From the biggest to the smallest black hole jets: Does size matter?

by Ioannis Liodakis

Ioannis Liodakis. (Credit: KIPAC.)

Is it possible to learn how the jets from small black holes a few solar masses in size behave by studying the biggest, most extreme, most relativistically distorted jets from full-fledged quasars, the monstrous billion solar mass black holes at the centers of giant galaxies? Remarkably the answer is YES!

Supermassive black holes in the centers of giant elliptical galaxies can sometimes produce powerful relativistic outflows called jets. Blazars are a special class of these galaxies with the unique property of having their jets oriented within a small angle from our line of sight. (For more on blazar jets, see the KIPAC blog post, Where have all the magnetic fields disappeared to?). Because of that preferential alignment and the fact that jets move with speeds close to the speed of light, extreme aberration of light and time dilation effects take place, distorting the observed properties of the objects.

Artist's depiction of jets from the black hole in galaxy 3C 120. (Credit: Cosmovision (led by Dr. Wolfgang Steffen of the Instituto de Astronomia, UNAM, Ensenada, Mexico) for A. Marscher; NRAO/AUI/NSF.)
Still image from an animation depicting the jets emitted by the black hole in galaxy 3C 120. (Credit: Cosmovision (led by Dr. Wolfgang Steffen of the Instituto de Astronomia, UNAM, Ensenada, Mexico) for A. Marscher; NRAO/AUI/NSF.)


These effects give rise to unique phenomena, such as enhanced emission throughout the electromagnetic spectrum and apparent superluminal motion, while at the same time severely complicating our understanding of their intrinsic properties and the processes relevant to their central engines. As a result, despite decades of systematic study little is known regarding the properties of blazars in the jet rest frame.

Figure demonstrating apparent superluminal motion.
Schematic showing the projection effect causing the jets to appear as if they are moving faster than the speed of light. (Credit: Jose L. Gómez​​​​​​​.)

To put it simply, what we observe with blazars is not what’s really going on.

Can these relativistic effects be accounted for? The relativistic effects are quantified by what we call the Doppler factor, which is a function of the velocity of the jet and the angle to the line of sight of an observer. Although there is no direct method to measure either velocity or angle, there are indirect ways to estimate the Doppler factor, the most accurate of which are the variability Doppler factors, or Doppler factors from variable sources. This method is based on the comparison between the observed energy output of the flares' observed radio wavelengths and the (theoretical) maximum of that energy output that can be achieved due to the synchrotron nature of the radiation in jets.

Using this method, an international team of astronomers I am a part of was able to estimate the intrinsic (relativistic effects-free) broad-band radio luminosity—i.e., the total radio output of the jets—in a number of blazars and radio galaxies, and found that it is strongly correlated with the mass of their supermassive black holes. This is the first ever relation to connect the intrinsic emission of the jets in blazars with the properties of their host galaxies. Can that tell us anything about microquasars, the name given to galactic black holes with jets?

As it turns out the smallest of black holes obey the relation derived from the most massive ones! The figure below shows the extrapolation of the relation from supermassive black holes to galactic black holes extending over more than nine orders of magnitude in both radio luminosity and black hole mass.

Intrinsic broad-band radio luminosity vs. black hole mass. (Credit: I. Liodakis.)
Intrinsic broad-band radio luminosity vs. MBH (luminosity is in watts, mass is in units of 1e8 solar masses). The dashed line represents the best-fit relation. The gray shaded areas are the 1σ, 2σ, and 3σ confidence regions respectively, taking into account the error on both slope and intercept. (Credit: I. Liodakis.)

The results, published in 2017 as "Scale invariant jets: from blazars to microquasars," clearly show that jets from black holes are indeed scale-invariant. The results also demonstrate clear evidence of the connection between the properties of the black holes and the jets they cause regardless of size. In addition, the results are some of the first direct observational evidence supporting the Blandford-Znajek mechanism: i.e, the extraction of angular momentum from the spinning black hole as the most likely mechanism for jet production in black hole-powered jets. They also set strong constraints on any other potential jet model since it would need to reproduce such a connection.

Our team concludes the paper by discussing how this universal relation could be used to look more closely into the environment surrounding a black hole and better understand the different accretion regimes operating with different sources. It could also help guide the search for the yet-elusive intermediate mass black holes—if they form jets—filling in the mysterious gap between the most and least massive black holes.