Our home galaxy, the Milky Way, is located in a small group of about 30 galaxies known as the Local Group, for lack of a better name. Groups of galaxies with more than hundreds or thousands of galaxies are known as clusters. The Virgo cluster of galaxies located 65 million years away from us has over 2,000 member galaxies.
Deep in the heart of the Virgo cluster of galaxies, an enormous black hole hides inside the largest galaxy in this cluster. This giant elliptical galaxy is named M87. It has a mass that is about 10 times larger than the Milky Way.
Images of the galaxy M87 show a long jet that is thousands of light years long, reaching out from the central region of the galaxy. This jet led astronomers to suspect that a supermassive black hole lies at the center of the galaxy.
In 1994, the then new Hubble Space Telescope was pointed at the nucleus of M87. At the core of the galaxy, the telescope was able to resolve a disk of gas, suggesting orbital motion around the galaxy center.
Hubble spectrograph was able to detect Doppler shifts and emission lines on either side of the disc at distances of 60 light years from the galaxy center. The red-shifted and blue-shifted spectral lines allow the measurement of the gases velocity, which was then used with Kepler's laws to find the mass of the center.
The resulting mass of 2.5 billion solar masses is large enough that the only sensible conclusion is that a supermassive black hole lies at the center of this galaxy.
허블 망원경으로 제트를 관측하고 분광 측정으로 강착원반의 적색 편이와 청색 편이를 측정하여 M87은하의 중심에 거대 블랙홀이 존재한다는 확신을 가졌다. 케플러 법칙에 따라 회전하는 (강착원반의) 가스구름을 계산해보니 질량이 태양의 25억배에 이르는 초거대 블랙홀이 중심에 있는 것으로 추정되었다.
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Back at this time, I was a student studying black holes, so it was really exciting to hear about this observational confirmation of the existence of supermassive black holes.
The gigantic mass of the black hole, and the relatively close distance on astronomical scales led the Event Horizon Telescope team of scientists to choose the black hole in M87 as their first target. In April 2017, they pointed radio telescopes located on multiple locations on the earth at M87.
The black hole at the center of the galaxy M87 is sometimes called M87 star. More recently, it has been renamed Powehi, which is a Hawaiian name, meaning the adorned, fathomless dark creation.
After two years of data analysis, they announced their results and images in April 2019. The orange donut image swept the Internet by storm. But what is this image actually showing us?
The Event Horizon Telescope observes radiowaves. So the orange color is a false color image that maps the brightest radio waves to yellow and dimmer radio waves to dark orange. The astronomers could have shown this picture in black and white, but the use of color does help bring out details to our eyes.
This set of images shows how the image of the black hole changes day by day. The general shape of a photon ring that is bright on one side and dim on the other, surrounding a black region known as the black hole shadow, stays constant. But you can see that the ring develops little bumps that move around a bit on timescales of days.
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The ring-shaped comes about as photons travel around the black hole on curved paths. A bright region on the far side of the black hole will emit light that travels around the black hole to the earth. Since the light can travel over, under, or beside the black hole, we see a ring of light surrounding a dark region.
The outer edge of the black hole shadow has radius, R_shadow is equal to the square root of 27 times GM over c squared, which is about 2.6 times the event horizon radius of a Schwarzschild black hole.
Measuring the size of the image allows an independent measurement of the black hole's mass.
위의 블랙홀 그림자의 반경은 블랙홀 질량 추정치와 별도로 관측 영상만으로 계산된 관측 추정치다.
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The Doppler-boost effect is responsible for the bright and dim region seen in this image. Regions of gas that are moving towards us are blue-shifted and appear brighter, while regions that are moving away from us are red-shifted and appear dimmer to us.
The astronomers computed thousands of model accretion disks, traced the light rays around the black hole, and produced images similar to this one.
Then the blurring effect of the gas between the black hole and the earth was added to the simulated image.
The simulated blurred image is then compared to the observed image in order to validate the physical interpretation of the image. This is a really remarkable observation and analysis.
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The next black hole that the Event Horizon Telescope collaboration plans to observe is Sagittarius A star, the supermassive black hole at the center of the Milky Way.
국제공조를 통한 EHT의 다음 관측 대상은 우리은하 중심의 초거대 블랙홀 궁수 A*다.
You might think that Sagittarius A star would be easier to observe than M87 star. The center of the Milky Way is 1,000 times closer to the earth than the galaxy M87, so surely the black hole shadow should look larger.
SGR A*는 M87보다 1000 배나 가깝지만,
However, the mass of Sgr A star is about 1,000 times smaller than M87 star's mass. This makes the event horizon and the size of the shadow smaller. The overall effect is that the shadow will appear to have a similar size.
크기는 1000분의 1에 불과하다.
The real problem is that Sagittarius A star is known to very rapidly on timescales shorter than a day. This will make the image jump around and blurry. The observation of our own black hole shadow will be even more difficult than the observation of M87 star's shadow.
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