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The galaxy with a wagging tail

One of the most massive galaxies at the centre of the Abell 1775 cluster has a 2.5 million light-year long tail – twice as long as previously thought – that appears to be “wagging”. The discovery was made by an international team of researchers, including several from the Italian National Institute for Astrophysics (INAF), analysing images of the cluster, located nearly one billion light-years away, obtained with the European LOFAR radio telescope and NASA’s Chandra X-ray space observatory

Published by the editorial team, 21 May 2021

Galaxy clusters are the most massive structures in the Universe. They enclose hundreds to thousands of galaxies held together by gravity. These orbit within clusters at impressive speeds, moving up to thousands of kilometres in just one second. The space through which they move is permeated by an extremely rarefied gas, with temperatures up to tens or hundreds of millions of degrees, reaching up to ten million light-years across. Astronomers resort to high-energy satellites to observe the gas that pervades galaxy clusters, in particular at X-ray energies, a type of radiation that is absorbed by Earth’s atmosphere. These observations reveal important information not only about galaxy clusters but also about the formation of some strange sources that were recently discovered by analysing these objects in the opposite portion of the electromagnetic spectrum: radio waves.

An international team of researchers has combined observations in these two portions of the spectrum to study galaxy cluster Abell 1775, unveiling previously undetected details in this system, located just under one billion light years from us. In radio waves, the team used data from three different radio telescopes, including LOFAR (Low Frequency Array). LOFAR is a large instrument consisting of thousands of antennas distributed across the Netherlands as well as various European countries, and is managed by ASTRON, the Netherlands Institute for Radio Astronomy, together with an international consortium of which INAF is also a member. In X-rays, they pointed NASA’s Chandra satellite towards the cluster for over one day in a row.

Previous radio observations had revealed one of the most spectacular phenomena in Abel 1775: a peculiar galaxy, with a morphology that astronomers call “head-tail”. This galaxy is one of the fastest in the cluster and hosts at its core an “active” black hole, which eats the surrounding matter at a swift pace and ejects some via jets that shine brightly in radio waves. Due to the galaxy’s high speed and the pressure the surrounding hot gas exerts on it, these jets are “bent” in the vicinity of the black hole, creating the “tail” – an extremely long trail of electrons and magnetic fields.

Radio emission from the huge “head-tail” galaxy at the centre of the Abell 1775 cluster, observed with LOFAR (in red) and overlaid on an optical image. Credits: LOFAR / Pan-STARRS / Botteon et al. 2021.

“In our study we realised that the tail of this galaxy in the Abell 1775 cluster spans about 2.5 million light-years: it is one of the largest ever observed, and twice as long as detected with previous observations,” says Andrea Botteon from Leiden University in the Netherlands and INAF associate, who is the lead author of the paper describing the results, published in Astronomy & Astrophysics. “This discovery was possible especially thanks to LOFAR, which explores the radio sky at wavelengths around two metres and is therefore sensitive to radiation emitted by the region farthest from the ‘head’, where the black hole giving rise to the jets is located”.


The astronomers then realised that the newly revealed region in the tail arises close to a point where the electron and magnetic field trail seems to break. It is here that the tail slightly changes orientation, as if the galaxy “wagged its tail”.

“Analysing the X-ray data from Chandra, we found that this transition point coincides with a region where the hot gas has a sudden change in density, and we believe that this is the origin of its tail-wagging”, adds co-author Fabio Gastaldello from INAF in Milan. “We think that the density jump is caused by the gas motions within the cluster Abell 1775. These motions also produced a structures with a mushroom-like morphology and forced the local gas into a spiral. Our extensive data processing, using the latest techniques, was required to bring out these structures”.


Hot gas at the centre of galaxy cluster Abell 1775 observed in X-rays with the Chandra space observatory (left). The images at the centre and right have been obtained using special filters to make the mushroom-like structure, density jump and gas spiral stand out. Credits: NASA / Chandra / Botteon et al. 2021.

According to the new study, the gas motions inside the cluster would be responsible for other structures detected by observing Abell 1775 in radio waves, such as the two filaments located near the head-tail galaxy. The large amount of data gathered with instruments as powerful as LOFAR and Chandra has also enabled the researchers to study in great detail the phenomena that contribute to accelerating electrons both in this galaxy’s tail and in the centre of the cluster.

“With its very high sensitivity and angular resolution in the frequency range of hundreds of MHz, LOFAR is contributing in a significant way to our understanding of radio emission from galaxies and galaxy clusters”, comments co-author Tiziana Venturi, who is the director of INAF’s Istituto di Radioastronomia in Bologna. “Italy has recognised from the beginning the revolutionary potential of this instrument and has been a member of the LOFAR consortium for years. Soon, a station of this radio telescope will be built at the Medicina Radio Astronomical Station and will be part of what is known as the International LOFAR Telescope”.

More information

The paper “Nonthermal phenomena in the center of Abell 1775: An 800 kpc head-tail, revived fossil plasma and slingshot radio halo”, by A. Botteon, S. Giacintucci, F. Gastaldello, T. Venturi, G. Brunetti, R. J. van Weeren, T. W. Shimwell, M. Rossetti, H. Akamatsu, M. Brüggen, R. Cassano, V. Cuciti, F. de Gasperin, A. Drabent, M. Hoeft, S. Mandal, H. J. A. Röttgering and C. Tasse is published online in the journal Astronomy & Astrophysics.

DOI: 10.1051/0004-6361/202040083



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