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The more distant a galaxy is in Space, the more ancient it is in Time. For this reason, extremely remote, ancient galaxies are usually too faint to be observed, even by astronomers using the largest and most powerful telescopes. But Nature has provided astronomers with a gift–gravitational lensing. Gravitational lenses can bend, warp, and distort streaming light in such a way that a remote object can be magnified by the gravity of a foreground object (the lens), thus making the background lensed galaxy easier for astronomers to see. In January 2018, an international team of astronomers led by Dr. Harald Ebeling from the University of Hawaii (Manoa), announced their important discovery of one of the most extreme examples of magnification of a remote object by a gravitational lens. Using the Hubble Space Telescope (HST) to survey a sample of vast clusters of galaxies, the team found a distant galaxy, dubbed eMACsJ1341-OG-1, that is magnified 30 times thanks to the distortion of Spacetime created by a foreground massive galaxy cluster that warps its traveling streams of light.
The term gravitational lensing itself refers to the path that traveling light has taken when it has been deflected. This occurs when the mass of an object situated in the foreground warps the light streaming out from a more remote object situated in the background. The light does not have to be visible–it can be any form of electromagnetic radiation. As a result of gravitational lensing, light beams that would normally not have been observable are bent in such a way that their paths wander towards the observer. Conversely, light can also be bent in such a way that its beams can wander away from the observer.
There are different types of gravitational lenses: strong lenses, weak lenses, and microlenses. The differences that exist between the three forms of gravitational lenses have to do with the position of the background object that is emitting the light, the foreground lens that is bending the light, and the position of the observer–as well as the shape and mass of the foreground lens. The foreground object determines how much of the background object’s light will be warped, as well as where this light will wander on its path through Spacetime.
The Cosmos that we observe today sparkles with the fabulous flames of billions and billions of stars that populate the more than 100 billion galaxies inhabiting the relatively small part of the Universe that we can observe. We cannot observe whatever may exist beyond the cosmological horizon–or edge–of the observable Universe because light streaming out from luminous objects, inhabiting those unimaginably distant regions, has not had enough time to reach us since the Big Bang. This is because of the expansion of the Universe. The speed of light–the universal speed limit–has made it impossible for us to observe what may exist beyond the cosmological horizon of our visibility. When we look deep into space, we look back in Time. This is because the more remote a shining object is in Space, the longer it has taken for its light to reach us.
No known signal in the Universe can travel faster than light in a vacuum, and the light flowing out from remote celestial objects cannot travel to us faster than this universal speed limit will permit. It is impossible to locate an object in Space without also locating it in Time. Hence, the term Spacetime. Time is the fourth dimension. The three spatial dimensions that characterize our familiar world are back-and-forth, side-to-side, and up-and-down.
Gravitational lensing was predicted by Albert Einstein in his Theory of General Relativity (1915) and has since been observed many times by astronomers. Einstein’s first theory of Relativity, the Special Theory of Relativity (1905), describes a Spacetime that is frequently likened to an artist’s canvas. The artist paints points and lines on this canvas which represents the stage where the universal drama is being played–and not the drama itself. The great achievement uniting the stage with the drama came a decade later with the Theory of General Relativity–where Space becomes a star in the universal drama itself. Space tells mass how to move, and mass tells space how to curve. Spacetime is as elastic as a child’s backyard trampoline. Imagine a little girl tossing a heavy bowling ball onto the trampoline. The ball represents a heavy mass, like that of a star. It creates a dimple–or a “gravitational well”–in the stretchy elastic fabric of the trampoline. Now, if the little girl then throws a handful of marbles onto the trampoline, they will wander down curved paths around the “star”–as if they were planets in orbit around a real star. If the bowling ball is then removed, the marbles will make straight paths, instead of curved ones. The marbles–or “planets”–travel according to the more massive “star’s” warpage of the flexible fabric of the trampoline, which represents Spacetime. The stage and the drama are united. The drama will continue until the show’s final curtain.
The first gravitational lens was discovered in 1979, and today lensing gives astronomers a remarkable view of the extremely dim Universe soon after its mysterious birth. Gravitational lensing was first validated during a solar eclipse in 1919 when background stars were seen to be offset in exactly the way that Albert Einstein had predicted. Astronomers now use these celestial magnifying glasses to learn about distant objects that would otherwise be so faint that they would almost be invisible. Indeed, remote and ancient galaxies may well reveal to astronomers a treasure trove of information about our own Milky Way Galaxy. These distortions of Spacetime caused by massive objects closer to Earth can be used by astronomers to study nearby stars and their retinues of planets.
Extreme Celestial Magnifying Glass Detects Dim Galaxies In The Primeval Universe
Gravitational lensing can dramatically magnify remote celestial sources in the primeval Universe, just so long as there is a sufficiently massive foreground object situated between the background source and the prying eyes of curious astronomers.
Clusters of galaxies are vast concentrations of dark matter and searing-hot gas surrounding hundreds–or even thousands–of individual galaxies. All of the constituent galaxies belonging to a cluster are linked together by their mutual gravitational attraction. These clusters are of great value to astronomers because they serve as powerful gravitational lenses. By functioning as magnifying glasses for faint objects that would otherwise be hidden behind them, massive galaxy clusters can serve as natural telescopes that enable astronomers to study objects long ago and far away in Spacetime–sources that would otherwise be beyond the reach of telescopes.
Dark matter is a ghostly and invisible form of matter that is thought to be composed of exotic non-atomic particles that do not interact with light or any other form of electromagnetic radiation–which is why it is transparent. The weird dark matter is much more abundant than the misnamed “ordinary” atomic matter that makes up our familiar world–the world that we can see. The so-called “ordinary” atomic matter is the stuff of stars, planets, moons, and people, and it accounts for literally all of the elements listed in the Periodic Table.
There is a picture depicting the quiescent galaxy eMACSJ1341, as captured by the HST. The picture shows a yellow dotted line that traces the boundaries of the galaxy’s gravitationally lensed image. An inset on the upper left of the image shows what eMACSJ1341 would look like if it were observed directly, without the invaluable aid of the foreground cluster lens. A very dramatic amplification and distortion caused by the intervening massive galaxy cluster can readily be seen.
Quiescent galaxies are those in which star-birth has all but ceased entirely. Therefore, quiescent galaxies represent the final phase of galaxy evolution. This is what makes eMACSJ1341 intriguingly unusual. Galaxies that are as ancient and remote as eMACSJ1341 is are usually youthful enough not to have depleted their supply of star-birthing gas. For this reason, learning about why eMACSJ1341 has stopped producing new baby stars has become a significant scientific quest.
Dr. Ebeling and his colleagues, working with the data obtained from HST, are continuing their research using both the HST data and ground-based instruments. The astronomers are conducting further analysis of the lens model, removing distortions from the magnified image.
“The very high magnification of the image provides us with a rare opportunity to investigate the stellar populations of the distant object and, ultimately, to reconstruct the undistorted shape and properties,” commented study team member, Dr. Johan Richard, in a January 31, 2018, University of Hawaii Press Release. Dr. Richard, who performed the lensing calculations, is of the University of Lyon in France.
Even though similarly extreme gravitational magnifications have been observed previously, this discovery sets a new record for the magnification of a rare quiescent galaxy. Dr. Ebeling explained on January 31, 2018, University of Hawaii Press Release that “We specialize in finding extremely massive clusters that act as natural telescopes and have already discovered many exciting cases of gravitational lensing. This discovery stands out though, as the huge magnification provided by eMACJ1341 allows us to study in detail a very rare type of galaxy.”
Representing the final phase of galaxy evolution, quiescent galaxies are an abundant population in the local Universe. “However, as we look at more distant galaxies, we are also looking back in time, so we are seeing objects that are young and should not yet have used up their gas supply. Understanding why this galaxy has already stopped forming stars may give us critical clues about the processes that govern how galaxies evolve,” explained Dr. Mikkel Stockmann, a study team member from the University of Copenhagen, and an expert in galaxy evolution.
Additional follow-up observations of eMACJ1341 are currently in progress by astronomers using telescopes in Chile and Mauna Kea in Hawaii. Details of the study are published in The Astrophysical Journal Letters.
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