The Galaxy That Wags Its Tail (Cosmology)

It has a tail long two and a half million light years – twice as long as previously thought – and it even seems to “wag” one of the most massive galaxies at the center of the cluster Abell 1775, almost a billion light years away from us. This was discovered by an international team that includes several INAF researchers by analyzing images collected with the European Lofar radio telescope and NASA’s Chandra X-ray satellite.

The Galaxy clusters are the most massive structures in the universe. They enclose hundreds to thousands of galaxies bound together by the force of gravity. These orbit inside clusters with remarkable speed, so much so that they can move even thousands of kilometers in a single second. The space in which galaxies make their orbits is permeated by an extremely rarefied gas that reaches temperatures of tens or even hundreds of millions of degrees and extends for tens of millions of light years.

To study the gas that pervades the clusters of galaxies, astronomers have to resort to satellites that scan the sky in high energy, particularly in X-rays , a radiation that is absorbed by the Earth’s atmosphere. These observations reveal important information not only about clusters of galaxies but also about the formation of some strange sources recently discovered by analyzing these objects on the opposite side of the electromagnetic spectrum, in the radio band .

An international team of researchers combined observations in these two bands to study the Abell 1775 galaxy cluster , uncovering previously unseen details in this system that is just under a billion light-years from Earth. In the radio band, the group used data from three different radio telescopes including Lofar ( Low Frequency Array ), a large instrument made up of thousands of antennas distributed in the Netherlands and several European countries, managed by the Dutch Astron institute together with a international consortium which also includes the National Institute of Astrophysics. In X-rays, they pointed NASA’s Chandra satellite continuously for over a day at the cluster.

In the past, radio observations had revealed one of Abell 1775’s most spectacular phenomena: the presence of a particular galaxy , with a morphology that astronomers call ” head-tail “. This galaxy is one of the fastest in the cluster and hosts an “active” black hole at its center , which swallows the surrounding matter at a sustained rate and at the same time expels a part of it in the form of jets with strong radio band emission. Due to the galaxy’s high speed and the pressure exerted on it by the surrounding hot gas, these jets “bend” near the black hole, forming the “tail”, which is a very long trail of electrons and magnetic fields.

“In this study we found that the tail of this galaxy in the Abell 1775 cluster has an extension of about 2.5 million light years, one of the largest ever observed, and double that of past observations,” he explains. Andrea Botteon , researcher at the University of Leiden, in the Netherlands, and associate Inaf, first author of the article describing the results, published in the journal Astronomy & Astrophysics . “This discovery was made possible especially thanks to Lofar who, scanning the radio sky at wavelengths of about two meters, is sensitive to the radiation emitted in the region farthest from the ‘head’, where the black hole that generates the radio jets ».

The hot gas at the center of the Abell 1775 galaxy cluster observed in the X-band with the Chandra satellite. Note the mushroom structure, the density jump and the spiral structure of the gas. Credits: Nasa / Chandra / Botteon et al. 2021

The astronomers then realized that the tail region revealed by the new observations arises near a point where the trail of electrons and magnetic fields seems to break. This is where the tail changes direction slightly , as if the galaxy is “wagging its tail”.

“By analyzing Chandra’s X-ray data, we found that this transition point coincides with a region where the hot gas has a sharp change in density, and we believe that this is precisely the cause of its tail wagging,” adds Fabio Gastaldello , researcher Inaf in Milan and co-author of the study. “We think that the density jump is due to the gas motions inside Abell 1775, also highlighted by a ‘mushroom’ structure and a gas spiral in the center of the cluster, which we were able to bring out using particular techniques for processing the images”.

According to the new study, the movements of the hot gas inside the cluster would be responsible for the formation of other structures discovered by observing Abell 1775 in the radio band, such as the two filaments that are located near the head-tail galaxy. The large amount of data collected with two powerful tools such as Lofar and Chandra also allowed researchers to study in great detail the phenomena that contribute to accelerating electrons both in the tail of this galaxy and in the central region of the cluster.

“Lofar, with its very high sensitivity and angular resolution at frequencies of the order of one hundred MHz, is contributing substantially to our understanding of radio emission from galaxies and galaxy clusters”, comments co-author Tiziana Venturi , director of the Institute of Radioastronomy of INAF in Bologna. «Italy immediately saw the revolutionary potential of this instrument, and it has been part of the Lofar consortium for years. Soon a station of this radio telescope will be built at the Medicine Radio Astronomy Station and will be part of what is known as the International Lofar Telescope ».

Featured image: The radio emission from the huge “head-to-tail” galaxy at the center of the Abell 1775 cluster, observed by Lofar (in red) and superimposed on an optical band image. Credits: Lofar / Pan-Starrs / Botteon et al. 2021


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