Meerkat and uGmrt Rediscover Historical Radio Galaxies (Cosmology)

A group of radio astronomers, including some from INAF, took a journey through time revisiting some of the most famous radio galaxies discovered 47 years ago by the famous duo Fanaroff-Riley. Using the incredible sensitivity of two of the most powerful radio telescopes in the world, the researchers highlighted new and unexpected details of four radio galaxies, presenting their results in the journal Mnras

Being a scientist is not just about hunting for the most sensational discovery. Sometimes, it also means reviewing past studies to try to get better results, reveal new details or confirm theories using modern and cutting-edge tools. This was the goal of a group of radio astronomers, including some from the National Institute of Astrophysics (INAF), who decided to revisit a set of radio galaxies discovered and classified almost 50 years ago by the famous duo Bernie Fanaroff-Julia Riley, traveling through time with two of the most powerful and sensitive radio telescopes in the world today: the Upgraded Giant Metrewave Radio Telescope (uGmrt) of the National Center for Radio Astrophysics(Ncra) in India and the MeerKat of the South African Radio Astronomy Observatory (Sarao) in South Africa. It was in fact 1974 when the South African astronomer and the English astrophysics revolutionized the radio astronomy of the time (but also the modern one) with an article in which they classified radio galaxies into two types based on their morphology (shape), the famous Fanaroff classification -Riley type I and II (FrI and FrII).

“Over the past 50 years, the observation potential in the radio band has improved by several orders of magnitude. With current radio telescopes it is possible to reveal both details that were indistinguishable in the images of the first interferometers, and regions of weak emission, which escaped their “sensitivity”. For this reason, our understanding of many astrophysical phenomena is in great evolution right now. Specifically, the objects chosen for our work are radio galaxies “, explains in an interview with Media Inaf Tiziana Venturi , second author of the study published in Monthly Notices of the Royal Astronomical Society and director of the Institute of Radio Astronomy of the INAF a Bologna and Italian project managerRadio Sky 2020 . « This work also has a historical flavor. The first author is Bernie Fanaroff, who, together with his colleague Riley, began the study of the “radio galaxy” phenomenon. Revisiting that classification in the light of modern radio interferometers can hold many surprises ». 

Tiziana Venturi, director of the INAF Radio Astronomy Institute in Bologna

What are radio galaxies? 

“Radio galaxies are radio emission sources that originate in the central regions of massive elliptical galaxies as a consequence of the black hole’s accretionwhich is located in the center of the same. They are spectacular objects, whose dimensions often exceed by several orders of magnitude those of the galaxies from which they originate, expanding in the intergalactic medium, or the gas that permeates the regions between galaxies. Being able to observe radio galaxies with current instruments means collecting extremely detailed information on the electrons and magnetic fields that produce radio emission from the regions close to the black hole from which everything originates, up to the most extreme external regions, covering a range of phenomena ranging from from relativistic astrophysics, to the interactions between the radio-emitting plasma and the surrounding environment, which in the case of radio galaxies changes a lot “.

Among the radio galaxies rediscovered and observed in this study, which is the most interesting and which are the unpublished details that you have brought to light? 

“In fact, each object in our study turned out to be interesting, and all showed some unexpected characteristics. Of the four presented in the work, the radio galaxy associated with the elliptical galaxy CGCG044-046 is perhaps the most spectacular. The object is located at the center of a group of galaxies, and its radio emission is affected by this somewhat special environment, in which the motions of galaxies and the presence of hot gas shape the shape of the radio galaxy itself. Our observations revealed all the emission regions in great detail, highlighting particularly relevant filaments and compression zones of the radio-emitting plasma. They also made it possible to understand how the radio emission is oriented in the plane of the sky, and to make hypotheses about the intergalactic medium that surrounds it ».

One of the GMRT antennas in Pune, India. Credits: Ncra-Tifr

For your time travel, you have exploited the enormous sensitivity of interferometers such as uGmrt and MeerKat. What is special about these arrays compared to others? 

‘UGmrt and MeerKat are used as pathfinder and precursor of the Ska project respectively, the radio telescope that is destined to revolutionize our knowledge of the Universe. uGmrt consists of 30 antennas of 45 meters in diameter each, works at frequencies ranging from about 100 MHz to about 1.5 GHz, while MeerKat, consisting of 64 antennas of 13.5 meters each, is already in fact a portion of the SKA project and works at frequencies ranging from about 900 MHz to about 3.5 GHz. Both instruments, thanks to the configuration of the distribution of the antennas, combined with the very high technology of the entire observation and signal recording chain, allow to simultaneously detect the radio emission on multiple angular scales, at the best sensitivity available at the moment. For comparison the Very Large Array, whose performance is exceptional, must observe the same area of ​​sky in three different configurations in order to achieve the richness of detail that MeerKat is able to reveal with a single observation. The advantage in terms of observation time and data analysis speaks for itself ».

How do these tools differ from those used nearly 50 years ago?

“Compared to 50 years ago, the differences are even more striking, starting from the wide range of observational frequencies available, which allow spectral analyzes unthinkable when the study of radio galaxies was born, to arrive at the number of antennas in each array and the large technological progress in the field of receivers, with the relative advantages both in terms of radio image quality and richness of detail ».  

A glimpse of the group of 64 antennas that make up MeerKat, in the Karoo desert, South Africa. Credits: Inaf / E. Bags

Inaf is a top player in radio astronomy worldwide: what is the role of Italian researchers in this survey?

“This collaboration was born thanks to Radio Sky 2020, a bilateral scientific and technological cooperation project between Italy and South Africa financed by the Ministry of Foreign Affairs and International Cooperation and by the National Research Foundation, having among its main drivers the exploitation of the scientific potential of MeerKat. The study of the origin and evolution of radio galaxies has been one of INAF’s leading researches since the 1970s, thanks to the important surveys of the northern hemisphere sky conducted with the Croce del Nord radio telescope., which is located at the Medicine Radio Astronomy Station. Thanks to the bilateral project, the conditions were created to deploy our scientific skills, which were fundamental in all phases of the project, from the preparation of the first observing time requests, to the interpretation of the results and the drafting of the article. , where the contribution of INAF was fundamental. The article represents the first piece of this new survey. Other observations with MeerKat are planned shortly ».

With the advent of radio telescopes such as those of the Ska project, how will the study of radio galaxies change?

«It is a question that is not easy to give a univocal answer. Recent experience shows that the most interesting results of the new observers were not foreseen. The antennas of the Ska project will have a much higher sensitivity than the best interferometers currently in operation, and a high ability to see details on many angular scales. This will make it possible to observe the population of radio galaxies up to still inaccessible distances, studying their cosmological evolution and other important aspects of the evolution of galaxies and radio galaxies, for now only hypothetical “. 

Featured image: In this graphic elaboration of the radio galaxy CGCG044-046 the radio data collected by the researchers with the MeerKat antennas at 1.28 GHz (in white) have been highlighted. In the background, the optical data of the Digital Sky Survey 2 in the infrared, red and blue bands. Credits: T. Venturi / O. Smirnov / MNRAS 2021


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