At the heart of every galaxy lies a supermassive black hole devouring everything that falls
within its gravitational pull, almost everything, that is.
While we often conceive of black holes as massive cosmic vacuum cleaners, they aren't
entirely one-way.
When gas falls into a black hole, it spirals inwards like water down a plug hole, but not
all of it is lost; some of it is blasted out in narrow, powerful beams known as jets by
astronomers.
Jets have been known to astronomers for nearly a century.
Heber Curtis identified the first jet in 1918, emanating from the core of the massive galaxy
M87.
What he initially thought was a "strange straight ray" of light turned out to be a powerful
source of radio waves.
Astronomers have detected comparable jets all around the universe since then.
However, despite decades of research, it is still unknown what these jets are made of.
In some instances, the jets travel outside the galaxy, resulting in massive radio lobes
far from the active galaxy center.
With the help of radio telescopes, it has been observed that galaxies can take on a
variety of shapes, the majority of which resemble dumbbells.
Depending on how bright they are and how quickly they consume the stuff around them, they are
known as radio galaxies or quasars.
As these gigantic black holes grow to a billion times the mass of the Sun, their jets become
powerful enough to sweep gas out of the galaxy, thus stopping the birth of new stars!
Only a tiny percentage of active galaxies with jets have their jets aimed straight at
Earth.
In those circumstances, we observe radiation across the electromagnetic spectrum, amplified
by the jet's incredible speed, and we refer to these sources as blazars.
We are learning about the physics of how powerful jets are produced and sustained by integrating
NuSTAR X-ray observations with observations in radio, visible light, and extremely energetic
gamma-rays.
One of the unsolved puzzles is how jets produce radiation with such a broad spectrum up to
very high energy.
These jets fly at near-lightspeed outwards, depositing massive amounts of energy in their
surroundings.
The jets' primary method of releasing this enormous amount of motion energy is by converting
it into extremely high-energy gamma rays.
How exactly this radiation is formed is an outstanding mystery.
The jet must release its energy somewhere, and prior research has not discovered where
it goes.
The leading candidates are the broad-line region and the molecular torus, which orbit
black holes and are made of gas and light.
By releasing some of its energy, a black hole's jet has the potential to convert visible and
infrared light in any region into high-energy gamma rays.
Recent research funded by NASA clarifies the debate by demonstrating that the jets mainly
release energy in the molecular torus, rather than the broad-line region.
The broad-line region is closer to the center of a black hole, at a distance of about 0.3
light-years.
The molecular torus is much farther out, more than three light-years away.
The bright radio waves seen from the jets by our radio telescopes on Earth indicate
that the jets must include electrons.
However, because electrons have a negative electric charge and the jets must be electrically
neutral, something else must balance that charge.
The two possibilities are positrons, the antimatter counterparts of electrons, or the atomic nuclei.
Because the amount of energy carried away from the black hole by jets is proportional
to the mass of the particles contained in them, a jet carrying the nuclei can carry
thousands of times more energy than a jet containing electrons and positrons.
The energy is finally deposited in the surrounding environment, where radio eyes may observe
its spectacular effects; in powerful galaxies, black hole jets are seen to blow gigantic
bubbles, thousands of light-years long.
As electrons in atoms hop between different energy levels (like rungs on a ladder), they
emit light with a distinct wavelength, in effect, a distinct color that is unique to
that atom.
Physicists have precisely measured these wavelengths in the lab, allowing us to directly correlate
light of a specific wavelength with a specific atom.
Scientists are still carrying out more research on black hole jets because knowing what black
hole jets are made of is one more piece of the puzzle that may help us finally unlock
the mystery of how the jets are actually launched.
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