Supermassive black holes with masses larger than one million solar masses are found at the centers of most galaxies. But what effect do these black holes have on the galaxies themselves?
Do the properties of the host galaxy affected central black hole? These questions are important areas of an active study in the field of black hole feedback, a forefront area for black hole physics today.
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Let's look again at Sagittarius A*, the black hole at the center of our own galaxy, the milky way. Sagittarius star has a mass of four million times the mass of our sun. Although this seems impressively large, it is small when compared to the total mass of the entire milky way galaxy. Sag A* weighs four million times the mass of our sun, but the milky way weighs close to one trillion solar masses. Therefore, our galaxy is one million times heavier than the black hole at its center. Sag A* contributes only a tiny amount of the total mass of the galaxy.
Similarly, the size of a black hole, Sag A*, is also tiny compared to the size of the galaxy. The radius of Sag A* event horizon extends about 12 million kilometers which itself is almost 30 times larger than the distance between the Moon and the Earth, a reasonable size. However, the event horizon is much smaller than the distance between the Earth and the Sun which is around 150 million kilometers. Black holes accretion disk is larger but it's still less than a light year across. In contrast, the milky way galaxy is huge, with a diameter of more than 100,000 light years. To give you a sense of the astronomical size scale.
If Sag A*'s accretion disk will shrink down to the size of a penny, our galaxy would still be as big as the Earth. Supermassive black holes living in the centers of other galaxies also have relatively tiny masses and sizes compared to their host galaxies.
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For instance, the galaxy M87 has a supermassive black hole with a mass of three billion solar masses. The massive its host galaxy M87, is more than two trillion solar masses, which is more than a 1,000 times larger than the central black holes mass. This suggests that the overall impact of a supermassive black hole on its host galaxy should be quite small.
The funny thing is, this doesn't seem to be the case. In the 1990s, astronomers measured the masses of many supermassive black holes along with the mass of their host galaxies. They found a strong correlation between the black holes mass and the galaxy's mass. Essentially, they found that larger mass galaxies have larger mass black holes at their centers.
They suggests that the galaxies and their central black holes have a symbiotic relationship, meaning that as one grows, so does the other. This opened up an interesting new area of study that is still under investigation today. How can they impact each other so much?
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The exchange of mass and energy between galaxies and their central black holes is called feedback. The galaxy supplies some gas and dust that accretes onto the central black hole causing the black hole to slowly grow. As the gas accretes, thermal heating causes some of the gas to be ejected from a region near the black hole in the form of high speed jets. These jets can extend thousands of light years which heats up the gas and dust in the central parts of the galaxy. So, as long as the galaxy is feeding the black hole, the black hole throws some of the gas back out at high speeds that feeds back into the galaxy.
The outflowing gas can affect the galaxy in two ways:
Firstly, the outflowing gas can push the interstellar gas outwards clearing large regions of gas and dust. By clearing the areas of gas and dust, there is a lack of material required to form new stars. So, this can slow down the birth rate of stars in this region. This may happen in the brightest active galactic nuclei helping regulate star formation.
The second effect of outflowing gas can strangely have the opposite effect. If the black holes jets interact with large gas clouds, they can compress the clouds, triggering formation of new stars. When this happens the black hole actually helps galaxies form new stars.
So, the central black hole of galaxy can suppress or promote the formation of stars that could potentially have planets and maybe even life. In other words, black holes aren't just doom, and gloom, or death through spaghettification.
은하 중심부의 블랙홀은 대재앙이 될수도 있고 새로운 생명력을 불어넣을 수도 있다.
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We think of astronomy as a science where cosmic objects, like stars or galaxies, are studied by observing the light that they emit. But there's more to the universe than just light. The universe consists of uncountable elementary particles; like electrons, protons, and neutrons that make up all the elements such as; hydrogen, helium, and so on. These in turn makeup all the molecules and matter.
Amazingly here on the Earth we often detect particles that originate from cosmic sources, particles that come from distant parts of the universe. These particles are called cosmic rays and carry tremendous amounts of energy.
The highest energy cosmic ray ever measured affectionately named the OMG particle carried 48 joules of energy. To put that into context, that's about the same as a baseball pitched at a speed of 28 meters per second. But since all our energy was carried in atomic nucleus, it was traveling at only a whisper slower than the speed of light. When astronauts travel beyond the protection of Earth's atmosphere, they report strange flashes of light visible even when their eyes are closed. Although this phenomenon hasn't been studied in detail, scientists think that astronauts are observing flashes of light when a cosmic ray travels through their eyes.
These high-energy particles emit Cherenkov radiation as they pass through. This phenomena is known as the cosmic ray visual phenomena.
In order to understand the nature of cosmic rays which can range from a single electron to nuclei of heavy atoms, we must identify the energy source capable of accelerating them. Lower energy cosmic rays like the ones we detect from ozone are accelerated by the sun's rapidly changing energetic magnetic fields.
High-energy cosmic rays originate outside our solar system and a common occurrence. So are accelerated during supernova explosions taking place in our own galaxy, but where else can they originate. Most cosmic rays are charged particles like protons or occasionally a heavy atomic nucleus like an atom of iron. But we also know about even smaller fundamental particles, like the neutrino.
We've already seen that neutrinos which are uncharged particles with tiny masses can travel incredible distances without being stopped by interactions with other types of matter. This makes neutrinos almost impossible to detect since it's extremely rare for them to participate in particle interactions.
This is why neutrino observatories are some of the most specialized detectors on the planet. Neutrino detectors regularly see neutrinos created in the sun's core. In 1987 the Kamiokande chain detector in Japan detected neutrinos from a supernova explosion called SN 1987A. This detection led to the award of the Nobel Prize in physics to Masatoshi Koshiba, the leader of the Kamiokande experiment in 2002.
More recently, the Ice Cube detector located in Antarctica detected some very high energy neutrinos that are unlikely to be emitted by supernova. The source of the ice cubes highest energy neutrinos is still a mystery, but one possibility is that the neutrinos may have come from the jets of a supermassive black hole.
Occasionally, detectors such as the Pierre Auger cosmic ray observatory in Argentina detect ultra-high energy particles, like the OMG particle The term ultra-high energy refers to particles whose energies are millions of times more energetic than anything humans can create. What we mean by this is that the highest energy particles that humans have created are 40 million times less energetic than the highest ultra-high cosmic rays ever seen.
Ultra-high energy cosmic rays can't originate from supernova explosions. Since scientists have determined through simulations that a supernova can't accelerate particle to ultra-high energies. The origin of ultra-high energy cosmic rays is still a deep mystery in astrophysics.
We do know that the highest energy cosmic rays almost certainly originate from sources vast distances beyond our galaxy. Again, one plausible explanation is that the jets of supermassive black holes at the centers of distant galaxies are capable of accelerating particles to incredibly high speeds. If this is true, then our detectors here on the Earth are occasionally registering matter which escaped from the neighborhood of a black hole.
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[인터뷰][커세라 페이지]
What is the impact of a supermassive black hole on a galaxy?
Interview with Dr. Sarah Gallagher, Professor at Western University
When supermassive black holes are growing and they have the accretion disk, you have material that's swirling in and all that light basically can drive a wind, and the way that works is the actual individual photons. The individual light particles have momentum that carry a punch, and so when they encounter gas they can blow these really energetic winds. The way they might impact their host galaxy is that depending on how much mass is in the wind so the mass could easily be a few times the mass of our sun, which is comparable to the amount of gas that's falling into the black hole per year. So few times the mass of the Sun per year could be coming out as winds and a few times the mass of the Sun could be falling into the black hole causing the mass to grow. So, if you have that much gas that's being blown out from the central part of the black hole system in the center of a galaxy and its going out at tens of thousands of kilometers per second that gas has a lot of energy and what it can do is it can impact the other gas in the galaxy and it can do. We're not quite sure what it could do, but what it could do there's sort of two things that it could do. One is that you have gas that's coming out at very high velocity from the center of the galaxy near the black hole and it can basically plow into other gas in the galaxy. If it has enough energy it can actually plow it out of the entire galaxy, and what happens is if it blows all the gas out of the galaxy, it can completely shut off the star formation because that gas is the fuel of star formation. What it could also do if it's not going as fast as it could plow into the gas in the galaxy and it can compress it, and if it can compress it it might actually trigger star formation and cause star formation to occur. So, either of those two is our plausible explanations, but what we really need to do is we need to learn more about the properties of the actual winds. What are the velocities? What are the geometries? How much mass is in them? Once we have a better handle of that then we'll have a better idea of how it actually could ease impacting the galaxies that it lives in. But those are sort of the most extreme scenarios that could enhance star formation by causing the gas clouds to compress or it could just blow out all the gas and shut off star formation, and that's why the event of being a quasar which is something that all massive galaxies go through, it's kind of like being a teenager. It's just like this phase of life. It's kind of dramatic. It's potentially unpleasant and you're definitely different afterwards, but it's not clear what the change actually will be. So that's something that I think is really fascinating and one reason why if you really want to understand how galaxies evolve, you have to understand how their black holes grow because this quasar phase could be really fundamentally important to shaping them over time.
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