What Are Black Hole Jets and How Do They Form?
Black holes are definitely one of the most complicated structures in the universe. They push the boundaries of physics to their breaking points and continue to intrigue us with new mysteries. One of these is the jets that shoot off from them, seemingly from the spinning madness near the center of the black hole. Recent research has shed light on the jets and how they work, as well as their implications to the universe.
Most jets that we see come from supermassive black holes (SMBH) located in the center of a galaxy, though stellar mass black holes have them as well but are harder to see. These jets shoot matter vertically from the galactic plane they reside in at speeds approaching those reached by light. Most theories predict that these jets arise from spinning matter in the accretion disc surrounding the SMBH and not from the actual black hole. As matter interacts with the magnetic field generated by the spinning material around the SMBH, it follows the field lines up or down, narrowing and heating up further until sufficient energy has been achieved for them to escape outward, avoiding the event horizon of the SMBH and thus being consumed. The matter that escapes in the jets also releases X-rays as it is energized.
A recent study seems to confirm the link between the jets and the accretion disc. Scientists looking at blazars, or active galactic nuclei that happen to have their jets pointed directly at Earth, examined the light from the jets and compared it to the light from the accretion disc. While many would think distinguishing between the two would be hard, the jets emit mostly gamma rays while the accretion disc is primarily in the X-ray/ visible portion. After examining 217 blazars using the Fermi observatory, scientists plotted the luminosity of the jets vs. the luminosity of the accretion disc. The data clearly shows a direct relation, with the jets having more power than the disc. This is likely because as more matter is present in the disc, a greater magnetic field is generated and thus the jet’s power is increased (Rzetelny "Black Hole", ICRAR).
How long does the transition take from being in the disc to becoming a part of the jet? A study done by Dr. Poshak Gandhi and team using NuSTAR and ULTRACAM looked at V404 Cygni and GX 339-4, both smaller binary systems located 7,800 light-years away that have activity but also good periods of rest, allowing for a good baseline. V404 has a 6 solar mass black hole while GX has a 12, allowing for properties about the disc to be easily discerned because of the energy output. Once an outburst occurred, NuSTAR looked for X-rays and ULTRACAM for visible light, then compared the signals during the entire event. From disc to jet, the difference between the signals was just 0.1 seconds, which at relativistic speeds is about a distance covered 19,000 miles - that happens to be the size of the accretion disc. Further observations have shown the jets of V404 actually rotate and not in alignment with the black hole's disc. Its possible that the mass of the disc could pull the jets courtesy of frame dragging of space time (Klesman "Astronomers," White, Haynes, Masterson).
An even cooler finding was that stellar-size black holes and SMBH both seem to have symmetrical jets. Scientists realized this after examining some gamma-ray sources in the sky using the SWIFT and Fermi space telescopes and finding that some came from SMBHs while others came from stellar-sized black holes. In total, 234 active galactic nuclei and 74 gamma-ray bursts were examined. Based on the speed of the rays leaving, they come from polar jets that have the roughly same output for their size. That is, if you plot the size of the black hole to the jet output, its a linear relation, according to the December 14, 2012 issue of Science (Scoles "Black Holes Big").
Ultimately, one of the best ways to make jets happen is to collide two galaxies together. A study using the Hubble Space Telescope examined merging galaxies in the process or just recently completed and found that relativistic jets traveling at nearly the speed of light and causing high-radio waves to be emitted were sourced from these mergers. However, not all mergers result in these special jets and other properties such as spin, mass, and orientation surely play a role (Hubble).
Different Sides of the Same Black Hole
The general amount of X-rays generated from the jets indicates the power of the jet flow and thus its size. But what is that relation? Scientists began to notice two general trends in 2003, but did not know how to reconcile them. Some were narrow beams and others were wide. Did they indicate different types of black holes? Was theory needing revision? As it turns out, it may be a simple case of black holes having behavioral changes that allow them to go between the two states. Michael Coriat from the University of Southampton and his team were able to witness a black hole going through such a change. Peter Jonker and Eva Ratti from the SRON were able to add even more data when they noticed more black holes exhibiting similar behavior, using data from Chandra and the Expanded Very Large Array. Now scientists have a better understanding of the relationship between narrow jets and wide jets, thus allowing scientists to develop even more detailed models (Netherlands Institute for Space Research).
Whats in a Jet?
Now, the material that is in the jet will determine how powerful they are. Heavier materials are difficult to accelerate, and many jets are leaving their galaxy at near light speeds. This is not to say that heavy materials cannot be in the jets, for they can be but move at a slower rate because of energy demands. This seems to be the case in system 4U 1630-47, which has a stellar mass black hole and a companion star. Maria Diaz Trigo and her team looked at X-rays and radio waves coming from it as recorded by the XMM-Newton Observatory in 2012 and compared them with current observations from the Australian Telescope Compact Array (ATCA). They found signatures of high speed and highly ionized iron atoms, specifically Fe-24 and Fe-25, though nickel was also detected in the jets. Scientists noticed the shifts in their spectrums corresponding to speeds of nearly 2/3 the speed of light, leading them to conclude that the material was in the jets. Since many black holes are in systems like this, it is possible that this is a common occurrence. Also of note was the amount of electrons present in the jet, for they are less massive and therefore carry less energy than the nuclei present (Francis, Wall, Scoles "Black Hole Jets").
This seems to resolve many mysteries about the jets. No one disputes that they were made of matter but whether it was predominantly light (electrons) or heavy (baryonic) was an important distinction to be had. Scientists could tell from other observations that the jets had electrons which are negatively charged. But the jets were positively charged based on EM readings, so some form of ions or positrons had to be included in them. Also, it takes more energy to launch heavier material at such speeds, so by knowing the composition scientists can get a better grasp on the power that the jets exhibit. Additionally, the jets seem to come from the disc around the black hole and not as a direct result of the spin of a black hole, as earlier research seemed to indicate. Finally, if most of the jet is heavier material then collisions with it and the outer gas could cause neutrinos to form, solving a partial mystery of where other neutrinos could be sourced from (Ibid).
So what do these jets do to their environment? Plenty. The gas, known as feedback. can collide with surrounding inert gas and heat it up, releasing huge bubbles into space while raising the temperature of the gas. In some cases, the jets can start star formation in places known as Hanny’s Voorwerp. Most of the time, huge amounts of gas leave the galaxy (Netherlands Institute for Space Research).
When scientists looked at M106 using the Spitzer telescope, they got a very good demonstration of this. They looked at heated hydrogen, a result of jet activity. Almost 2/3 of the gas around the SMBH was being ejected from the galaxy, and thus its ability to make new stars is being diminished. In addition to this, spiral arms not like those seen at visible wavelengths were detected and found to have formed from shockwaves of the jets as they hit cooler gas. These could be reasons why galaxies become elliptical, or old and full of red stars but not producing new stars (JPL “Black Hole”).
More evidence for this potential result was found when ALMA looked at NGC 1433 and PKS 1830-221. In the case of 1433, ALMA found jets extending over 150 light-years from the center of the SMBH, carrying much material with it. Interpreting the data from 1830-221 proved challenging because it is a distant object and has been gravitationally lensed by a foreground galaxy. But Ivan Marti-Vidal and his team from the Chalmers University of Technology at the Onsala Space Observatory, FERMI, and ALMA were up to the challenge. Together, they found that changes in the gamma rays and submillimeter radio spectrums corresponded to matter falling near the base of the jets. How these affect their surroundings remains unknown (ESO).
One possible outcome is that the jets prevent future star growth in elliptical galaxies. Plenty of them have cold enough gas that they should be able to resume star growth, but the central jets may actually kick up the temperature of the gas high enough to prevent condensation of the gas into a proto-star. Scientists arrived at this conclusion after looking at observations from the Herschel Space Observatory comparing elliptical galaxies with active and non-active SMBHs. Those that were churning gas about with their jets had too much warm material to form stars, as opposed to those more quiet galaxies. It seems as though the fast-radio waves formed by the jets also create a feedback pulse of sorts that further prevents star formation. The only places where star formation did occur was on the periphery of the bubbles, according to observations by ALMA of the Phoenix cluster of galaxies. There, cold gas is condensing and with the star-forming gases pushed out there by the jets it can create a right environment for new stars to form (ESA, John Hopkins, Blue).
In fact, the jets of a SMBH can not only create these bubbles but possibly impact the rotation of stars near them in the central bulge. This is a close-proximity area of a galaxy to its SMBH and scientists have known for years that the larger the bulge the faster the stars in it move. Researchers led by Fransesco Tombesi at the Goddard Space Flight Center figured out the culprit after looking at 42 galaxies with XMM-Newton. Yep, you guessed it: those jets. They figured this out when they spotted those iron isotopes in gas from the bulge, indicating the link. As the jets hit the gas nearby, the energy and material cause an outflow that impacts star motion through transference of energy, leading to an increased speed (Goddard).
But wait! This picture of jets impacting formation by starting or stunting isn't as clear cut as we may think it is. Evidence from ALMA's observations of WISE1029, a dust obscured galaxy, shows that the jets from its SMBH was made of ionized gas that should have impacted the carbon monoxide around it, generating star growth. But it didn't. Does this change our understanding of jets? Maybe, maybe not. It is a singular outlier, and until more are found the consensus is not universal (Klesman "Can")
Want more? Scientists found in NGC 1377 a jet leaving a supermassive black hole. It totaled in length at 500 light years, was 60 light years wide, and was traveling at 500,000 miles per hour. Nothing major here at first glance, but when examined further the jet was found to be cool, dense, and exiting in a spiral, spray like manner. Scientists postulate that gas could have flowed in at an unsteady rate or that another black hole could have tugged and caused the weird pattern (CUiT).
How Much Energy?
Of course, any discussion on black holes would not be complete unless something which counters expectations was found. Enter MQ1, a stellar-mass black hole found in the Southern Pinwheel Galaxy (M 83). This black hole seems to have a shortcut around the Eddington Limit, or the amount of energy a black hole can export before cutting off too much of its own fuel. It is based on the huge amount of radiation that leaves a black hole impacting how much matter can fall into it, thus reducing the radiation after a certain amount of energy is leaving the black hole. The limit was based on calculations involving the mass of the black hole but based on how much energy was seen leaving this black hole some revisions will be needed. The study, led by Roberto Soriaof the International Center for Radio Astronomy Research, was based on data from Chandra which helped find the mass of the black hole. Radio emmisions resulting from the shockwave of matter being impacted by the jets helped calculate the net kinetic energy of the jets and were recorded by Hubble and the Australia Telescope Compact Array. The brighter the radio waves, the higher the energy of the impact of the jets with the surrounding material. They found that 2-5 times as much energy was being sent into space than should be possible. How the black hole cheated remains unknown (Timmer, Choi).
Another consideration is the material exiting the black hole. Does it leave at the same rate, or does it fluctuate? Does faster portions collide or overtake slower pieces? This is what the internal shock model of black hole jets predicts, but evidence is tough to find. Scientists needed to spot some fluctuation in the jets themselves and track any changes in brightness along with it. Galaxy 3C 264 (NGC 3862) provided that chance when over a span of 20 years scientists tracked clumps of matter as they left at nearly 98% the speed of light. After faster moving clumps caught up with drag-reduced slower clumps, they collided and caused a 40 percent increase in brightness. A shock wave-like feature was spotted and indeed validated the model and can partially explain erratic energy readings seen up until now (Rzetelny "Knots," STScl).
Jets Bouncing Around
Cygnus A has presented astrophysicists with a pleasant surprise: Inside this elliptical galaxy located 600 million light-years away lies a SMBH whose jets are bouncing around inside it! According to observations from Chandra, hotspots along the edges of the galaxy are a result of the jets hitting material that is highly charged. Somehow, the SMBH has created a void around it as big as 100,000 light-years long by 26,000 light years wide and the charged material is outside of it as lobes, creating a dense region. This can redirect the jets hitting it to a secondary location, creating multiple hotspots along the edges (Klesman "This").
A Different Approach?
It should be noted that recent observations from ALMA of the Circhinus Galaxy, 14 million light-years away, hint at a different model for jets than is traditionally accepted. It would seem that cold gas around the black hole is heated up as it approaches the event horizon, but after a certain point gains enough heat to become ionized and escape as a jet. However, the material cools off and can fall back into the disc, repeating the process in a cycle that is perpendicular to the disc of rotation. Whether this is a rare or common event remains to be seen (Klesman "Black").
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© 2015 Leonard Kelley