Every galaxy has a black hole at its centre [0]
Reminder: Have you read the preceding page "Black holes and Hawking radiation"?
It now appears that every galaxy has a black hole at its centre. These black holes have the capability of exercising a benevolent effect on life in the galaxies they inhabit, but much will depend upon their activity level, balanced as it is between the gravitational agglomeration of matter and the disruptive energy blasting from matter-swallowing black holes. Too much black hole activity, and there would be little star formation, and the production of heavy elements would cease to occur. Too little black hole activity, and the galactic environments might be overly full of young an exploding stars, or too little stirred up to produce anything. Change the balance at all, and you change the whole pathway of star and galaxy formation[1].
Our own Milky Way occupies a galactic sweet spot, with a black hole, having a mass approximately 4 million times that of the sun, that appears to act out just often enough to stir things up “and keep the galaxy’s stellar population at a perfect simmer”[2]. We can only detect the fact that it exists by its influence on the orbits of stars in its vicinity. For the first picture of a black hole detected by the Event Horizon telescope (EHT) detected in April 1987 but only revealed in April 2019 after all the information gathered was finally processed, see the following page.
In some galaxies, the black holes are being fed by material falling in, and are characterised by material being ejected therefrom at nearly the speed of light. They are known as radio galaxies and quasars (quasi-stellar radio sources), depending on the direction in which they are pointing. Quasars were originally discovered by astronomer Maarten Schmidt in 1963. They are cosmic beacons, the ultraluminous centres of active galaxies that can be seen all the way to the outskirts of the visible universe, each shining brighter than billions of suns.
They are thought to occur when massive clouds of gas and dust plunge onto a supermassive black hole for hundreds of thousands or millions of years, compressing, heating and glowing as they circle the black hole’s maw or gullet. This notwithstanding, they constitute only a small portion of any supermassive black hole’s lifetime and are too rare, distant and sluggish to wholly reveal how such giant black holes feed. More nuanced detail can come from watching black holes snack on entire stars. [2.1]
Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies[3]. Both are an explosive result of the overfeeding of the black holes at their centres.
The fact that we see quasars at high redshift means that massive black holes already existed and were growing less than a billion years after the big bang. So the black holes must have been formed very early on in the universe. Did they exist first and then galaxies grew around them? Or did the galaxy and the black hole both form together? We still don’t know. We do know, however, that almost every galaxy has a massive black hole at its heart, and that the bigger the galaxy, the bigger the black hole. This suggests that the growth of the galaxy and the black hole are somehow intimately linked.
We think that the link is that both galaxies and black holes grow through the merging of galaxies. Everywhere we look, we see signs of galaxies in the process of colliding or showing evidence of collisions in the not-too-distant past. What’s more, the further back we look the more common these collisions seem to be. This suggests that gas falls in to build the galaxy, and at the same time some gets funnelled in to the centre where it feeds and grows the black hole [4]. On this view, a quasar is just a particularly violent stage in the life of a galaxy, one that many, perhaps most, galaxies have gone through.
One recent hypothesis (2013 ff) in fact suggests that the earliest supermassive black holes may have formed much earlier than conventional theories about black hole formation - that they were formed from the remnants of the first stars after they exploded into supernovae about 400 million years after the universe was born - would suggest. Under this recent scenario, rather than being born in the deaths of massive stars, the seeds of the most ancient black holes might have collapsed directly from gas clouds much earlier in the universe’s history within a few hundred million years after the bang.
A DCBH (direct-collapse black hole) then merged with a nearby starry galaxy, gorging itself on the contents, and growing rapidly causing an obese black hole galaxy (OBG) to form. DCBHs are expected to form typically in regions adjacent to satellite galaxies that orbit around larger parent galaxies where Population III stars (those formed from the universe’s first stars) have already formed. The brightest quasars are the result of this process. The James Webb Space Telescope, anticipated to launch in 2019 with the capability to see farther back in time than any instrument before it, may be able to identify OBGs by their unique signatures in infrared light, providing evidence for the direct-collapse scenario.[5]
Save dwarf galaxy mergers such as those the subject of investigation by Anna Frebel, the Milky Way hasn’t had a major merger with another galaxy for about 10 billion years. Perhaps this is why it has a relatively small black hole at its centre. On the other hand, we now know that the Milky Way and Andromeda will collide and merge in about 3.75 billion years. During the interaction, the likelihood that any stars will collide is vanishingly small.
However, the gas clouds will collide and the galaxies will merge. Some stars will be thrown out of the galaxy in long tidal streams. This could well be the fate of our sun [6]. After several diminishing orbits, the remains of the two galaxies will coalesce into a giant elliptical galaxy, with all spiral structure destroyed. This has been called Milkomeda (an anagram for Milky Way and Andromeda). It may also have a dust lane running through it as a reminder of its merger past.
Supermassive black holes may inhibit star formation [7]
Scientists recently (2019) trained Hawaii’s Gemini North telescope on a galaxy known as 4C 31.04 far, far away from Earth which has at its centre a supermassive black hole 400 times the diameter and more than 100 million times the mass of our sun. The hole was spitting out jets of destructive super-heated plasma from its accretion disk. The jets were composed of tiny particles such as protons fuelled by the crushed up remnants of stars sucked into the black hole’s vortex and reacting violently to the extremely powerful magnetic fields thus generated.
The jets being spat out of galaxy 4C's black hole are so big and their magnetic fields so powerful, that they reach some 5,000 light years away through the galaxy, in the process, so it is thought, inhibiting star formation across thousands of light years. A distinct absence of stars is in evidence in the region conflicting with computer simulations as to what the case should be. This is thought to be because the gas clouds so essential to star formation have been shredded and blasted away, destroying the capability for new stars to be formed. The hunt is on the way to see if this phenomenon is replicated in other galaxies.
[0] Unless otherwise indicated, the substance of this section is drawn from Dr Helen Johnston’s Sydney University Centre for Continuing Education (CCE) course: Origins: From the Big Bang to Life, 16.3.11. Her permission to publish in this form is gratefully acknowledged.
[1] Caleb Scharf, “The benevolence of black holes – the Black Hole at the Heart of the Milky Way Galaxy”, Scientific American, Aug 2012, 22, 27.
[2] Ibid, 24.
[2.1] S Bradley Cenko and the late Neil Gehrels, How to swallow a sun", Scientific American, April 2017, 28-35 at 30-31.
[3] http://en.wikipedia.org/wiki/Quasar.
[4] The question of whether and how these seed black holes grew into the supermassive black holes, comprising millions to billions of suns, found in the cores of all big elliptical or bulged galaxies is the subject of an article in the Scientific American, January 2012, “Goldilocks Black Holes”, 28-35. The article concludes that the most likely explanation favours gas collapse over star mergers. The article explores the role played by intermediate black holes in this process.
[5] Priyamvada Natarajan, “The first monster Black holes”, Scientific American, February 2018, 18-23.
[6] If so, being presently half way through its cycle, this would occur before it sees out its remaining complement of a further approximate 4.6 billion years.
[7] Liam Mannix, “Absent stars blamed on plasma trail from distant galaxy's black heart”, SMH, 22 April, 2019: https://www.smh.com.au/national/absent-stars-blamed-on-plasma-trail-from-distant-galaxy-s-black-heart-20190422-p51g74.html
It now appears that every galaxy has a black hole at its centre. These black holes have the capability of exercising a benevolent effect on life in the galaxies they inhabit, but much will depend upon their activity level, balanced as it is between the gravitational agglomeration of matter and the disruptive energy blasting from matter-swallowing black holes. Too much black hole activity, and there would be little star formation, and the production of heavy elements would cease to occur. Too little black hole activity, and the galactic environments might be overly full of young an exploding stars, or too little stirred up to produce anything. Change the balance at all, and you change the whole pathway of star and galaxy formation[1].
Our own Milky Way occupies a galactic sweet spot, with a black hole, having a mass approximately 4 million times that of the sun, that appears to act out just often enough to stir things up “and keep the galaxy’s stellar population at a perfect simmer”[2]. We can only detect the fact that it exists by its influence on the orbits of stars in its vicinity. For the first picture of a black hole detected by the Event Horizon telescope (EHT) detected in April 1987 but only revealed in April 2019 after all the information gathered was finally processed, see the following page.
In some galaxies, the black holes are being fed by material falling in, and are characterised by material being ejected therefrom at nearly the speed of light. They are known as radio galaxies and quasars (quasi-stellar radio sources), depending on the direction in which they are pointing. Quasars were originally discovered by astronomer Maarten Schmidt in 1963. They are cosmic beacons, the ultraluminous centres of active galaxies that can be seen all the way to the outskirts of the visible universe, each shining brighter than billions of suns.
They are thought to occur when massive clouds of gas and dust plunge onto a supermassive black hole for hundreds of thousands or millions of years, compressing, heating and glowing as they circle the black hole’s maw or gullet. This notwithstanding, they constitute only a small portion of any supermassive black hole’s lifetime and are too rare, distant and sluggish to wholly reveal how such giant black holes feed. More nuanced detail can come from watching black holes snack on entire stars. [2.1]
Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended sources similar to galaxies[3]. Both are an explosive result of the overfeeding of the black holes at their centres.
The fact that we see quasars at high redshift means that massive black holes already existed and were growing less than a billion years after the big bang. So the black holes must have been formed very early on in the universe. Did they exist first and then galaxies grew around them? Or did the galaxy and the black hole both form together? We still don’t know. We do know, however, that almost every galaxy has a massive black hole at its heart, and that the bigger the galaxy, the bigger the black hole. This suggests that the growth of the galaxy and the black hole are somehow intimately linked.
We think that the link is that both galaxies and black holes grow through the merging of galaxies. Everywhere we look, we see signs of galaxies in the process of colliding or showing evidence of collisions in the not-too-distant past. What’s more, the further back we look the more common these collisions seem to be. This suggests that gas falls in to build the galaxy, and at the same time some gets funnelled in to the centre where it feeds and grows the black hole [4]. On this view, a quasar is just a particularly violent stage in the life of a galaxy, one that many, perhaps most, galaxies have gone through.
One recent hypothesis (2013 ff) in fact suggests that the earliest supermassive black holes may have formed much earlier than conventional theories about black hole formation - that they were formed from the remnants of the first stars after they exploded into supernovae about 400 million years after the universe was born - would suggest. Under this recent scenario, rather than being born in the deaths of massive stars, the seeds of the most ancient black holes might have collapsed directly from gas clouds much earlier in the universe’s history within a few hundred million years after the bang.
A DCBH (direct-collapse black hole) then merged with a nearby starry galaxy, gorging itself on the contents, and growing rapidly causing an obese black hole galaxy (OBG) to form. DCBHs are expected to form typically in regions adjacent to satellite galaxies that orbit around larger parent galaxies where Population III stars (those formed from the universe’s first stars) have already formed. The brightest quasars are the result of this process. The James Webb Space Telescope, anticipated to launch in 2019 with the capability to see farther back in time than any instrument before it, may be able to identify OBGs by their unique signatures in infrared light, providing evidence for the direct-collapse scenario.[5]
Save dwarf galaxy mergers such as those the subject of investigation by Anna Frebel, the Milky Way hasn’t had a major merger with another galaxy for about 10 billion years. Perhaps this is why it has a relatively small black hole at its centre. On the other hand, we now know that the Milky Way and Andromeda will collide and merge in about 3.75 billion years. During the interaction, the likelihood that any stars will collide is vanishingly small.
However, the gas clouds will collide and the galaxies will merge. Some stars will be thrown out of the galaxy in long tidal streams. This could well be the fate of our sun [6]. After several diminishing orbits, the remains of the two galaxies will coalesce into a giant elliptical galaxy, with all spiral structure destroyed. This has been called Milkomeda (an anagram for Milky Way and Andromeda). It may also have a dust lane running through it as a reminder of its merger past.
Supermassive black holes may inhibit star formation [7]
Scientists recently (2019) trained Hawaii’s Gemini North telescope on a galaxy known as 4C 31.04 far, far away from Earth which has at its centre a supermassive black hole 400 times the diameter and more than 100 million times the mass of our sun. The hole was spitting out jets of destructive super-heated plasma from its accretion disk. The jets were composed of tiny particles such as protons fuelled by the crushed up remnants of stars sucked into the black hole’s vortex and reacting violently to the extremely powerful magnetic fields thus generated.
The jets being spat out of galaxy 4C's black hole are so big and their magnetic fields so powerful, that they reach some 5,000 light years away through the galaxy, in the process, so it is thought, inhibiting star formation across thousands of light years. A distinct absence of stars is in evidence in the region conflicting with computer simulations as to what the case should be. This is thought to be because the gas clouds so essential to star formation have been shredded and blasted away, destroying the capability for new stars to be formed. The hunt is on the way to see if this phenomenon is replicated in other galaxies.
[0] Unless otherwise indicated, the substance of this section is drawn from Dr Helen Johnston’s Sydney University Centre for Continuing Education (CCE) course: Origins: From the Big Bang to Life, 16.3.11. Her permission to publish in this form is gratefully acknowledged.
[1] Caleb Scharf, “The benevolence of black holes – the Black Hole at the Heart of the Milky Way Galaxy”, Scientific American, Aug 2012, 22, 27.
[2] Ibid, 24.
[2.1] S Bradley Cenko and the late Neil Gehrels, How to swallow a sun", Scientific American, April 2017, 28-35 at 30-31.
[3] http://en.wikipedia.org/wiki/Quasar.
[4] The question of whether and how these seed black holes grew into the supermassive black holes, comprising millions to billions of suns, found in the cores of all big elliptical or bulged galaxies is the subject of an article in the Scientific American, January 2012, “Goldilocks Black Holes”, 28-35. The article concludes that the most likely explanation favours gas collapse over star mergers. The article explores the role played by intermediate black holes in this process.
[5] Priyamvada Natarajan, “The first monster Black holes”, Scientific American, February 2018, 18-23.
[6] If so, being presently half way through its cycle, this would occur before it sees out its remaining complement of a further approximate 4.6 billion years.
[7] Liam Mannix, “Absent stars blamed on plasma trail from distant galaxy's black heart”, SMH, 22 April, 2019: https://www.smh.com.au/national/absent-stars-blamed-on-plasma-trail-from-distant-galaxy-s-black-heart-20190422-p51g74.html