do black holes power quasars?” (retrieved 2023)
Quote: “Quasi-stellar radio sources, or quasars for short, are the ultra-luminous cores of very distant galaxies, blowing opposing jets of charged particles and energetic radiation into space. They are powered by supermassive black holes, weighing in at billions of solar masses. Gas falling into the black hole accumulates in a bright, whirling accretion disc; the jets are probably produced by strong magnetic fields. Quasars are most conspicuous when one of the jets is more or less aimed at the observer. Seen edge-on, their energetic radiation can be obscured by thick clouds of surrounding dust.
–Quasars use the same fuel as stars to shine: Matter. It is just that black holes are the most efficient engines for converting matter into energy in the universe. The energy released by matter falling into a black hole can be 60 times greater than that released by nuclear fusion in the core of a star. Because the energy released by a black hole comes not from nuclear reactions, but from gravity.
We can see a quasar as a “machine” (a supermassive black hole) that absorbs matter and spits energy. Energy and mass are related through the famous formula E = mc2. The energy released by quasars can be up to 40% of the mass that they consume. In comparison, the energy produced by nuclear fusion in stars is just 0.7% of the mass that they use to produce that energy. This makes quasars up to 40/0.7 ~ 60 times more efficient than stars at converting mass into energy. Let’s see why.
In the main fusion process taking place in stars, 4 hydrogen nuclei are converted into 1 helium nucleus through a chain of fusion-reactions. In this process, a little mass is lost and converted into energy:
#ESA (2002): “Measuring the Distance to the Cat’s Eye Nebula”, The ESA/ESO Astronomy Exercise Series.
https://sci.esa.int/documents/34439/36575/1567254560036-5-exercise3-low.pdf
We can therefore ask how efficient this process is; i.e. what percentage of the initial mass is converted into energy.
In atomic units of mass, the mass of one hydrogen nucleus is 1.007825:
#PubChem: “Protio”, National Library of Medicine (retrieved 2023)
https://pubchem.ncbi.nlm.nih.gov/compound/Hydrogen-1
Quote: “Protium atom is the stable isotope of hydrogen with relative atomic mass 1.007825”
while that of one helium nucleus is 4.002603:
#PubChem: “Helium-4 atom”, National Library of Medicine (retrieved 2023)
https://pubchem.ncbi.nlm.nih.gov/compound/Helium-4-atom
Quote: “Helium-4 atom is the stable isotope of helium with relative atomic mass 4.002603”
So we see that, in stars
(4 × 1.007825 – 4.002603) / (4 × 1.007825) = 0.7%
of the initial hydrogen mass is converted into energy. The remaining 99.3% stays as mass in the form of helium.
What would be the equivalent in a quasar? The efficiency of a quasar, usually denoted by η, is defined as the energy output per unit of accreted mass (in units of E/c2). Depending on the mass and the accretion rate of the supermassive black hole, the efficiencies of quasars have been found to be between η = 3% and η = 40%:
#Davis, Shane W. (2011): “The radiative efficiency of accretion flows in individual active galactic nuclei”, The Astrophysical Journal, Volume 728, Number 2
https://iopscience.iop.org/article/10.1088/0004-637X/728/2/98/meta
Quote: “The radiative efficiency of active galactic nucleus (AGN) is commonly estimated based on the total mass accreted and the total AGN light emitted per unit volume in the universe integrated over time (the Soltan argument). In individual AGNs, thin accretion disk model spectral fits can be used to deduce the absolute accretion rate Ṁ, if the black hole mass M is known. The radiative efficiency η is then set by the ratio of the bolometric luminosity Lbol to Ṁc2. We apply this method to determine η in a sample of 80 Palomar–Green quasars with well-determined Lbol, where Ṁ is set by thin accretion disk model fits to the optical luminosity density, and the M determination based on the bulge stellar velocity dispersion (13 objects) or the broad line region. We derive a mean log η = −1.05 ± 0.52 consistent with the Soltan-argument-based estimates. We find a strong correlation of η with M, rising from η ~ 0.03 at M = 107 M☉ and L/LEdd ~ 1 to η ~ 0.4 at M = 109 M☉ and L/LEdd ~ 0.3.“
–Matter falling into a black hole speeds up to almost the speed of light before it crosses the event horizon, buzzing with an incredible amount of kinetic energy.
When approaching the event horizon of a black hole, matter moves at nearly the speed of light:
#NASA’s Goddard Space Flight Center (2019): “Black Hole Accretion Disk Visualization” (retrieved 2023)
https://svs.gsfc.nasa.gov/13326
Quote: “Nearest the black hole, the gas orbits at close to the speed of light, while the outer portions spin a bit more slowly. This difference stretches and shears the bright knots, producing light and dark lanes in the disk.”
–Of course, once inside the black hole, it takes that energy with it. You only get to witness this energy if you drop your matter in the right way. Fall straight down and the outside universe gets nothing. But when you have a lot of matter, it spirals in incredibly fast towards the event horizon forming a disk. Collisions between particles and friction heat it up to millions of degrees.
If, instead of falling into the BH following a perfectly straight line, the infalling matter follows inspiralling orbits (which in practice is always the case, since galaxies always rotate and so do their central black holes), a disk of matter will form and the particles will collide with each other. This creates friction which, due to the high speed of the particles, will heat up the infalling matter to very high temperatures:
#NASA (2016): “Active Galaxies”, Imagine the Universe (retrieved 2023)
https://imagine.gsfc.nasa.gov/science/objects/active_galaxies1.html
Quote: “In an active galaxy, its supermassive black hole is accreting material from the galaxy's dense central region. As the material falls in toward the black hole, angular momentum will cause it to spiral in and form into a disk. This disk, called an accretion disk, heats up due to the gravitational and frictional forces at work.”
The temperature in the accretion disk of a supermassive black hole depends on the mass of the black hole. For a supermassive black hole of one billion solar masses, theoretical estimates give a temperature of the order of 100,000 kelvin:
#Maoz, Dan (2016): “Astrophysics in a Nutshell”. Princeton University Press.
https://press.princeton.edu/books/hardcover/9780691164793/astrophysics-in-a-nutshell
Quote: “Thus, a 109 M☉ black hole radiating at the Eddington luminosity must accrete 40 M☉ per
Year. [...] For the above black-hole mass and accretion rate, the inner temperature will thus be
T = 1.7 × 109 K × 401/4 × (109)−1/2 = 1.4 × 105 K, (7.43)
producing thermal radiation that peaks in the extreme UV, between the UV and the X-ray
ranges. Quasars, in fact, have such a UV “bump” in their spectral energy distributions
(see Fig. 7.12), which is thought to be the signature of the accretion disk.”
–In a space not much bigger than our Solar System, the core of a galaxy can release many times more energy than all its stars combined.
As explained above, quasars can shine with the luminosity of thousands of galaxies. However, the active galactic nucleus radiating so much energy is pretty small in astronomical terms. Although their size is not known precisely (they are too far away to be resolved directly by telescopes), theoretical models of accretion discs and indirect measures of their size suggest typical sizes of a few light-days (lt-days), i.e. a few times bigger than our Solar System:
#Jiménez-Vicente, J. (2014): “The average size and temperature profile of quasar accretion disks”, The Astrophysical Journal, Volume 783, Number 1
https://iopscience.iop.org/article/10.1088/0004-637X/783/1/47
Quote: “We have also obtained a maximum likelihood estimate for the average quasar accretion disk size of rs = 4.5+1.5–1.2 lt-day at a rest frame wavelength of λ = 1026 Å for microlenses with a mean mass of M = 1 M☉, in agreement with previous results, and larger than expected from thin disk theory.”
#Wei-Jian Guo (2022): “Accretion Disk Size Measurements of Active Galactic Nuclei Monitored by the Zwicky Transient Facility”, The Astrophysical Journal, Volume 929, Number 1
https://iopscience.iop.org/article/10.3847/1538-4357/ac4e84
Quote: “The best-estimated disk size is τ0 = 4.56+0.69–0.69 lt-days. This is 1.36 times as large as the theoretical expectation 3.35 lt-days of the standard accretion disk model.”
One light day is about 2.6·1010 km, or 173 astronomical units (AU). If we define the size of the solar system as the location of the heliopause (the border marking the region of influence of the solar wind), we get a radius of 123 AU, or about 0.7 light years.
#Encyclopaedia Britannica: “Heliopause” (retrieved 2023)
https://www.britannica.com/science/heliopause
Quote: “heliopause, boundary of the heliosphere, the spherical region around the Sun that is filled with solar magnetic fields and the outward-moving solar wind consisting of protons and electrons. [...] The heliopause is about 123 astronomical units (AU; 18 billion km [11 billion miles]) from the Sun.”
(This is much further away than the orbit of Neptune, which is about 30 AU from the Sun, and beyond the main region of the Kuiper belt.)
Therefore, the typical accretion disk of a quasar will roughly be about 5-6 times bigger than our Solar System (defined by the size of the heliopause, not by the orbit of Neptune).
These sizes were confirmed in order of magnitude by the famous picture of M87*, a supermassive black hole of 6.5 billion solar masses that also has an accretion disc (the image at the left in the picture below):
#ESO (2022): “Comparison of the sizes of two black holes: M87* and Sagittarius A*” (retrieved 2023)
–And these black holes eat a lot. Typical quasars consume one to a hundred Earth masses of gas per minute!
In order of magnitude, typical accretion rates of quasars are in the range of one solar mass per year:
#Wei-Hao Bian et al. (2003): “Accretion Rates and the Accretion Efficiency in AGNs”, Publications of the Astronomical Society of Japan, Volume 55, Issue 3
https://academic.oup.com/pasj/article/55/3/599/1549665
Quote: “From table 1 we can find that the mean accretion rate in quasars is larger than in Seyfert 1 galaxies. Our calculated accretion rates in AGNs are about one solar mass per year.”
The mass of the Sun is about 333,000 times that of Earth:
#NASA (2022): “Sun Fact Sheet” (retrieved 2023)
https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html
And therefore one solar mass per year amounts to more than 0.6 Earth masses per minute, which we’ve rounded to 1 in order of magnitude.
However, higher accretion rates have been determined for other active galactic nuclei:
#Daly, Ruth A. (2020): “Black hole mass accretion rates and efficiency factors for over 750 AGN and multiple GBH”, Monthly Notices of the Royal Astronomical Society, Volume 500, Issue 1.
https://academic.oup.com/mnras/article-abstract/500/1/215/5936639
https://arxiv.org/abs/2010.06908 (open access version)
This table shows accretion rates for several supermassive black holes. The logarithm of the accretion rate (in solar masses per year) is given in column 5, and the logarithm of the mass of the black hole (in solar masses) is given in column 10. We see that the highest accretion rate in this list is that of 3C 268.4, a black hole with a mass of 6 billion solar masses:
Mass = 109.80 M⊙ = 6.3·109 M⊙
And with an accretion rate of more than 12 solar masses per year:
Accretion rate = 101.10 M⊙ / year = 12.6 M⊙ / year
which translates into 8 Earth masses per minute.
However, some very distant quasars (i.e. quasars that existed when the universe was much younger than today) may have had accretion rates of up to ~190 solar masses per year:
#Trakhtenbrot, Benny (2017): “On the Accretion Rates and Radiative Efficiencies of the Highest-redshift Quasars”, The Astrophysical Journal Letters, Volume 836, Number 1
https://iopscience.iop.org/article/10.3847/2041-8213/836/1/L1/meta
Quote: “Figure 1 presents the derived accretion rates through the disks, Ṁdisk, for all the sources in our sample, and based on the two different (rest-frame) optical luminosities. The accretion rates we obtain using L4.5 are in the range Ṁdisk ≃ 3.6−187 M⊙ yr−1”
Which translates into more than 100 Earth masses per minute.
–Ten billion years ago, the universe was about a third of its current size, so the intergalactic medium was much less spread out, meaning the filaments of gas around quasars could feed them a banquet, making them vomit insane amounts of light and radiation.
The age of the universe and the relative size of the universe are related by the “redshift”: a directly measurable quantity (usually denoted by the letter z) that reflects how the color of the light has changed due to the expansion of the universe. Basically, if a given wavelength emitted many billions of years ago has become 3 times longer today, it is because in that time the observable universe has become 3 times bigger. The precise formula is given by
1 + z = size(today) / size(then)
#Sloan Digital Sky Survey: “Redshifts” (retrieved 2023)
https://skyserver.sdss.org/dr1/en/proj/basic/universe/redshifts.asp
Quote: “The redshift tells us the size of the universe at the time the light left the galaxy. Because the universe is billions of light-years across, it takes billions of years for light from distant galaxies to reach us. Suppose the distance to galaxy 587731512071880746 was d(z) at the time the light left it that we are now observing (for z = 0.1, this time was roughly a billion years ago). In those billion years, the space in the universe has expanded, so that now the distance between our galaxy and it is d(0). Then
1 + z = d(0) / d(z).”
The current age of the universe is about 14 billion years. When it was about 3 billion years old (i.e. 11 billion years ago, or 10 for the sake of simplicity) it had an associated to a redshift of z = 2, which means that it was 1 + 2 = 3 times smaller:
#Wright, Edward L. (2006): “Ned Wright’s Javascript Cosmology Calculator”, UCLA Division of Astronomy and Astrophysics (retrieved 2023)
There is another reason for which quasars were more abundant in the past. Back then the universe was not only smaller and therefore more “dense”. Also, the total amount of gas available was higher, since a smaller fraction of it had been converted into stars.
–The brightest quasars power jets, tangling the magnetic field of the matter around them into a narrow cone. Like a particle accelerator they launch enormous beams of matter out, plowing through the circumgalactic medium, forming plumes of matter that grow to hundreds of thousands of lightyears in size. It’s almost unfathomable in scale. A tiny spot in a galaxy carving out patches of the universe 100,000s of light years long.
It is well known that some quasars eject powerful jets of high-speed particles along the poles of the black hole:
#NASA (2016): “Active Galaxies”, Imagine the Universe (retrieved 2023)
https://imagine.gsfc.nasa.gov/science/objects/active_galaxies1.html
Quote: “Models of active galaxies also include a region of cold gas and dust, thought to be in the shape of a giant donut with the black hole and accretion disk nestled in the donut's hole. In about one out of ten AGN, the black hole and accretion disk produce narrow beams of energetic particles and ejects them outward in opposite directions away from the disk. These jets, which emerge at nearly the speed of light, become a powerful source of radio wave emission.”
These jets are typically hundreds of thousands of light-years long (i.e. much bigger than the linear dimensions of a typical galaxy), and are thought to be one of the main mechanisms of redistribution of matter in the universe at the largest scales:
#NASA (2006): “NASA Scientists Determine the Nature of Black Hole Jets”, Goddard Space Flight Center (retrieved 2023)
https://www.nasa.gov/centers/goddard/news/topstory/2006/swift_blazars.html
Quote: “Black hole particle jets typically escape the confines of their host galaxies and flow for hundreds of thousands of light years. They are a primary means of redistributing matter and energy in the universe.”
#Tucker, Wallace et al (2007): “Black Hole Blowback”. Scientific American, vol. 296
https://www.scientificamerican.com/article/black-hole-blowback/
Quote: “A single black hole, smaller than the solar system, can control the destiny of an entire cluster of galaxies”
How exactly these jets are formed is an astrophysical enigma, but magnetic fields are known to play a pivotal role:
#NASA (2020): “The Recipe for Powerful Quasar Jets”, Chandra X-Ray Observatory (retrieved 2023)
https://www.nasa.gov/mission_pages/chandra/images/the-recipe-for-powerful-quasar-jets.html
Quote: “In astronomy, the term “corona” is commonly associated with the outer atmosphere of the Sun. Black hole coronas, on the other hand, are regions of diffuse hot gas that lie above and below a much denser disk of material swirling around the gravitational sinkhole. Like the corona around the Sun, black hole coronas are threaded with strong magnetic fields. [...]
An important implication of their work is that to produce powerful jets a quasar must have a bright black hole corona, threaded by strong magnetic fields, in addition to a rapidly spinning black hole. Quasars with fainter black hole coronas and weaker magnetic fields have less powerful or non-existent jets whether or not their supermassive black holes are spinning quickly.
“Both a quasar’s powerful jets and bright corona occurring together may be fundamentally driven by magnetic fields”, said Zhu.
Stronger magnetic fields may result from a thicker disk caused by a higher rate of matter falling into the black hole.”
–But quasars can’t eat for long, maybe a few million years, because their feast ultimately kills their galaxy.
This frenetic activity doesn’t last for long, though. As quasars shake their cosmic environment, they also push out the gas that is feeding them and keeping them active. The total lifecycle of a quasar lasts for just a few million years (a short time in astronomical terms), with the most active episodes of activity lasting perhaps just tens or hundreds of thousands of years:
#Nature Astronomy Editorial (2019): “The relevance of being active”, Nature Astronomy, volume 3.
https://www.nature.com/articles/s41550-019-0730-2
Quote: “Perhaps most interestingly and despite early evidence to the contrary, we now believe that AGN activity is highly temperamental. While the full lifecycle of an AGN can be up to tens to hundreds of millions of years (based, for example, on the extent of their radio jets), the actual duty cycle of each activity episode might be far shorter than that (of the order of tens to hundreds of thousands of years, sometimes even shorter). This temporal volatility is probably due to chaotic accretion onto the supermassive black hole.”
–Okay, maybe “killing” is a bit of an exaggeration. A galaxy is still there after its quasar turns off. But it will never be the same again. Because the light and heat of a quasar is so great that it makes it hard for the rest of the galaxy to make new stars.
The energy released by quasar activity expels gas from the host galaxy, which partially shuts down new star formation and, in addition, limits the future growth of the black hole itself:
#Di Matteo, Tiziana et al. (2005): “Energy input from quasars regulates the growth and activity of black holes and their host galaxies”. Nature, vol. 433.
https://www.nature.com/articles/nature03335
https://arxiv.org/abs/astro-ph/0502199 (open-access version)
Quote: “In the early Universe, while galaxies were still forming, black holes as massive as a billion solar masses powered quasars. Supermassive black holes are found at the centres of most galaxies today1,2,3, where their masses are related to the velocity dispersions of stars in their host galaxies and hence to the mass of the central bulge of the galaxy4,5. This suggests a link between the growth of the black holes and their host galaxies6,7,8,9, which has indeed been assumed for a number of years. [...] Here we report simulations that simultaneously follow star formation and the growth of black holes during galaxy–galaxy collisions [...] The energy released by the quasar expels enough gas to quench both star formation and further black hole growth. This determines the lifetime of the quasar phase (approaching 100 million years) and explains the relationship between the black hole mass and the stellar velocity dispersion.”
–Hot gas cannot form stars. This sounds odd, because stars are gas that collapsed in on itself and then got really hot. But to form a star, gas must first get cold: in a cloud of hot gas, atoms are moving quickly. When they collide, they hit hard, exerting pressure that resists gravity’s squeeze. Instead, the best gas for forming stars is already cold, and won’t put up a fight when it’s time to collapse into a star.
Stars are born from the gravitational collapse of gigantic gas clouds. For the gas to condense and form a star, it has to be very cold, or otherwise it would tend to expand.
#Schombert, James: “Star Formation”. Department of Physics, University of Oregon (retrieved 2023)
http://abyss.uoregon.edu/~js/ast122/lectures/lec13.html
Quote: “Stars form inside relatively dense concentrations of interstellar gas and dust known as molecular clouds. These regions are extremely cold (temperature about 10 to 20K, just above absolute zero). At these temperatures, gases become molecular meaning that atoms bind together. CO and H2 are the most common molecules in interstellar gas clouds. The deep cold also causes the gas to clump to high densities. When the density reaches a certain point, stars form.”
–As sad as this sounds, it might be a good thing for life. The alternative can be far more dangerous: too many stars. New stars forming is usually followed by massive stars exploding in supernovae, so planets would be burned sterile.
Very active star formation is known to create a hostile environment for life due to an excessive radiation and frequent supernova explosions:
#Scharf, Caleb (2012): "The Benevolence of Black Holes". Scientific American, vol. 307
https://www.scientificamerican.com/article/how-black-holes-shape-galaxies-stars-planets-around-them/
Quote: “Very active star formation produces an awfully messy environment. It builds the massive stars that burn through their nuclear fuel the fastest, ending up as colossal supernova explosions. Planetary atmospheres can be blasted away or chemically altered by radiation. Fast-moving energetic particles and gamma rays can pummel the surface of a world. Even the flux of ghostly neutrinos released in stellar implosion is intense enough to damage delicate biology. And those are just the moderate effects. Too close to a supernova, and there is a good chance your entire system will be vaporized.”
–But of course it's more complicated. Like the intricacies of our own planet’s biosphere, every piece of the galaxy is dependent on and influencing every other part of the galactic environment. While hot things, like quasars and supernovae, tend to push gas out of the galaxy, shockwaves and quasar jets can also compress gas, making new stars at least for a short time.
Supernova explosions can be dangerous for life if life happens to be nearby. However, they are also crucial for life in at least two ways. First, they synthesize and spread heavy chemical elements that are essential for life. Secondly, the energy released in these explosions can compress the gas in the interstellar medium, causing its collapse into protostars and therefore leading to the birth of new stars and planetary systems.
#NASA (2017): “The Dispersion of Elements”. Goddard Space Flight Center, Imagine the Universe (retrieved 2023)
https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-elements.html
Quote: “The most common elements, like carbon and nitrogen, are created in the cores of most stars, fused from lighter elements like hydrogen and helium. The heaviest elements, like iron, however, are only formed in the massive stars which end their lives in supernova explosions. Still other elements are born in the extreme conditions of the explosion itself. Without supernovae, life would not be possible. Our blood has iron in the hemoglobin which is vital to our ability to breath. We need oxygen in our atmosphere to breathe. Nitrogen enriches our planet's soil. Earth itself would be a very different place without the elements created in stars and supernova explosions. [...] Supernovae change the chemical composition of the ISM [interstellar medium], by adding elements which were not present before, or were only present in trace amounts. Though these explosions only occur a few times a century in our Galaxy, they are responsible for the synthesis of all the elements heavier than iron, including many we come across in daily life, like copper, mercury, gold, iodine and lead. Most of the elements which are produced in supernovae have small cosmic abundances and very few have been directly detected in the interstellar medium. [...] The chaos caused by supernovae, like the one that created the Crab Nebula (shown at left), is also responsible for the complex structure of the ISM. A supernovae creates shock waves through the interstellar medium, compressing the material there, heating it up to millions of degrees. Astronomers believe that these shock waves are vital to the process of star formation, causing large clouds of gas to collapse and form new stars. No supernovae, no new stars.”
Although their role in star formation is less clear, quasar jets have been proposed as enhancers of star formation, too:
#Berardelli, Phil (2009): “The Quasar That Built a Galaxy”. Science (retrieved 2023)
https://www.science.org/content/article/quasar-built-galaxy
Quote: “HE0450-2958 is a typical quasar in some respects. Discovered in 2005 and located about 5 billion light-years away, it's as massive and energetic as other quasars. But one thing about the object piqued the curiosity of a team of astronomers: HE0450-2958 was shrouded within a dust cloud that appeared to be too small to hide a surrounding galaxy. Their new observations, which they gathered from infrared light, produced an unexpected result. The surrounding galaxy wasn't there. Even more remarkable, the astronomers observed a small companion galaxy, only about 22,000 light-years away, whose new stars are being formed at an extremely rapid clip. One of the quasar's jets is aimed directly at the galaxy, and the team thinks it's likely that the jet is driving the star-making process by blasting matter into the galaxy.”
#Elbaz, David et al. (2009): “Quasar induced galaxy formation: a new paradigm?”. Astronomy & Astrophysics, vol. 507.
https://www.aanda.org/articles/aa/abs/2009/45/aa12848-09/aa12848-09.html
Quote: “We discuss observational evidence that quasars play a key role in the formation of galaxies, starting from the detailed study of the quasar HE0450-2958 and extending the discussion to a series of converging evidence that radio jets may trigger galaxy formation. [...] Comparison to other systems suggest that an inside-out formation of quasar host galaxies and jet-induced galaxy formation may be a common process.”
–It’s possible the Milky Way was once a quasar, which may have allowed our supermassive black hole Sagittarius A star to have grown to 4 million times the mass of the sun. But sadly we don’t know its ancient history.
At 4 million solar masses, Sgr A* is pretty small when compared to other supermassive black holes, which as documented above can easily reach billions of solar masses. Therefore it is not clear if the Milky Way experienced a real quasar phase in the past or not. However, a few years ago scientists discovered two giant “gamma-ray bubbles” emanating from the poles of the Milky Way:
#NASA(2010): "NASA's Fermi Telescope Finds Giant Structure in our Galaxy”. Fermi Space Telescope (retrieved 2023)
https://www.nasa.gov/mission_pages/GLAST/news/new-structure.html
Quote: “NASA's Fermi Gamma-ray Space Telescope has unveiled a previously unseen structure centered in the Milky Way. The feature spans 50,000 light-years and may be the remnant of an eruption from a supersized black hole at the center of our galaxy. [...] The structure spans more than half of the visible sky, from the constellation Virgo to the constellation Grus, and it may be millions of years old.”
Such bubbles have been hypothesized to have been created by quasar activity just a few million years ago:
#Zubovas, K. et al. (2011): “The Milky Way's Fermi bubbles: echoes of the last quasar outburst?”. Monthly Notices of the Royal Astronomical Society: Letters, vol. 415
https://academic.oup.com/mnrasl/article/415/1/L21/965171
Quote: “Here we propose an alternative picture where the bubbles are the remnants of a large-scale wide-angle outflow from Sgr A*, the supermassive black hole (SMBH) of our Galaxy. Such an outflow would be a natural consequence of a short but bright accretion event on to Sgr A* if it happened concurrently with the well-known star formation event in the inner 0.5 pc of the Milky Way ∼6 Myr ago.”
–And as dormant as it is now, Sagittarius A star could turn into a quasar in the future. In a few billion years the Milky Way will merge with Andromeda. We’ve seen over a hundred ‘double quasars’ in galaxies smashing together, where fresh gas is provided for the central black holes. But it won’t last for long. When galaxies merge, so do their super massive black holes, sinking into the center of their new galaxy, kicking up dust and stars in every direction.
It has long been known that the Milky Way and Andromeda (our closest big galaxy) are headed towards each other, being expected to collide in about 4 billion years from now:
#Cowen, Ron (2012): “Andromeda on collision course with the Milky Way”. Nature (retrieved 2023)
https://www.nature.com/articles/nature.2012.10765
Quote: “It’s a definite hit. The Andromeda galaxy will collide with the Milky Way about 4 billion years from now, astronomers announced today. Although the Sun and other stars will remain intact, the titanic tumult is likely to shove the Solar System to the outskirts of the merged galaxies.”
The future merging of the Milky Way and Andromeda has been simulated by NASA scientists:
#NASA (2018): “Crash of the Titans: Milky Way & Andromeda Collision”. Scientific Visualization Studio (retrieved 2023)
Such a collision will lead to the merging of both galaxis, and probably to the merging of their respective supermassive black holes, leading to quasar activity:
#Cox, T. J. et al. (2008): “The collision between the Milky Way and Andromeda”. Monthly Notices of the Royal Astronomical Society, vol. 386
https://academic.oup.com/mnras/article/386/1/461/978865
Quote: “While we have not explicitly tracked the black holes at the centre of the MW and Andromeda, it is interesting to speculate whether the merger will produce a luminous quasar, which many models argue is intricately linked to galaxy mergers and starbursts (Hopkins et al. 2006). Even though Fig. 11 demonstrates that there is not enough gas to fuel a powerful starburst, this gas content is clearly sufficient to ignite a luminous quasar if ∼1 per cent of it is accreted by the black hole.”
This is how the merger of the corresponding black holes may look like:
#NASA (2018): “New Simulation Sheds Light on Spiraling Supermassive Black Holes” (retrieved 2018)
Such “double quasar” activity has already been detected in about 100 distant galaxies:
#NASA (2021): “Hubble Spots Double Quasars in Merging Galaxies” (retrieved 2023)
https://www.nasa.gov/feature/goddard/2021/hubble-spots-double-quasars-in-merging-galaxies
Quote: “Astronomers have discovered more than 100 double quasars in merging galaxies so far. However, none of them is as old as the two double quasars in this study.”