The galaxy Fornax A is probably the result of repeated nuclear activity

Fornax A with its lobes
Fornax A with its lobes

An article published in the journal “Astronomy & Astrophysics” reports a study of the galaxy known as Fornax A or as NGC 1316. A team of researchers led by Filippo Maccagni of the Italian National Institute of Astrophysics, Cagliari, used data collected by five different telescopes to find evidence of repeated nuclear activity that formed what were called radio lobes, gigantic plasma halos invisible at optical frequencies but bright at radio frequencies. For this reason, the MeerKat radio telescope was crucial in obtaining the best observations ever obtained by Fornax A.

Fornax A, also known by a series of abbreviations dependent on the catalog in addition to NGC 1316, is a lenticular galaxy distant just over 60 million light years from Earth. It’s part of the Fornax galaxy cluster and is a radio galaxy, a class of galaxies with strong radio emissions. The image (Courtesy F. Maccagni, D. Kleiner, INAF-OAC, Sarao. All rights reserved) shows Fornax A with the huge red radio halo that surrounds the galaxy with an extension of about one million light years. Above Fornax A another smaller galaxy is visible, cataloged as NGC 1317, more or less as large as the Milky Way.

The deformations present in the optical frequency images of the galaxy Fornax A indicate that it’s the result of various galactic mergers that occurred between 1 and 3 billion years ago. Its brightness at radio waves, however, makes it an object of study especially with radio telescopes and was recently included among the targets of the MeerKat Fornax Survey project, started in 2018 to study in detail the whole Fornax cluster.

The characteristics of the galaxy Fornax A were the subject of discussions regarding the processes taking place inside it. For example, a mystery concerned the abundance of dust in the materials that fills its interstellar space, of which it generally forms about 2% while the rest of those materials is composed of hydrogen and helium gas. According to a research published in the journal “Astronomy & Astrophysics” in August 2019, part of the hydrogen was hidden. Already in that case the MeerKat radio telescope offered the information that indicated a solution to the mystery, in the new research just published it gave more useful information to better understand the history of Fornax A.

Filippo Maccagni’s team added data from four other telescopes to those collected with the MeerKat radio telescope: the Murchison Widefield Array (MWA) in Australia, the Planck Surveyor space probe, the Very Large Array (VLA) in the USA and the Sardinia Radio Telescope (SRT), in Italy. This made it possible to obtain a picture of the galaxy Fornax A at different wavelengths that revealed traces of repeated nuclear activity. That’s because the radio lobes are formed by synchrotron radiation in which particles travel at speeds close to the speed of light in a trajectory that depends on the power of the magnetic field that determines it.

According to the researchers, the shape of the lobes and the properties of their synchrotron radiation at different frequencies indicate that they must have formed through more than one episode of nuclear activity. The last of these episodes might have started about 24 million years ago and might have continued for about 12 million years. A much more limited episode might have started about 3 million years ago and might have lasted about a million years, forming central jets that are visible thanks to the sensitivity of the MeerKat radio telescope.

One object of the research concerns the heating of interstellar gas and its consequence on star formation. What’s called in jargon the active galactic nucleus feedback is at the center of various research into the influence of supermassive black hole’s activity at the center of galaxies. Interstellar gas must be cold in order to coalesce and form new stars. In short, the study of the galaxy Fornax A is interesting for astronomers for various reasons and the contribution of the MeerKat radio telescope is proving to be really important.

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