Physicists revisit the Fanaroff-Riley classification system

Rhodes University honorary doctorate Dr Bernie Fanaroff, and several physicists associated with Rhodes University, have recently revisited the Fanaroff-Riley classification (FRI/FRII morphological classification) in a paper called ‘A new look at old friends. I. Imaging classical radio galaxies with uGMRT and MeerKAT’. 

The paper, which has just been accepted by Monthly Notices of the Royal Astronomical Society (MNRAS), studies four radio galaxies using the improved imaging capabilities of the Giant Metrewave Radio Telescope (uGMRT) in India and the MeerKAT telescope in the South African Karoo to determine whether newly-discovered radio emission features require the current FRI/FRII morphological classification scheme, which was established in 1974, to be revised.

“[Radio galaxy emission details that were not visible before include] filamentary structures in the emission from the lobes, radio emission beyond the hot-spots in three sources, and misalignments," the paper explains.

According to Dr Fanaroff, the sensitivity and ability to image diffuse areas of emission around radio galaxies was one of the design objectives of the MeerKAT telescope. “We wanted to see whether this new capability would reveal new features in and around the jets of radio emission from the supermassive black holes in the centres of these galaxies, and whether this would require and assist astrophysicists to carry out more work on the physics of the jets: how they form, how they propagate through the gas in and around the galaxies and why they look as they do,” he explained.

Published 47 years ago, Dr Fanaroff’s last first-author paper established the Fanaroff-Riley classification that we use to this day.

“The Fanaroff-Riley system was based on the early radio maps of the brightest radio galaxies and quasars in the Northern sky, from the Third Cambridge Survey (3CR),” he explained.

The classification notes that there are two kinds of large-scale jets from radio galaxies and quasars: FRII jets, where there is a hot-spot of radiation at the end of the jet as it sweeps aside the inter-galactic gas like a snowplough; and FRI jets, where the jet is brighter towards the centre of the galaxy and becomes more diffuse and fainter as it goes further out into and often beyond the host galaxy.

In the 1974 sample of bright radio sources, there was an apparent link between the radio power of the galaxy or quasar and the shape of the jet, which indicated FRII galaxies were more powerful than FRI galaxies.

“This link is less clear as the new uGMRT and MeerKAT telescopes have been able to study fainter radio sources, but there is still a clear tendency for FRII sources to be more powerful than FRI sources,” Dr Fanaroff explained. 

He said that when astrophysicists try to explain the physics of the things they see in the Universe, it helps to link different characteristics of the radio galaxies, such as the shape and the power.

“Although many different characteristics have been found since 1974, they have not yet shown unambiguous associations between each other, so the Fanaroff-Riley classification has remained an important guide to the phenomena being studied,” added Dr Fanaroff. 

Professor Oleg Smirnov, SKA Research Chair at Rhodes University and head of the Radio Astronomy Research Group at SARAO, who led the imaging for the paper, has worked closely with Dr Fanaroff. 

He said, “In a fast-moving field like ours, it's very unusual for a problem to remain relevant for this long. I wasn't even one year old when Bernie's first paper was published. When I was just starting out in the field, the Fanaroff-Riley classification was hallowed, old-school radio astronomy from some ancient Golden Era. So you can imagine what a special privilege it has been for me to work with Bernie on this problem again, after so many years.”

 Additional authors of the paper, associated with Rhodes University, include:

• Professor Oleg Smirnov, SKA Research Chair at Rhodes University and head of the Radio Astronomy Research Group at SARAO, led the imaging for the paper
• Dr Landman Bester, Data Scientist at SARAO, Rhodes University alumnus (PhD) and a research associate at Rhodes University
• Dr Gyula Józsa, Senior Astronomer at SARAO, Visiting Professor at Rhodes University
• Dr Sarah White, SARAO postdoctoral fellow at Rhodes University
• Dr Sphesihle Makhathini, Rhodes University PhD graduate and former Research Fellow
• Dr Kshitij Thorat, a former postdoctoral fellow at Rhodes University

 

All of the above were involved in developing the Caracal software pipeline (https://caracal.readthedocs.io/en/latest/) used to process the MeerKAT data.