from Sagittarius A*, a supermassive black hole
in the center of the Milky Way galaxy.
Credit: NASA/CXC/Stanford/I. Zhuravleva et al.
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.
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.
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.
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.
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
Over time, it rapidly accreted mass in
order to become an intermediate, and then supermassive, black hole.
These and other theories remain theoretical for the time being.
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.
And from the orbital parameters of S2,
astronomers have been able to produce estimates on the size and mass
of the object.
Furthermore, the radius of this object
would have to be less than 120 AU, otherwise S2 would collide with
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:
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.
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,
...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...?