The unseen universe: Radio galaxies, black holes and synchronized telescopes

By Tiziana Venturi, Senior Researcher, Institute of Radio Astronomy (INAF), Italy, and Michael Lindqvist, Senior Research Engineer, Onsala Space Obervatory (OSO), Sweden

Radio astronomy is a comparatively young, yet profound, discipline within the vast landscape of celestial exploration compared to optical astronomy. It offers a more energetic view of the cosmos with unprecedented insights into phenomena that remain entirely invisible to both traditional optical instruments and high-energy X‑ray observatories.

These are important considerations for the International Telecommunication Union (ITU) and its Radiocommunication Sector (ITU-R) in managing the increasingly crowded radio-frequency spectrum on Earth and in space.

The radio sky

Among the highly energetic astrophysical manifestations of the radio sky are radio galaxies, breath-taking objects which unveil the intense activity of supermassive black holes residing at the centre of massive elliptical galaxies.

Black holes may reach masses as high as billions of solar masses and manifest themselves launching two colossal, oppositely directed “jets” of highly magnetized relativistic plasma that propagate far beyond the host galaxy into the surrounding intergalactic space. As these jets fluctuate and interact with the tenuous external medium, they decelerate and take fascinating shapes, known as “radio lobes”, which make them resemble “dancers in the radio sky”.

Radio galaxies are true cosmic laboratories and deliver unique information on the central black holes themselves, on the magnetic fields responsible for the radio emission, and on the density and temperature gradients of the surrounding gas as the jets traverse from the dense, hot inner regions of the galaxy out to the extremely rarefied and colder intergalactic medium. Radio galaxies are primarily studied from a few hundred Megahertz (MHz) to a few tens of Gigahertz (GHz), and each frequency within this range carries unique, non-redundant information that, once lost to interference or instrumental limitations, is unrecoverable.

Very long baseline interferometry

The quest to reach the very heart of these energetic systems, the immediate vicinity of the central black holes, is the domain of Very Long Baseline Interferometry (VLBI), a modern observational technique involving radio telescopes separated hundreds to thousands of kilometres operating simultaneously. Angular resolution – our capacity to discern fine details in the sky – improves proportionally with the distance, or “baseline,” between two individual radio telescopes in an interferometer array, as well as with increasing observing frequency.

Angular resolution has been a major issue for radio astronomy, due to the much lower frequency of the radio waves compared for instance to the optical light. This limitation, however, has turned into an invaluable advantage. Thanks to decades of technological advances, VLBI has overcome geographic limits, simulating a single virtual radio telescope with a diameter that spans the Earth, and even extends into space, reaching the highest angular resolution possible in modern astronomy, which will be matched by optical telescopes only at the end of the present decade. 

In VLBI, a radio source must be observed at the same time and frequency by all participating telescopes. Each telescope uses a hydrogen maser to tag the data with exact arrival times, a mandatory step for the following complex processing. The coordination in performing VLBI observations, as well as the need for compatible technology and clear skies at each individual station make VLBI a key successful example of global collaboration and coordination.

To “zoom in” to the innermost regions of galaxies, reaching the very edge of black holes, VLBI observations need to be carried out at hundreds of GHz, the current published record is at 345 GHz. This introduces a further complication to the operations, due to the very sparse distribution of radio telescopes operating at such high frequencies, and to the water vapour conditions in the troposphere.

A telescopic breakthrough

Despite all these difficulties, this incredible adventure culminated in the Event Horizon Telescope (EHT) collaboration, with the delivery of the stunning first images of the plasma’s final, relativistic orbit around a black hole before it plunges past the event horizon. The visible “shadow” captured in these images of M87 and Sgr A* is the ultimate confirmation of Einstein’s General Relativity in the strong gravitational field regime.

The direct measure of the black hole’s gravity affecting spacetime thanks to the EHT’s unique combination of baseline length and observing frequencies is not the end of the journey. Filming the time evolution of the radio emission in the closest proximity of the black holes under the effects of general relativity took place between March-May 2026.

VLBI also remains a vital tool for studying the relatively nearby universe, particularly the mechanism triggering the formation of massive stars, which is still largely unknown. This can be investigated through the observation of the methanol maser line at 6.7 GHz.

This molecule, abundant in the dense environments of massive star-forming regions, acts as a precise beacon for these areas. It is further a bridge towards astrochemistry, a discipline through which high angular resolution and high frequency observations of carbon chemistry molecular lines open the door to our understanding of the formation of planetary systems and ultimately brings us to the origin of humankind. 

Preserving the quality of observations

As we approach the next World Radiocommunication Conference, WRC-27, this treasure for humanity is facing a major threat. While the United Nations Educational, Scientific and Cultural Organization (UNESCO) is working closely with the International Astronomical Union (IAU) to support the unique importance of astronomy as a world heritage asset, the radio portion of the electromagnetic spectrum has become highly coveted by telecommunication companies for mobile and satellite applications.

Spectrum allocations to preserve radio astronomy, therefore, is a crucial topic for WRC-27 and subsequent discussions in the ITU conference cycle.

High-tech solutions to mitigate the impact of human-made radio frequency interference (RFI) are in place at each observatory but these are not enough to preserve the quality of the observations.  

Losing even small slivers of this spectrum to human-generated interference causes enormous and irreparable damage to scientific progress. Protecting the radio quietness of Earth is not merely a scientific necessity but a matter of preserving humanity’s connection to its cosmic origins and the mysteries that define our existence.

Header image credit: South African Radio Astronomy Observatory (SARAO), SSS, S. Dagnello and W. Cotton (U.S. National Radio Astronomy Observatory (NRAO)/AUI/National Science Foundation (NSF))

Radio Galaxy PKS1333-33: The Fanaroff-Riley Type I radio galaxy IC 4296 dominates this spectacular vista, wider than the full moon on the sky. MeerKAT radio data are represented in red/orange hues in this composite view. The visible light image from the SuperCOSMOS Sky Survey (SSS) shows the central giant elliptical galaxy, as well as numerous unrelated galaxies and foreground stars in the Milky Way.