What is the structural relationship between the Super Massive Black Hole at Sgr A* and the rest of the Galaxy? Why is it there? Where did it come from?
It seems like such a obvious question, and very often science articles and scientists themselves get caught up in the details and virtually ignore the larger questions like WHY we see the current structure that we do, as if that was an irrelevant philosophical question, when in fact it's an analytical one. I read an interesting explanation on Quora that fits something I have been wondering about the origin of Globular Clusters for a while now. There is more and more evidence that the milky way galaxy is made up of cannibalized smaller galaxies, and was not formed as we see it now directly from the Big Bang material. And this begs the question, where did the SMBH at the center of the galaxy come from? How do active galactic neuclei shape the galaxy when they contain such a small fraction of the total mass? For one possible explanation read on...
This explains why Sgr A* is not quite at the center, and also what the Intermediate Mass Black Hole "The Great Annihilator" may have come from. hint: it is the core remnant of another smaller mass galaxy that was absorbed by the Milky Way and may one day merge with Sgr A*.
Anyway, it's just something cool to ponder while on my way to the center of the Galaxy, and doing my first large scale galactic survey. Stuff like this really helps to put what we see in the game in a larger perspective, and helps to keep me in a sense of awe and wonder while in the Black.
It seems like such a obvious question, and very often science articles and scientists themselves get caught up in the details and virtually ignore the larger questions like WHY we see the current structure that we do, as if that was an irrelevant philosophical question, when in fact it's an analytical one. I read an interesting explanation on Quora that fits something I have been wondering about the origin of Globular Clusters for a while now. There is more and more evidence that the milky way galaxy is made up of cannibalized smaller galaxies, and was not formed as we see it now directly from the Big Bang material. And this begs the question, where did the SMBH at the center of the galaxy come from? How do active galactic neuclei shape the galaxy when they contain such a small fraction of the total mass? For one possible explanation read on...
FROM QUORA: Abhijeet Borkar, PhD student in Physics (Astrophysics)Why would there be supermassive black holes (SMBHs) at the centers of galaxies? Why are they there? What is their relation with their host galaxy? How does their presence affect the host galaxy?
These were the questions that were asked when different types of active galactic nuclei (AGN) were discovered. It was discovered that each galaxy that has a bulge in the center, has a SMBH sitting there. It was also discovered that most of the galaxies host SMBH. Some weird relationships were also discovered between the host galaxy and the central SMBH. For example, the [FONT=MathJax_Math-italic]M[FONT=MathJax_Math-italic]B[FONT=MathJax_Math-italic]H[/FONT][FONT=MathJax_Main]−[/FONT][FONT=MathJax_Math-italic]σ[/FONT][/FONT]relation, the black-hole mass - velocity dispersion relation. These relations are difficult to grasp. Why does a supermassive black hole, which is a teeny-tiny object, the size of a solar system or less, with mass just 0.1% that of the galaxy affect something on the scale of several thousand light years? This is not at all like the solar system. In the solar system, 99% of the mass is in the Sun so it controls the system. But in case of the galaxies, the mass of the SMBH is less then 1% of the bulge mass.
The answer was found to be in the formation of the galaxies. A model, called the hierarchical galaxy formation model, was proposed to explain these relations. The model assumes that the galaxies we see today are formed by merging of the small galaxies. Two smaller gas-rich galaxies come together, colliding with each other, which drives the gas in the interstellar medium towards the center, helping the black holes in the center grow, which in turn gives out massive bursts of energy that affect the whole galaxy. Eventually these galaxies will merge together, and their central black holes will merge into one giant BH which will sit at the center of the new combined galaxy. Repeat this procedure for a few times and you have a large galaxy with a few million solar mass BH sitting in the middle.
But wait, you might wonder, where did the BHs come to be there in those smaller galaxies in the first place?
These come from the smaller, the so called intermediate-mass black holes (IMBH) which sit in the middle of (some of) the dwarf galaxies and globular clusters.
[*]
But where do these IMBH come from?
They, most likely, come from the remnants of the first stars, which were very massive, of the mass of [FONT=MathJax_Main]100[/FONT][FONT=MathJax_Math-italic]M[/FONT][FONT=MathJax_Main]⊙[/FONT], which collapsed into massive black holes, which merge together to form IMBH. They could also have formed from direct collapse of massive nebulae in the very early Universe, forming BHs of the mass[FONT=MathJax_Main]∼[/FONT][FONT=MathJax_Main]10[/FONT][FONT=MathJax_Main]3[/FONT][FONT=MathJax_Main]−[/FONT][FONT=MathJax_Main]10[/FONT][FONT=MathJax_Main]4[/FONT][FONT=MathJax_Math-italic]M[/FONT][FONT=MathJax_Main]⊙[/FONT].
But why are they always in the center?
Because friction.
Due to the gravitational interaction and "collisions" [**], massive objects always end up at the center of the system. This is just the result of kinematics.
So there you go, now you know why there are supermassive black holes at the centers of galaxies.
Just to be clear - The existence of SMBH at the centers of galaxies is not a necessary condition, i.e., there is no reason SMBH "should" exist in the centers of all galaxies, but it's just the fact of the matter that majority (but not all) galaxies host a SMBH at their centers.
[*] There are some observational evidences for the IMBHs, but these are not as ubiquitous as the SMBH or stellar mass BHs. There are several reasons behind this, but this isn't the place to talk about them.
[**] yes, the word is in quotes because the stars and BHs don't actually collide head-on, but pass close to each other causing change in speed and direction of their motion[/FONT]
These were the questions that were asked when different types of active galactic nuclei (AGN) were discovered. It was discovered that each galaxy that has a bulge in the center, has a SMBH sitting there. It was also discovered that most of the galaxies host SMBH. Some weird relationships were also discovered between the host galaxy and the central SMBH. For example, the [FONT=MathJax_Math-italic]M[FONT=MathJax_Math-italic]B[FONT=MathJax_Math-italic]H[/FONT][FONT=MathJax_Main]−[/FONT][FONT=MathJax_Math-italic]σ[/FONT][/FONT]relation, the black-hole mass - velocity dispersion relation. These relations are difficult to grasp. Why does a supermassive black hole, which is a teeny-tiny object, the size of a solar system or less, with mass just 0.1% that of the galaxy affect something on the scale of several thousand light years? This is not at all like the solar system. In the solar system, 99% of the mass is in the Sun so it controls the system. But in case of the galaxies, the mass of the SMBH is less then 1% of the bulge mass.
The answer was found to be in the formation of the galaxies. A model, called the hierarchical galaxy formation model, was proposed to explain these relations. The model assumes that the galaxies we see today are formed by merging of the small galaxies. Two smaller gas-rich galaxies come together, colliding with each other, which drives the gas in the interstellar medium towards the center, helping the black holes in the center grow, which in turn gives out massive bursts of energy that affect the whole galaxy. Eventually these galaxies will merge together, and their central black holes will merge into one giant BH which will sit at the center of the new combined galaxy. Repeat this procedure for a few times and you have a large galaxy with a few million solar mass BH sitting in the middle.
But wait, you might wonder, where did the BHs come to be there in those smaller galaxies in the first place?
These come from the smaller, the so called intermediate-mass black holes (IMBH) which sit in the middle of (some of) the dwarf galaxies and globular clusters.
[*]
But where do these IMBH come from?
They, most likely, come from the remnants of the first stars, which were very massive, of the mass of [FONT=MathJax_Main]100[/FONT][FONT=MathJax_Math-italic]M[/FONT][FONT=MathJax_Main]⊙[/FONT], which collapsed into massive black holes, which merge together to form IMBH. They could also have formed from direct collapse of massive nebulae in the very early Universe, forming BHs of the mass[FONT=MathJax_Main]∼[/FONT][FONT=MathJax_Main]10[/FONT][FONT=MathJax_Main]3[/FONT][FONT=MathJax_Main]−[/FONT][FONT=MathJax_Main]10[/FONT][FONT=MathJax_Main]4[/FONT][FONT=MathJax_Math-italic]M[/FONT][FONT=MathJax_Main]⊙[/FONT].
But why are they always in the center?
Because friction.
Due to the gravitational interaction and "collisions" [**], massive objects always end up at the center of the system. This is just the result of kinematics.
So there you go, now you know why there are supermassive black holes at the centers of galaxies.
Just to be clear - The existence of SMBH at the centers of galaxies is not a necessary condition, i.e., there is no reason SMBH "should" exist in the centers of all galaxies, but it's just the fact of the matter that majority (but not all) galaxies host a SMBH at their centers.
[*] There are some observational evidences for the IMBHs, but these are not as ubiquitous as the SMBH or stellar mass BHs. There are several reasons behind this, but this isn't the place to talk about them.
[**] yes, the word is in quotes because the stars and BHs don't actually collide head-on, but pass close to each other causing change in speed and direction of their motion[/FONT]
This explains why Sgr A* is not quite at the center, and also what the Intermediate Mass Black Hole "The Great Annihilator" may have come from. hint: it is the core remnant of another smaller mass galaxy that was absorbed by the Milky Way and may one day merge with Sgr A*.
Anyway, it's just something cool to ponder while on my way to the center of the Galaxy, and doing my first large scale galactic survey. Stuff like this really helps to put what we see in the game in a larger perspective, and helps to keep me in a sense of awe and wonder while in the Black.
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