Ever since the understanding of the phenomenon of the Big Bang, scientists have put I all their energies to understand the functioning of all the objects that make our universe. While most of these objects can be seen and observed, Black Holes make for a mystery much difficult to discern. Nevertheless, after years of hard work and energy, scientists and astronomers have managed to establish certain facets of black holes. One of the most important discovery in this respect has been the relationship between Black Holes and the Galaxies. It has now become an established fact that most galaxies, if not all, have a Black Hole at their centre and certain attributes and features of the galaxies depend on these black holes that reside in the centre and form the major mass of the central galactic bulk.
Keywords: Black Holes, Galaxies
The field of astronomy is one mystery that brings something new to light every day. While most objects in the universe can be checked, researched and studied through the telescopes (special thanks to Hubble), Black Holes happen to be the most difficult to observe. For those uninitiated, black holes are massive objects, which have immense densities and gravitational pull, such that even light cannot escape these. The only way, therefore, to study black holes is by studying their surrounding objects and events. Now there are two types of black holes: Stellar Black Holes, which have masses upto ten times that of our sun; and Massive Black Holes (MBH), which, as the name suggests, have masses upto billions of time that of our sun. For past several years, scientists and astronomers were working towards establishing the relationship between a black hole and a galaxy. It has been found that in most huge galaxies, much like our Milky Way, black holes exist in the centre of these galaxies. By observing the movements of gas clouds and stars, the presence of black holes within the centre of most of the galaxies, if not all, has been established (Macchetto, n.d, n.pag.).
The astronomers took 15 years and use of special imaging techniques such as Adaptive optics to establish that a black hole exists in the centre of our galaxy, the Milky Way, as well. By studying the paths of the stars at the centre of the galaxy, they were able to establish that there a gravitational pull of huge intensity that is keeping these stars in place. This could only happen in the presence of a super massive black hole, which was 3 million times larger than the sun. Further, there were regular flashes of infrared rays at the centre, further establishing the presence of a black hole (ESO, n.d., n.pag.).
The relationship between black holes and galaxies go even further than simple cohabiting. It has now been established that the colour of a galaxy and its size depends on the size of the black hole at its centre (Royal Astronomical Society, 2014, n.pag.).
In the figure above, the galaxies have been scaled on two parameters: the mass of the stars and the bulge to stellar mass ratio. It has been noticed that as the mass of the stars increased along with the ratio, the galaxies became redder. The central bulge is the black hole. Therefore, as the bulge or the size of the black hole increased, the galaxies got redder in colour.
Further, it was found that no new stars are formed above the central bulge of the galaxies. To understand the reason for this, let us understand the way a black hole works. Black hole is the source of immense energy release in the form of jets and X-rays. These rays heat up the surrounding gases to such extent that these gas clouds blow up. This, therefore, stops from any new stars to be formed.
Another relation between black holes and galaxy was on the speed of the stars orbiting. It was found that where there was a bigger and stronger black hole, the orbiting speed of the stars in the outer reaches of the galaxy (where the gravitational pull of the black hole was lesser) was faster (A Singular Place, n.d, n.pag.).
Just the way there are two types of black holes, there are also two types of galaxies depending on their shape. It has been found that galaxies which are elliptical in shape generally have no new stars forming and are generally red in colour. This means that they have a super massive black hole in the centre and hence its presence has stopped the making of new stars, also giving the galaxy its red colour. The second type of galaxy is a spiral shape galaxy which has arms winding out in a disc formation. This type of galaxy is generally new and is still in the process of making newer stars (Royal Astronomical Society, 2014, n.pag.).
The fact is that even though it is easier to study the presence of a black hole in an active galaxy, it becomes more difficult to study the presence of a black hole in a normal galaxy. In what can be taken as providence, scientists at European Space Agency (ESA), discovered two black holes cohabiting in a normal or dormant galaxy. This can be understood to be a result of the merger of two galaxies. It so happened that the scientists saw the ripping of an existing star within that galaxy due to the gravitational pull of the two cohabiting super massive black holes (Hays, n.pag.).
Why the discovery above was considered so special was because the only way that a black hole presence can be verified in a normal galaxy is in the "event of tidal disruption". This cannot be predicted and hence depends on your luck (or providence as some might say). Whereas in an active galaxy, the black hole presence can be verified by the rays it emits and the regular consumption of the gas clouds around it. Therefore, when it comes to black holes, the discovery of this magnitude becomes even more unique simply because the scientists were looking in the right direction at the right time. While the scope of understanding the relationship of black holes and galaxies and the impact that each has on the other is huge, it will take the scientists some more time, hard work and serendipity to get the complete picture. Nevertheless, one has to admit that with the magnitude of the nature of the universe, some mysteries might remain mysteries, as certain unexplained forces work behind the working of the very nerve of each cell of the universe.
Hays, Brooks (2014, April 23). "European astronomers spot pair of supermassive black holes." United Press International (UPI). Retrieved from: http://www.upi.com/Science_News/2014/04/23/European-astronomers-spot-pair-of-supermassive-black-holes/1341398266477/
Macchetto, Duccio (n.d.). Black holes, quasars and active galaxies. Spacetelescope.org. Retrieved from: https://owl.english.purdue.edu/owl/resource/560/10/
(n.d.). "A Black Hole at the Centre of our Galaxy." European Southern Observatory. Retrieved from: https://www.eso.org/public/science/gc/
(n.d.). "A Singular Place." Cosmotography. Retrieved from: http://www.cosmotography.com/images/supermassive_blackholes_drive_galaxy_evolution_2.html
Royal Astronomical Society (2014, April 23). "How black holes shape galaxies." Astronomy Magazine. Retrieved from: http://www.astronomy.com/news/2014/04/how-black-holes-shape-galaxies