African Physics Newsletter

Two Giant Radio Galaxies Discovered by MeerKAT

The two giant radio galaxies found with the MeerKAT telescope. In the background is the sky as seen in optical light. Overlaid in red is the radio light from the enormous radio galaxies, as seen by MeerKAT. North is at the top of the images. Left: “GRG1,” Right: “GRG2.” Credit: I. Heywood (Oxford/Rhodes/SARAO)

The discovery of two radio emitting sources in South Africa questions what was previously throught about these objects.

The discovery of two unusually large giant radio galaxies, or GRGs, has been reported by a team using MeerKAT. Such galaxies have been thought of as relatively scarce, but MeerKAT has enough surface brightness sensitivity to see previously invisible large-scale, diffuse radio emission. Their discovery in the early phase of a large sky survey invites the thought that GRGs may not be uncommon at all.

The authors of the paper, more than half of whom have African affiliations, were surprised by the detection. Jacinta Delhaize, SARAO post-doc at the University of Cape Town and first author of the paper, says “We knew that we would discover more radio galaxies with the MIGHTEE (MeerKAT International GHz Tiered Extragalactic Exploration) survey, but we did not expect to immediately find two huge beasts!”

Three reasons for importance

“The discovery of these two GRGs is important for several reasons. Reason number one is simply that they had not, and could not, have been discovered before due to technical telescope limitations. Reason number two: these two GRGs are fainter than any other giant radio galaxies of roughly the same size. This means that we are probing a previously unexplored region of GRG parameter space. Reason number three: We should not have found TWO giant radio galaxies in a patch of the sky this small, based on our current knowledge of the sky density of GRGs. In fact, we shouldn’t have even found one!”

Data was used from the MIGHTEE survey of galaxies using MeerKAT, which has excellent sensitivity to large-scale emission, and compared with data from the Very Large Array COSMOS 3 GHz Large Project, which has better angular resolution. The cores of the galaxies are identifiable in the latter survey, but only MeerKAT shows that each of these galaxies has two extremely long jets with diffuse lobes and hot spots at the ends (top figure).

A special part of the sky

The study of GRGs is important because it helps us gain insight into the evolution of galaxies which may be undergoing extreme disruption. Galaxies are the ‘lighthouses’ of the Universe, illuminating the dark. We want to understand how they have changed and evolved over the history of the universe. Ultimately, this could help us understand the past and future of our own galaxy, the Milky Way. The galaxies were found in the COSMOS field, which is a patch of sky chosen for having few stars or obscuring clouds of gas from our own galaxy – a window through the Milky Way in the constellation Sextans.

It is a part of the night sky that looks relatively empty and dark to the naked eye, but that is what makes it an exceptionally rich field of discovery for extragalactic astronomers. It has been surveyed in the optical, infrared, and radio wavelengths, and X-ray sources have been mapped by the Chandra satellite. The availability of this high quality multi-wavelength data is essential for the MIGHTEE analysis.

Characteristic long lobes 

In the MeerKAT maps, the characteristic long radio lobes were found by visual inspection while the radio images were being matched with their visual counterparts. They are formed by synchrotron radiation emitted by relativistic electrons slowing down in the intergalactic medium; a good example of a radio galaxy is Centaurus A, in figure 1, which shows how multiwavelength imaging is used to untangle the components of a radio galaxy.

Centaurus A

Figure 1: False-color image of the nearby radio galaxy Centaurus A, showing radio (red), 24-micrometer infrared (green), and 0.5-5 keV X-ray emission (blue). The jet can be seen to emit synchrotron radiation in all three wavebands. The lobes only emit in the radio frequency range, and so appear red. Gas and dust in the galaxy emits thermal radiation in the infrared. Thermal X-ray radiation from hot gas and non-thermal emission from relativistic electrons can be seen in the blue 'shells' around the lobes, particularly to the south
(bottom).  Credit: Martin Hardcastle CC BY-SA 3.0


Both of the GRGs are located in small groups of galaxies, and this is consistent with lobe-bending in the intergalactic medium. (For interest: the morphology of the northern lobe of GRG1 is typical of a Fanaroff-Riley Type II radio source. The classification was co-authored in 1974 by Bernard Fanaroff, who has the distinction of being respected both as an astronomer and as a trade union leader, thereby creating an unusually significant impact for South Africa.)

700 million years old

Both radio sources appear to be embedded in early-type elliptical galaxies with low-excitation active galactic nuclei. These galaxies are known as “red and dead” as they have little to no ongoing star formation.

Radio galaxies evolve along trajectories in a power-size diagram that takes them from smaller size and higher radio jet power to larger size and lower power. These two GRGs are on the edge of the largest and lowest power region of this diagram, consistent with ages of roughly 700 million years.

Unexpected discovery

Dr. Delhaize says, “This shows that with telescopes like MeerKAT, we are starting to probe a previously unexplored part of galaxy parameter space.” Based on the GRG sky density measured by the LOFAR telescope in the Netherlands, the probability of finding two such enormous GRGs in this small COSMOS field is only 0.0027%.

“So either we have been stupidly, ridiculously lucky, or GRGs are much more common than we previously thought! And this is the big zinger of a result from our work” says Dr. Delhaize. The latter possibility would be an important conclusion.

Waiting for SKA for more

The collaborating authors remark, with more gravity, “… our findings provide strong evidence that GRGs may be far more numerous than previously thought … These are tantalizing hints of what the future SKA will ultimately uncover with its simultaneously excellent angular resolution and surface brightness sensitivity."

What does this mean for Africa? Discoveries of fundamental significance are being made across African countries and by African scientists. Young astrophysicists are making great achievements as well as having aspirations.

Igle Gledhill

This article has first been published by the African Physics Newsletter - © American Physical Society, 2021


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