- 3723 - QUASAR - where did they come from? Quasars are extremely bright and extremely distant objects. Their huge energy output is thought to be due to activity around the central supermassive black hole in young galaxies, near the edge of the observable universe.
--------------------- 3723 - QUASAR - where did they come from?
- 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. The light paths that created each image traveled through different parts of the cluster. One path took a little longer than the other.
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- That meant a flicker in one image of the quasar occurred 14 months later in the other. The cluster’s mass distribution formed a lens that distorted the light and drastically affected the two paths.
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- In 2022 astronomers had spent fourteen years measuring an even longer time delay between multiple images of their 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. The result is the same strange time-delayed effect.
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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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- Gravitational lensing creates an optical effect as light passes through a region of space with a strong gravitational influence.
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- Galaxy clusters are astonishingly 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. The mass in the cluster is spread out unevenly. That affects each path of light through the cluster.
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- Measuring these time delays helps to better understand the properties of galaxies and clusters of galaxies, their mass, and its distribution. This data helps us understand other characteristics of the lensing cluster. 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.
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- Calculating the time delay allows other discoveries, including the distribution of stars and other objects in the area of space between galaxies in the cluster. It will help astronomers to calculate the size of the distant quasar’s accretion disk.
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- These observations occurred over 14.5 years at the 1.2-meter telescope located at the Fred Lawrence Whipple Observatory.
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- The word quasar stands for “quasi-stellar radio source“. Quasars got that name because they looked starlike when astronomers first began to notice them in the late 1950s and early 60s. But quasars aren’t stars.
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- Scientists now know they are young galaxies, located at vast distances from us, with their numbers increasing towards the edge of the visible universe. Quasars are extremely bright, up to 1,000 times brighter than our Milky Way galaxy. They are highly active, emitting staggering amounts of radiation across the entire electromagnetic spectrum.
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- Because they’re far away, we’re seeing these objects as they were when our universe was young. The oldest quasar is approximately 13.03 billion light-years away, and therefore we see it as it was just 670,000,000 years after the Big Bang.
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- Quasars are the extremely luminous centers of galaxies in their infancy. A quasar is a type of “active galactic nucleus“. The intense radiation released by an AGN powers a supermassive black hole at its center. The radiation comes from material in the accretion disk surrounding the black hole when it is superheated to millions of degrees by the intense friction generated by the particles of dust, gas and other matter in the disk colliding countless times with each other.
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- The inward spiral of matter in a supermassive black hole’s accretion disk at the center of a quasar is the result of particles colliding and bouncing against each other and losing momentum. That material came from the enormous clouds of gas, mainly consisting of molecular hydrogen, which filled the universe in the era shortly after the Big Bang. In the early universe, quasars had a vast supply of matter to feed on.
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- As matter in a quasar’s black hole accretion disk heats up, it generates radio waves, X-rays, ultraviolet and visible light. The quasar becomes so bright that it’s able to outshine entire galaxies. They are so far from us that we only observe the active nucleus, or core, of the galaxy in which they reside. We see nothing of the galaxy apart from its bright center. It’s like seeing a distant car headlight at night: you have no idea of which type of car you are looking at, as everything apart from the headlight is in darkness.
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- “Seyfert galaxies” are not classed as quasars but that still have bright, active centers where we can see the rest of the galaxy. Seyfert galaxies make up perhaps 10% of all the galaxies in the universe. They are not classed as quasars because they are much younger and have well-defined structures. Quasar-containing galaxies are young and formless.
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- Consider the amounts of energy required to illuminate an object sufficiently to make it visible in radio waves from the farthest reaches of the universe. Quasars can emit up to a thousand times the energy of the combined luminosity of the 200 billion or so stars in our own Milky Way galaxy.
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- A typical quasar is 27 trillion times brighter than our sun! Most large galaxies went through a so-called “quasar phase” in their youth, soon after their formation. They subsided in brightness when they ran out of matter to feed the accretion disk surrounding their supermassive black holes.
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- After this epoch, galaxies settled into “quiescence“, their central black holes starved of material to feed on. The black hole at the center of our own galaxy has been seen to flare up briefly, however, as passing material strays into it, releasing radio waves and X-rays. It’s conceivable that a black hole can tear apart entire stars and consume them as they cross its event horizon, the point of no return.
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- We know that 3.5 million years ago there was a gigantic explosion known as a Seyfert flare at the center of our galaxy. It was apparently centered on Sagittarius A*, the Milky Way’s supermassive black hole, producing two huge lobes of superheated plasma extending some 25,000 light years from the north and south galactic poles. These are huge lobes Fermi bubbles and they are visible today at gamma and X-ray wavelengths.
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- Huge lobes extend above and below the plane of our Milky Way galaxy. They shine in gamma rays and X-rays and thus are invisible to the human eye. A quasar’s light passed through one of these bubbles. Imprinted on that light is information about the outflow’s speed, composition, and eventually mass.
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- Quasars were first discoveries in the late 1950s from astronomers using radio telescopes. They saw starlike objects that radiated radio waves ( “quasi-stellar radio objects“), but which were not visible in optical telescopes. Their resemblance to stars, their brightness and small angular diameters understandably led astronomers of the time to assume they were looking at objects within our own galaxy.
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- Many early observations of quasars, including those of “3C48” and “3C273“, the first two quasars to be discovered, took place in the early 1960s by British-Australian astronomer John Bolton. He and his colleagues found it puzzling that quasars were not visible in optical telescopes. They wanted to find quasars’ so-called “optical counterparts,” that is, a quasar which would be visible to their eyes in a telescope rather than only being detectable with radio instruments.
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- Astronomers simply didn’t know at that time that quasars were extremely distant, too distant for their optical counterparts to be visible from Earth at that time, despite being intrinsically brilliant objects. In 1963, astronomers Allan Sandage and Thomas A. Matthews found what they were looking for, what appeared to be a faint, blue star at the location of a known quasar.
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- Then, using the 200-inch Hale telescope, Bolton and his team observed quasar 3C273 as it passed behind the moon. These observations also let them obtain spectra. And again the spectra looked strange, showing unrecognizable emission lines. These lines tell astronomers which chemical elements are present in the object they are examining. But the quasar’s spectral lines were nonsensical, seeming to indicate elements which should not be present.
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- The hydrogen emission lines fall farther to the right, toward longer wavelengths, compared to where hydrogen emission lines would normally be located on the spectrum. They are “redshifted“, indicating that the quasar is located at an extreme distance from us.
Astronomer Maarten Schmidt, after examining the strange emission lines in the spectra of quasars, suggested that astronomers were seeing normal emission lines that were highly shifted towards the red end of the electromagnetic spectrum!
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- The redshift was due to the quasar’s great distance. Light being stretched by the expansion of the universe during its long journey to us from the edge of the visible cosmos appears much redder.
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- But if it were really true that quasars were as far away as towards the edge of the visible universe, how could they have generated such enormous quantities of energy? In 1964, even the existence of black holes caused hot debate. Many scientists considered them nothing more than mathematical freaks, because surely they could not exist in the real universe.
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- The debate about the nature of quasars raged on until the 1970s when a new generation of Earth- and space-based telescopes established beyond reasonable doubt that quasars do indeed lie at vast distances, that we are seeing galaxies when they were young, that the quasar stage is a natural phase of their growth.
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- With black holes finally being taken seriously too, astronomers could now finally model the identity of the almost incomprehensible powerhouse behind quasars: supermassive black holes consuming stupendous amounts of gas and radiating vast amounts of energy across the spectrum as a result.
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- This is why quasars sit towards the edge of the visible universe and why we don’t see them closer: because quasars are young galaxies, seen not long after their formation in the early universe.
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- Quasars are extremely bright and extremely distant objects. Their huge energy output is thought to be due to activity around the central supermassive black hole in young galaxies, near the edge of the observable universe.
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October 29, 2022 QUASAR - where did they come from? 3722
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