In 1971, English astronomers Donald Lynden-Bell and Martin Rees hypothesized that a supermassive black hole (SMBH) resides at the center of our Milky Way Galaxy. This was based on their work with radio galaxies, which showed that the massive amounts of energy radiated by these objects was due to gas and matter being accreted onto a black hole at their center.
By 1974, the first evidence for this SMBH was found when astronomers detected a massive radio source coming from the center of our galaxy. This region, which they named Sagittarius A*, is over 10 million times as massive as our own Sun. Since its discovery, astronomers have found evidence that there are supermassive black holes at the centers of most spiral and elliptical galaxies in the observable Universe.
Description:
Supermassive black holes (SMBH) are distinct from lower-mass black holes in a number of ways. For starters, since SMBH have a much higher mass than smaller black holes, they also have a lower average density. This is due to the fact that with all spherical objects, volume is directly proportional to the cube of the radius, while the minimum density of a black hole is inversely proportional to the square of the mass.
In addition, the tidal forces in the vicinity of the event horizon are significantly weaker for massive black holes. As with density, the tidal force on a body at the event horizon is inversely proportional to the square of the mass. As such, an object would not experience significant tidal force until it was very deep into the black hole.
Formation:
How SMBHs are formed remains the subject of much scholarly debate. Astrophysicists largely believe that they are the result of black hole mergers and the accretion of matter. But where the "seeds" (i.e. progenitors) of these black holes came from is where disagreement occurs. Currently, the most obvious hypothesis is that they are the remnants of several massive stars that exploded, which were formed by the accretion of matter in the galactic center.
Another theory is that before the first stars formed in our galaxy, a large gas cloud collapsed into a "qausi-star" that became unstable to radial perturbations. It then turned into a black hole of about 20 Solar Masses without the need for a supernova explosion. Over time, it rapidly accreted mass in order to become an intermediate, and then supermassive, black hole.
In yet another model, a dense stellar cluster experienced core-collapse as the as a result of velocity dispersion in its core, which happened at relativistic speeds due to negative heat capacity. Last, there is the theory that primordial black holes may have been produced directly by external pressure immediately after the Big Bang. These and other theories remain theoretical for the time being.
Sagittarius A*:
Multiple lines of evidence point towards the existence of a SMBH at the center of our galaxy. While no direct observations have been made of Sagittarius A*, its presence has been inferred from the influence it has on surrounding objects. The most notable of these is S2, a star that flows an elliptical orbit around the Sagittarius A* radio source.
S2 has an orbital period of 15.2 years and reaches a minimal distance of 18 billion km (11.18 billion mi, 120 AU) from the center of the central object. Only a supermassive object could account for this, since no other cause can be discerned. And from the orbital parameters of S2, astronomers have been able to produce estimates on the size and mass of the object.
For instance, S2s motions have led astronomers to calculated that the object at the center of its orbit must have no less than 4.1 million Solar Masses (8.2 × 10³³ metric tons; 9.04 × 10³³ US tons). Furthermore, the radius of this object would have to be less than 120 AU, otherwise S2 would collide with it.
However, the best evidence to date was provided in 2008 by the Max Planck Institute for Extraterrestrial Physics and UCLAs Galactic Center Group. Using data obtained over a 16 year period by the ESO's Very Large Telescope and Keck Telescope, they were able to not only accurately estimate the distance to the center of our galaxy (27,000 light years from Earth), but also track the orbits of the stars there with immense precision.
As Reinhard Genzel, the team leader from the Max-Planck-Institute for Extraterrestrial Physics said:
"Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt."
Another indication of Sagittarius A*s presence came on January 5th, 2015, when NASA reported a record-breaking X-ray flare coming from the center of our galaxy. Based on readings from the Chandra X-ray Observatory, they reported emissions that were 400 times brighter than usual. These were thought to be the result of an asteroid falling into the black hole, or by the entanglement of magnetic field lines within the gas flowing into it.
Other Galaxies:
Astronomers have also found evidence of SMBHs at the center of other galaxies within the Local Group and beyond. These include the nearby Andromeda Galaxy (M31) and elliptical galaxy M32, and the distant spiral galaxy NGC 4395. This is based on the fact that stars and gas clouds near the center of these galaxies show an observable increase in velocity.
Another indication is Active Galactic Nuclei (AGN), where massive bursts of radio, microwave, infrared, optical, ultra-violet (UV), X-ray and gamma ray wavebands are periodically detected coming from the regions of cold matter (gas and dust) at the center of larger galaxies. While the radiation is not coming from the black holes themselves, the influence such a massive object would have on surrounding matter is believed to be the cause.
In short, gas and dust form accretion disks at the center of galaxies that orbit supermassive black holes, gradually feeding them matter. The incredible force of gravity in this region compresses the disk's material until it reaches millions of degrees kelvin, generating bright radiation and electromagnetic energy. A corona of hot material forms above the accretion disc as well, and can scatter photons up to X-ray energies.
The interaction between the SMBH rotating magnetic field and the accretion disk also creates powerful magnetic jets that fire material above and below the black hole at relativistic speeds (i.e. at a significant fraction of the speed of light). These jets can extend for hundreds of thousands of light-years, and are a second potential source of observed radiation.
When the Andromeda Galaxy merges with our own in a few billion years, the supermassive black hole that is at its center will merge with our own, producing a much more massive and powerful one. This interaction is likely to kick several stars out of our combined galaxy (producing rogue stars), and is also likely to cause our galactic nucleus (which is currently inactive) to become active one again.
The study of black holes is still in its infancy. And what we have learned over the past few decades alone has been both exciting and awe-inspiring. Whether they are lower-mass or supermassive, black holes are an integral part of our Universe and play an active role in its evolution.
Who knows what we will find as we peer deeper into the Universe? Perhaps some day we the technology, and sheer audacity, will exist so that we might attempt to peak beneath the veil of an event horizon. Can you imagine that happening?
We have written many interesting articles about black holes here at Universe Today. Here's Beyond Any Reasonable Doubt: A Supermassive Black Hole Lives in Centre of Our Galaxy, X-Ray Flare Echo Reveals Supermassive Black Hole Torus, How Do You Weigh a Supermassive Black Hole? Take its Temperature, and What Happens When Supermassive Black Holes Collide?
Astronomy Cast also some relevant episodes on the subject. Here's Episode 18: Black Holes Big and Small, and Episode 98: Quasars.
More to explore: Astronomy Cast's episodes Quasars, and Black Holes Big and Small.
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