We are told there is a supermassive black hole living in the centre of our galaxy. Apparently, supermassive black holes can be found in the centre of most galactic nuclei, and all the stars within the surrounding galactic disk will orbit around it. But how do we know there is a huge black hole in the centre of the Milky Way? What evidence is there? It turns out there is quite a lot, actually.
In a recent review of the subject, the radio emissions observed since the 1950’s are examined. However, probably the most striking piece of evidence is the figure to the left. Of course, we know black holes exert a massive gravitational pull on local space, and by observing the centre of our galaxy, we find there is a huge gravitational influence over a compact cluster of stars, all orbiting a common point, reaching orbital velocities of 5000 km/s…
As I briefly touched on in a previous Astroengine article No, An Alien Radio Signal Has Not Been Detected (Aug. 13th), the intense radio emissions picked up by a new South African radio telescope was not generated by an alien civilization. The 1420 MHz radio signal was actually generated by neutral hydrogen atoms, a plentiful source of radio emissions, in the constellation of Sagittarius, at the centre of the galaxy. Specifically, the emissions emanated from Sagittarius A, a three-component radio source. The three components include Sagittarius A East (a supernova remnant), Sagittarius A West (a spiral structure of hot gas) and Sagittarius A*, an intense source and the location for the supermassive black hole.
The Sagittarius A radio source has been known for decades, since the very first radio telescopes of the 1950’s, but it wasn’t until the 1970’s that astronomers realised how compact the Sagittarius A* radio source was. By the end of the ’70’s, the dynamic behaviour of the gas surrounding Sagittarius A* indicated that there must be a compact, gravitationally dominated object within this intense radio source. Although it had been speculated that there might be a supermassive black hole in the centre, very little supporting evidence had been discovered. For example, there was little X-ray and optical observational evidence to back up the radio emissions.
Regardless, the radio emissions suggested the gas was circling a compact mass, several million times more massive than our Sun. Then observations of the orbits of stars close to Sagittarius A* indicated they were orbiting a common point of some six million solar masses. Following this, astronomers worked out that the gravitational and radio emission evidence correlated. Then infrared observations appeared to overlap the same point.
Since the 1970’s it has been shown that Sagittarius A* appears to be stationary to all other objects in the galactic centre, leaving astronomers in little doubt we have a massive, compact object of several million solar masses living in the centre of our galaxy.
So, that’s Sagittarius A*, our very own supermassive black hole.
For more details about the search for a supermassive black hole at the centre of the Milky Way, check out the arXivblog or go straight to the source and read Mark Reid’s publication Is there a supermassive black hole at the centre of the Milky Way?
(Note: I would like to go into more detail about this string of evidence for Sagittarius A*, but the Starbucks at which I’m writing this, waiting for my girlfriend to finish up at a meeting, has just cranked up the air conditioning and my fingers have frozen! Time to go…)