March 29, 2017

Galaxy Merger

The gravitationally bound system that binds all the stars visible is referred to as galaxy. It also consists of various other components such as stellar remnants, interstellar gas, dust, and dark matter. Galaxies are categorized according to their visual morphology as elliptical, spiral, and irregular. Many galaxies are thought to have black holes at their active centers, for example; The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. Our main aim at this passage is to understand why different galaxies could merge, interact together or form starburst galaxies; to this day, we people have come across the theory of their probability to collide with one another due to their large separation. This process is their basic way of growth, to become bigger, leading to massive expansion. When the two colliding galaxies both have central black holes, they will tend to sink toward the common center and eventually join together. This process even provides enough energy for the black holes to cause the active nucleus. The further process suggests that the gravitational field of the stars produced in the bulge influences how much mass can be collected in the center and assembled into the black hole. It has also been observed that jets and winds from the active nucleus can disrupt the star formation in the rest of the galaxy and that this happens in a fashion that helps regulate the mass in stars to stay proportional to that in the black hole. Taking a closer look at individual galaxies; The Magellanic Clouds are two small galaxies (LMC & SMC) within the Local Group and very close to the Milky Way (Galactic Plane). It is likely that the Milky Way is in the early stages of such an event with them, where it has been set to begin the process of absorption of the two local galaxies. A similar fate may await many other small members of the Local Group, merging either with the Andromeda Galaxy or with the Milky Way. Andromeda lies approximately 220,000 light-years across, and it is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and other smaller galaxies. Andromeda galaxy is a spiral galaxy 780 kiloparsecs away from Earth. Andromeda galaxy and Milky Way themselves are speeding towards each other at 300 miles per hour. This might even change the way our sky appears to be, replacing the way it already looks with an image of merging galaxies might feed possible data to our imagination at this point. When elliptical galaxies collide with each other, the smaller galaxy losing its energy as a whole tends to come closer and at reaching it's maximum, gets captured within and starts to spiral inwards. As a result, they often tend to have multiple nuclei. Various other results from their collision involve large amounts of star formation, high-density gas production that provides a lot of fuel for star formation, nucleus absorbs energy hence absorbing young stars which makes it brighter at the infrared region, Because of the dust, the star-forming region is hidden in the optical, etc. All of these results comprise to be known as “starburst galaxies.” Examples; Stephan's Quintet (visual grouping of five galaxies of which four forms the first compact galaxy group ever discovered) is one of the famous examples of interacting galaxies, theories of merging of disc galaxies (galaxy characterized by a disc, a flattened circular volume of stars), etc. On a recent note, NuSTAR (NASA's Nuclear Spectroscopic Telescope Array; NuSTAR is a Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington, NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI)) mission, observed theories contrary to our understanding about merging galaxies till date. It gave information over merging of large galaxy 49b and dwarf galaxy 49a, where 49a consists of the supermassive black hole that is way powerful than ever imagined. Normally, when two galaxies start to merge, the larger galaxy's central black hole becomes active, voraciously gobbling gas and dust, and spewing out high-energy X-rays as matter gets converted into energy. That is because, as galaxies approach each other, their gravitational interactions create a torque that funnels gas into the larger galaxy's central black hole. But in this case, the smaller galaxy hosts a more energetic and luminous X-rays with a highly active supermassive black hole, and the larger galaxy's central black hole remained relatively quiet. This study should be considered important along with the other theories stated above because it may give new insight into how supermassive black holes form and grow in such huge systems that collide with one another. By examining these formed galaxies, we may also find clues as to how our own galaxy’s supermassive black hole is formed. In several hundred million years, yet another hypothesis states that the black holes of the large and small galaxies will merge into one enormous beast, leaving more room for further discovery.

* - NGC 7714 is a spiral galaxy 100 million light-years from Earth — a relatively close neighbor in cosmic terms. The universe has witnessed some violent and dramatic events in its recent past. Tell-tale signs of this brutality can be seen in NGC 7714's strangely shaped arms, and in the smoky golden haze that stretches out from the galactic center — caused by an ongoing merger with its smaller galactic companion NGC 7715, which is out of the frame of this image. Credits: ESA, NASA

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