- 3702 - QUASARS - are blackholes? - In 1979 astronomers spotted two nearly identical quasars that seemed close to each other in the sky. These “Twin Quasars” are actually separate images of the same object. Even more intriguing the light paths that created each image traveled through different parts of the cluster.
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- One path took a little longer than the other. That meant a “flicker’ in one image of the quasar occurred 14 months later in the other. The reason? The cluster’s mass distribution formed a lens that distorted the light and drastically affected the two paths that the light traveled.
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- In 2022 astronomers spotted a similar effect with another distant quasar. They spent fourteen years measuring an even longer time delay between multiple images of this target quasar. The galaxy cluster “SDSS J1004+4112” plays a role in the delay. The combo of galaxies and dark matter in the cluster is really entangling the quasar light as it passes through. That’s causing the light to travel different trajectories through the gravitational lens.
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- The four images of the quasar actually correspond to a single quasar whose light is curved on its path towards us by the gravitational field of the galaxy cluster. Since the trajectory followed by the light rays to form each image is different, we observe them at different instants of time; in this case, we have to wait 6.73 years for the signal we observed in the first image to be reproduced in the fourth one.
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- The Sloan Digital Sky Survey first discovered cluster “SDSS J1004+4112“. Hubble Space Telescope imaged it in 2006. It was the first image of a single quasar with its light split into five images by lensing.
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- Galaxy clusters are massive and the largest gravitationally bound structures we know of in the universe. Some contain thousands of galaxies. The combined gravity of the galaxies, plus the intermingled dark matter in the cluster can entangle light from more distant objects as it passes through or near the cluster.
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- It has been possible to constrain the distribution of dark matter in the inner region of the cluster, since the lensing effect is sensitive not only to ordinary matter but also to dark matter. Calculating the time delay also allows other discoveries, including the distribution of stars and other objects in the area of space between galaxies in the cluster.
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- Nearly all large galaxies contain central black holes, and the galaxy in the middle of “Abell 2261” is expected to contain a particularly massive one. Scientists think this galaxy underwent a merger with another galaxy in the past, which could have caused a newly formed larger black hole to be ejected.
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- Despite searching with NASA's Chandra X-ray Observatory and Hubble Space Telescope, astronomers have no evidence that a distant black hole estimated to weigh between 3 billion and 100 billion times the mass of the Sun is anywhere to be found. This missing black hole should be in the enormous galaxy in the center of the galaxy cluster Abell 2261, located about 2.7 billion light years from Earth.
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- Nearly every large galaxy in the Universe contains a supermassive black hole in their center, with a mass that is millions or billions of times that of the Sun. Since the mass of a central black hole usually tracks with the mass of the galaxy itself, astronomers expect the galaxy in the center of Abell 2261 to contain a supermassive black hole that rivals the heft of some of the largest known black holes in the Universe.
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- Using Chandra data obtained in 1999 and 2004 astronomers had already searched the center of Abell 2261's large central galaxy for signs of a supermassive black hole. They looked for material that has been superheated as it fell towards the black hole and produced X-rays, but did not detect such a source.
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- Now, with new, longer Chandra observations obtained in 2018 a deeper search for the black hole in the center of the galaxy. Astronomers have also considered an alternative explanation, in which the black hole was ejected from the host galaxy's center. This violent event may have resulted from two galaxies merging to form the observed galaxy, accompanied by the central black hole in each galaxy merging to form one enormous black hole.
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- When black holes merge, they produce ripples in spacetime called “gravitational waves“. If the huge amount of gravitational waves generated by such an event were stronger in one direction than another, the theory predicts that the new, even more massive black hole would have been sent careening away from the center of the galaxy in the opposite direction. This is called a “recoiling black hole“.
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- Astronomers have not found definitive evidence for recoiling black holes and it is not known whether supermassive black holes even get close enough to each other to produce gravitational waves and merge. Astronomers have only verified the mergers of much smaller black holes.
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- There are two indirect signs that a merger between two massive black holes might have taken place. First, data from the Hubble and Subaru optical observations reveal a galactic core, the central region where the number of stars in the galaxy in a given patch of the galaxy is at or close to the maximum value, that is much larger than expected for a galaxy of its size.
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- The second sign is that the densest concentration of stars in the galaxy is over 2,000 light years away from the center of the galaxy. During a merger, the supermassive black hole in each galaxy sinks toward the center of the newly coalesced galaxy.
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- If they become bound to each other by gravity and their orbit begins to shrink, the black holes are expected to interact with surrounding stars and eject them from the center of the galaxy. This would explain Abell 2261's large core. The off-center concentration of stars may also have been caused by a violent event such as the merger of two super massive black holes and subsequent recoil of single, larger black hole that results.
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- Radio emission detected near the center of the galaxy showed evidence that supermassive black hole activity had occurred there 50 million years ago, but does not indicate that the center of the galaxy currently contains such a black hole.
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- Chandra was used to look for material that had been superheated and produced X-rays as it fell towards the black hole. While the Chandra data did reveal that the densest hot gas was not in the center of the galaxy, they did not reveal any possible X-ray signatures of a growing supermassive blackhole. No X-ray source was found in the center of the cluster, or in any of the clumps of stars, or at the site of the radio emission.
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- The mystery of this gigantic black hole's location therefore continues. Although the search was unsuccessful, hope remains for astronomers looking for this supermassive black hole in the future.
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- The James Webb Space Telescope may be able to reveal the presence of a supermassive black hole in the center of the galaxy or one of the clumps of stars. If Webb is unable to find the black hole, then the best explanation is that the black hole has recoiled well out of the center of the galaxy. The mystery continues:
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October 8, 2022 QUASARS - are blackholes? 3702
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