- 3587 - MILKY WAY - black hole centers our galaxy? The first image of our Galaxy’s supermassive black hole, released May, 2022, has already begun to explain some enduring mysteries about the heart of our Milky Way. The wealth of new information about the black hole, called Sagittarius A*, joins many other lines of evidence that are now painting a detailed picture of the center of our galaxy.
--------------------- 3587 - MILKY WAY - black hole centers our galaxy?
- Sagittarius A* black hole is sucking in matter at a slow pace, making it unusually dim compared with the central black holes of other galaxies. Sagittarius A* could have been spectacularly active only a few million years ago.
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- The blackhole picture shows a glowing ring of radio emissions surrounding a dark shadow. This shadow lies just beyond the black hole’s event horizon which is the intangible sphere that marks a point of no return for anything that crosses it.
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- As matter spirals into the black hole at nearly the speed of light, it forms an ‘accretion disk’ that emits radiation across the electromagnetic spectrum, including radio waves that the telescopes can detect.
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- This data show that the accretion disk is shaped more like a puffed-up doughnut than a flat pancake. This fattened shape means that the disk supplies the black hole with scraps of matter at a leisurely pace, which makes it relatively dim compared with other, greedier black holes.
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- Although the shape of the accretion disk met expectations, many astrophysicists were surprised that the data showed the disk ‘face on’. This means its axis of rotation is angled at less than 50° degrees from our line of sight from Earth.
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- Scientists had expected that the disk’s axis of rotation would instead point vertically, showing the accretion disk ‘edge on’ from Earth’s point of view. This orientation would arise from the interplay of three separate rotations: the stately turn of the Galaxy’s spiral arms, the infalling matter supplying the accretion disk, and the rapidly spinning black hole itself.
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- Sagittarius A* probably formed from the merger of two black holes, when a pair of galaxies combined to formed the Milky Way. Initially, the spin of the new black hole could have pointed in any direction. But as it grew by feeding on dust and gas, the momentum of infalling matter would have slowly aligned the black hole’s spin with that of the Galaxy.
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- This study ruled out a vertical spin axis for the accretion disk, and perhaps also for the black hole itself. This matches observations made in 2018 by the Very Large Telescope (VLT), a facility on the mountain Cerro Paranal in Chile, which saw flares from matter orbiting very close to the black hole’s event horizon in a clockwise direction, just where the “Event Horizon Telescope“, EHT saw its ring.
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- The “GRAVITY instrument“ collects infrared light from the VLT’s four 8-metre dishes to achieve a resolution comparable to that of a single 130-meter-wide telescope. Like the EHT, GRAVITY found that the accretion disk has a face-on orientation, with its axis of rotation angled 20 to 30° degrees from our line of sight.
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- This face-on orientation is also consistent with decades of observations of the structure of the Milky Way’s central region. The black hole’s accretion disk is supplied by matter flowing from stars that orbit Sagittarius A* in a disk about one light year across. So the orientation of the accretion disk should match the disk of stars, rather than the larger-scale structure of the Galaxy.
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- Zooming out from the center of the Galaxy, astronomers have previously mapped several other larger structures up to a few parsecs across. These include a ‘mini-spiral’ made of streams of gas that are reminiscent of the Milky Way’s spiral arms, but 10,000 times smaller.
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- There does not seem to be much matter falling inwards from the spiral, but in the past it could have fed the black hole during periods of much more intense activity. This spiral does not align with the disk of stars around Sagittarius A*, nor with its accretion disk or with the Galaxy itself.
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- Models predict a gradual alignment of the black hole’s spin might apply only to galaxies that supply a steady stream of matter to the black hole over a long time. That doesn’t seem to be the case for the Milky Way, nor for many other galaxies that seem to contain misaligned central black holes.
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- The reason must be that the gas feeding the black hole is not directed in an orderly way, but comes in separate episodes whose directions are arranged completely randomly compared with the black-hole spin axis.
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- This kind of chaotic feeding could keep the black hole spinning at a fairly slow rate, which would allow it to accrete enough matter to grow rapidly. That could help to explain how some black holes grew so big, so quickly: some were already billions of times as massive as the Sun when the Universe was one-tenth of its current age.
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- Although all of these pieces of evidence seem to agree on the orientation of Sagittarius A*, there are still big questions about a possible connection between the black hole and other huge features seen around the Galaxy’s center.
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- In 2010, astronomers using NASA’s Fermi Gamma-ray Space Telescope mapped two enormous lobes of gas extending directly above and below the central region of the Galaxy, each 7,700 parsecs long.
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- These lobes glow in X-rays, and have become known as the “Fermi bubbles“. And in 2020, the “eROSITA X-ray telescope” detected even larger bubbles in the same region of space.
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- This suggests that these bubbles are the afterglow from shock waves that jutted out of the Galactic Center in the past 20 million years. A plausible source for such a shock wave could be a burst of star-forming activity, leading to a large number of stellar explosions, supernovae. Another major suspect is a period of intense feeding from Sagittarius A*.
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- Researchers have also found glowing columns of gas extending more than 150 parsecs from the Galactic Center, which might indicate that Sagittarius A* created the Fermi bubbles. Like a chimney that’s still hot from smoke and heat that just went through it, these chimneys could be a relic of the outflow that inflated the Fermi and eROSITA bubbles.
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- But the bubbles seem to be aligned vertically with the Milky Way’s axis, so it’s unclear how they could have originated from a black hole that is tilted in a different direction. One possibility is that the bubbles are the end result of many separate periods of intense feeding, each spewing out matter in a different direction.
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- A super-sensitive South African telescope has studied these two giant ‘radio bubbles’ above and below the central region of the Milky Way. The features stretch over a total of 430 parsecs (1,400 light years), about 5% of the distance between the Solar System and the Galaxy’s center.
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- The bubbles are gas structures that can be observed because electrons stirring inside them produce radio waves as they are accelerated by magnetic fields. This activity suggests that the bubbles are the remnants of an energetic eruption of hot gas several millions years ago.
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- One possible explanation is that the super-massive black hole at the center of the Galaxy underwent a period of intense matter-gobbling that created the outburst.
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- Another could be a ‘starburst’ event, the near-simultaneous formation and subsequent fiery death of around 100 large stars. The shock waves of their explosions could have combined to blow a hole through the thick interstellar matter of the Galaxy’s central region.
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- Both starburst and black-hole activity might have been at play, even reinforcing each other. And researchers know of a starburst that took place in the region around 7 million years ago.
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- The bubbles could also solve an old puzzle in radio astronomy. It’s possible that the electrons accelerating inside them are the source of bright ‘filaments’ of matter tens of parsecs long that stretch out of the Galactic center. Even larger bubbles, towering over those seen by MeerKAT’s, have been seen in the γ-ray part of the spectrum , and could have a similar origin.
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- The $330-million MeerKAT is an array of 64 radio dishes, each 13.5 meters across, at a remote site in Northern Cape province. It will form the core of the South African part of the SKA, due to be built in the 2020s. The observatory’s second section will be in Australia.
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May 26, 2022 MILKY WAY - black hole centers our galaxy? 3587
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