Tag Archives: #galaxy

Cosmic Galaxy Assembly and the Evolution of Metals (Cosmology)

Astronomers refer to all the elements heavier than helium as “metals,” even elements that are typically found in gaseous form. In the big bang only hydrogen and helium (and a trace of lithium) were created while the “metals” were all made subsequently in stellar processes. The abundance of metals in the interstellar medium (ISM) of galaxies – the metallicity of the galaxies – thus quantifies the collective stellar processes that govern galactic evolution. The metallicity of the gaseous phase of the ISM (excluding particulates) has been found to be closely related to the history of a galaxy’s star formation and can be determined using optical spectroscopic observations of atomic lines, especially bright ones from ionized oxygen and neon. Another pivotal process in setting the metallicity is gas flow both out of the galaxy, driven by supernovae or other processes, and into the galaxy from the intergalactic medium.

How the metallicity of galaxies has evolved over cosmic time has become one of the most interesting questions in cosmology because it traces how stars have influenced the elemental composition of the universe in the roughly thirteen billion years since they first appeared, roughly a hundred or more million years after the big bang. CfA astronomer Mojegan Azadi is a member of team performing the MOSDEF survey, a four year program using the Multi-Object Spectrometer For Infrared Exploration (MOSFIRE) on the Keck I telescope to obtain optical spectra of about 450 galaxies in epochs from 1.7 to 4.5 billion years after the big bang. The astronomers measured the metallicity of each galaxy in the sample and concluded that many of the relations involving metallicity in the local universe also apply at these earlier times. For example, the relationship between a galaxy’s metallicity and stellar mass is about the same, as is the correlation between metallicity and star formation rate. These important new results signal that the processes that govern the growth of the element abundances in galaxies, whether by gas flows or star formation, have held in about the same form for at least the past twelve billion years.

Featured image: A schematic of the evolution of the universe. Astronomers have measured the elemental abundances of galaxies that date from about 1.7 to 4.5 billion years after the big bang, and found that overall the processes producing the elements follow about the same scaling relationships as seen in the local universe. © Argelander-Institut für Astronomie


Reference: “The MOSDEF Survey: The Evolution of the Mass–Metallicity Relation from z=0 to z∼3.3,” Ryan L. Sanders, Alice E. Shapley, Tucker Jones, Naveen A. Reddy, Mariska Kriek, Brian Siana, Alison L. Coil, Bahram Mobasher, Irene Shivaei, Romeel Davé, Mojegan Azadi, Sedona H. Price, Gene Leung, William R. Freeman, Tara Fetherolf, Laura de Groot, Tom Zick, and Guillermo Barro, The Astrophysical Journal 914, 19, 2021.


Provided by CFA Harvard

Researchers Created The Most Detailed Map Ever Of The Spiral Structure Of The Milky Way (Cosmology)

An international team led by researchers from the National Institute of Astrophysics has created the most detailed map ever produced of the spiral structure of the Milky Way within 16,000 light years of the Sun, using the position and brightness of about 600,000 young stars measured by the Gaia satellite. of the European Space Agency

The Milky Way, a galaxy of which the Sun is part along with hundreds of billions of other stars, is formed by a disk in which most of these stars reside, arranged in the form of a spiral . Since the 1950s, astronomers have been trying to define the number and structure of the “arms” of this spiral, which is no easy feat due to our position as observers within the disk itself. This scenario is now changing thanks to Gaia , the satellite of the European Space Agency which is carrying out a cosmic census of the positions, distances and motions of nearly two billion stars with unparalleled precision.

The new map, based on the latest data from Gaia , released last December, indicates that the geometric shape of the spiral arms of the Milky Way is different from that predicted by many of the models proposed in the past. The results concern the arm of Perseus , one of the main structures that form the galactic spiral, and the local arm , a less pronounced structure, where – among other stars – is also the Sun.

«The local arm appears to be more extensive than previously thought, reaching an extension of at least 26 thousand light years. The Perseus arm, on the other hand, has a different geometry compared to that envisaged by many previous models, with a larger opening angle “, explains Eloisa Poggio , first author of the article published in Astronomy & Astrophysics , researcher at the Côte d’Azur Observatory, in France, and associated with the National Institute of Astrophysics (INAF) of Turin.

The study is based on the largest sample of young stars – about 600,000 – ever used to map the spiral arms within 16,000 light-years of the Sun, a distance equal to about one-sixth the diameter of the Milky Way, which measures 100,000 light-years. . By analyzing the spatial distribution of populations of young stars in the galaxy’s disk, the team created maps of areas that are more or less star-rich than average density. The densest areas, also called “overdensity”, appear organized in a coherent way, tracing the segments of the spiral arms closest to the solar system.

“This study would not have been possible before Gaia,” adds co-author Ronald Drimmel of INAF of Turin. “Gaia has provided us with precise position measurements for an unprecedented amount of data, with a number of objects that have enough statistics to be able to apply the overdensity mapping method in the disk.”

The team compiled the map using mainly young, bright stars, which map out the areas where star-forming activity is most active, such as the spiral arms. In addition to these, they also analyzed the distribution of other young stellar components, such as variable stars called Cepheids and open star clusters.

The new map with the names of the main structures that form the galactic spiral. As the dotted line indicates, the new study redraws the geometry of Perseus’ arm. Credits: Poggio et al. 2021

“We know that the Milky Way is a spiral galaxy,” Poggio emphasizes. “However, even if this may seem paradoxical, in our galaxy we do not know exactly how many spiral arms there are, where they are located exactly, what their geometric shape is, because the Solar System is immersed in the galactic disk, making the mapping of the galaxy ‘on a large scale’ much more difficult. Yet having a map of the spiral arms is crucial for many aspects: for example to study the different physical phenomena that occur in the disk in relation to them ».

The new galactic spiral map in our cosmic neighborhood represents a first step towards understanding these structures, whose origin and dynamic nature have long been – and still are – much debated.

“Our study helps to delineate an increasingly detailed map of the spiral structure of the Milky Way,” concludes Drimmel. “With the future data of Gaia we plan to extend this map to greater distances, and to compare the position of the spiral arms obtained with the motions of the stars in the disk.”

To know more:

  • Read on Astronomy & Astrophysics the article ” Galactic spiral structure revealed by Gaia EDR3 “, by E. Poggio, R. Drimmel, T. Cantat-Gaudin, P. Ramos, V. Ripepi, E. Zari, R. Andrae, R . Blomme, L. Chemin, G. Clementini, F. Figueras, M. Fouesneau, Y. Frémat, A. Lobel, DJ Marshall, T. Muraveva and M. Romero-Gómez

Watch Claudia Mignone’s interview with Eloisa Poggio on MediaInaf Tv :

Featured image: The new map of our cosmic neighborhood: the areas in red show the regions richest in stars compared to the average density, and the areas in blue show the regions poorest in stars compared to the average. The black curves indicate the spiral structure of the Milky Way. Distances are given in kiloparsecs: 1 kpc = 3,260 light-years (click to enlarge). Credits: Poggio et al. 2021


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Hubble Views a Faraway Galaxy Through a Cosmic Lens (Cosmology)

The center of this image from the NASA/ESA Hubble Space Telescope is framed by the tell-tale arcs that result from strong gravitational lensing, a striking astronomical phenomenon which can warp, magnify, or even duplicate the appearance of distant galaxies. 

Gravitational lensing occurs when light from a distant galaxy is subtly distorted by the gravitational pull of an intervening astronomical object. In this case, the relatively nearby galaxy cluster MACSJ0138.0-2155 has lensed a significantly more distant inactive galaxy – a slumbering giant known as MRG-M0138 which has run out of the gas required to form new stars and is located 10 billion light-years away. Astronomers can use gravitational lensing as a natural magnifying glass, allowing them to inspect objects like distant dormant galaxies which would usually be too difficult for even Hubble to resolve.

This image was made using observations from eight different infrared filters spread across two of Hubble’s most advanced astronomical instruments: the Advanced Camera for Surveys and the Wide Field Camera 3. These instruments were installed by astronauts during the final two servicing missions to Hubble and provide astronomers with superbly detailed observations across a large area of sky and a wide range of wavelengths.

Featured image: The center of this image from the NASA / ESA Hubble Space Telescope is framed by the tell – tale arcs that result from strong gravitational lensing. Credit: ESA/Hubble & NASA, A. Newman, M. Akhshik, K. Whitaker


Provided by NASA

A Large Tidal Stream Observed in the Sombrero Galaxy (Cosmology)

According to the latest cosmological models, large spiral galaxies such as the Milky Way grew by absorbing smaller galaxies, by a sort of galactic cannibalism. Evidence for this is given by very large structures, the tidal stellar streams, which are observed around them, which are the remains of these satellite galaxies. But the full histories of the majority of these cases are hard to study, because these flows of stars are very faint, and only the remains of the most recent mergers have been detected.

A study led by the Instituto de Astrofísica de Andalucía (IAA-CSIC), with the participation of the Instituto de Astrofísica de Canarias (IAC), has made detailed observations of a large tidal flow around the Sombrero galaxy, whose strange morphology has still not been definitively explained. The results are published today in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

The Sombrero galaxy (Messier 104) is a galaxy some thirty million light years away, which is part of the Local Supercluster (a group of galaxies which includes the Virgo cluster and the Local Group containing the Milky Way). It has roughly one third of the diameter of the Milky Way, and shows characteristics of both of the dominant types of galaxies in the Universe, the spirals and the ellipticals. It has spiral arms, and a very large bright central bulge, which makes it look like a hybrid of the two types.

“Our motive for obtaining these very deep images of the Sombrero galaxy (Messier 104) was to look for the remains of its merger with a very massive galaxy. This possible collision was recently suggested on the basis of studies of the stellar population of its strange halo obtained with the Hubble Space Telescope”, says David Martínez-Delgado, a researcher at the IAA-CSIC and first author of the paper reporting the work.

Artist's conception of the tidal stream of the Sombrero galaxy (M104). Credit: Jon Lomberg for the Stellar Tidal Stream Survey.
Artist’s conception of the tidal stream of the Sombrero galaxy (M104). Credit: Jon Lomberg for the Stellar Tidal Stream Survey.

The observations with the Hubble, in 2020, showed that the halo, an extensive and faint region surrounding the Sombrero galaxy, shows many stars rich in metals, elements heavier than hydrogen and helium. This is a feature to typical of new generations of stars, which are normally found in the discs of galaxies, and are quite unusual in galactic halos, which are populated by old stars. To explain their presence astronomers suggested what is known as “a wet merger”, a scenario in which a large elliptical galaxy is rejuvenated by large quantities of gas and dust from another massive galaxy, which went into the formation of the disc which we now observe.

“In our images we have not found any evidence to support this hypothesis, although we cannot rule out that it could have happened several thousand million years ago, and the debris is completely dissipated by now -explains David Martínez-Delgado-. In our search we have in fact been able to trace for the first time the complete tidal stream which surrounds the disc of this galaxy, and our theoretical simulations have let us reconstruct its formation in the last three thousand million years, by cannibalism of a satellite dwarf galaxy”.

“Observational techniques in present day Astrophysics need advanced image processing. Our modelling of the bright stars around the Sombrero galaxy, and at the same time of the halo light of the galaxy itself has enabled us to unveil the nature of this tidal stream. It is remarkable that thanks to these advanced photometric techniques we have been able to do front line science with a Messier object using only an 18 cm (diameter) telescope”, explains Javier Román, a postdoctoral researcher at the IAC and a co-author of the study.

The research team rejects the idea that the large stellar tidal stream, known for more than three decades, could be related to the event which produced the strange morphology of the Sombrero galaxy which, if it was caused by a wet merger, would need the interaction of two galaxies with large masses.

The work has been possible thanks to the collaboration between professional and amateur astronomers. “We have collaborated with the Spanish astrophotographer Manuel Jiménez, who took the images with a robotic telescope of 18 centimetre diameter, and the well-known australian astrophotographer David Malin, who discovered this tidal stream on photographic plates taken in the 90’s of the last century. This collaboration shows the potential of amateur telescopes to take deep images of nearby galaxies which give important clues about the process of their assembly which is continuing until the present epoch”, concludes Martínez-Delgado.

Featured image: Sombrero galaxy (M104) © Manuel Jiménez/Giuseppe Donatiello


Article: D. Martínez Delgado et al. “A feather on the hat: Tracing the giant stellar stream around the Sombrero galaxy”. MNRAS, July 21, 2021. DOI: https://doi.org/10.1093/mnras/stab1874


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A Star in a Distant Galaxy Blew Up in A Powerful Explosion, Solving An Astronomical Mystery (Planetary Science)

Giant explosion in space illuminates thousand-year mystery

Dr. Iair Arcavi, a Tel Aviv University researcher at the Raymond and Beverly Sackler Faculty of Exact Sciences, participated in a study that discovered a new type of stellar explosion – an electron-capture supernova. While they have been theorized for 40 years, real-world examples have been elusive. Such supernovas arise from the explosions of stars 8-9 times the mass of the sun. The discovery also sheds new light on the thousand-year mystery of the supernova from A.D. 1054 that was seen by ancient astronomers, before eventually becoming the Crab Nebula, that we know today.

A supernova is the explosion of a star following a sudden imbalance between two opposing forces that shaped the star throughout its life. Gravity tries to contract every star. Our sun, for example, counter balances this force through nuclear fusion in its core, which produces pressure that opposes the gravitational pull. As long as there is enough nuclear fusion, gravity will not be able to collapse the star. However, eventually, nuclear fusion will stop, just like gas runs out in a car, and the star will collapse. For stars like the sun, the collapsed core is called a white dwarf. This material in white dwarfs is so dense that quantum forces between electrons prevent further collapse.

For stars 10 times more massive than our sun, however, electron quantum forces are not enough to stop the gravitational pull, and the core continues to collapse until it becomes a neutron star or a black hole, accompanied by a giant explosion. In the intermediate mass range, the electrons are squeezed (or more accurately, captured) onto atomic nuclei. This removes the electron quantum forces, and causes the star to collapse and then explode.

Historically, there have been two main supernova types. One is a thermonuclear supernova — the explosion of a white dwarf star after it gains matter in a binary star system. These white dwarfs are the dense cores of ash that remain after a low-mass star (one up to about 8 times the mass of the sun) reaches the end of its life. Another main supernova type is a core-collapse supernova where a massive star — one more than about 10 times the mass of the sun — runs out of nuclear fuel and has its core collapsed, creating a black hole or a neutron star. Theoretical work suggested that electron-capture supernovae would occur on the borderline between these two types of supernovae.

That’s the theory that was developed in the 1980’s by Ken’ichi Nomoto of the University of Tokyo, and others. Over the decades, theorists have formulated predictions of what to look for in an electron-capture supernova. The stars should lose a lot of mass of particular composition before exploding, and the supernova itself should be relatively weak, have little radioactive fallout, and produce neutron-rich elements.

The new study, published in Nature Astronomy, focuses on the supernova SN2018zd, discovered in 2018 by Japanese amateur astronomer Koihchi Itagaki. Dr. Iair Arcavi, of the astrophysics department at Tel Aviv University, also took part in the study. This supernova, located in the galaxy NGC 2146, has all of the properties expected from an electron-capture supernova, which were not seen in any other supernova. In addition, because the supernova is relatively nearby – only 31 million light years away – the researchers were able to identify the star in pre-explosion archival images taken by the Hubble Space Telescope. Indeed, the star itself also fits the predictions of the type of star that should explode as an electron-capture supernovae, and is unlike stars that were seen to explode as the other types of supernovae.

While some supernovae discovered in the past had a few of the indicators predicted for electron-capture supernovae, only SN2018zd had all six – a progenitor star that fits within the expected mass range, strong pre-supernova mass loss, an unusual chemical composition, a weak explosion, little radioactivity, and neutron-rich material. “We started by asking ‘what’s this weirdo?'” said Daichi Hiramatsu of the University of California Santa Barbara and Las Cumbres Observatory, who led the study. “Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario.”

The new discoveries also illuminate some mysteries of one of the most famous supernovae of the past. In A.D. 1054 a supernova happened in our own Milky Way Galaxy, and according to Chinese and Japanese records, it was so bright that it could be seen in the daytime and cast shadows at night. The resulting remnant, the Crab Nebula, has been studied in great detail, and was found to have an unusual composition. It was previously the best candidate for an electron-capture supernova, but this was uncertain partly because the explosion happened nearly a thousand years ago. The new result increases the confidence that the historic 1054 supernova was an electron-capture supernova.

“It’s amazing that we can shed light on historical events in the Universe with modern instruments,” says Dr. Arcavi. “Today, with robotic telescopes that scan the sky in unprecedented efficiency, we can discover more and more rare events which are critical for understanding the laws of nature, without having to wait 1000 years between one event and the next.”

Dr. Arcavi is a member of the Global Supernova Project, and makes use of the Las Cumbres telescope network to study rare transient phenomena like supernovae, neutron star mergers, and stars torn apart by black holes.

Link to the original article: https://www.nature.com/articles/s41550-021-01384-2

Featured image: Hubble Space Telescope color composite of the electron-capture supernova 2018zd and the host starburst galaxy NGC 2146 © NASA/STScI/J. DePasquale; Las Cumbres Observatory


Reference: Hiramatsu, D., Howell, D.A., Van Dyk, S.D. et al. The electron-capture origin of supernova 2018zd. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01384-2


Provided by Tel-Aviv University

Which Profile Is More Appropriate To Represent Dark Matter Haloes & Estimating Galaxy Total Mass? (Cosmology)

Rotation curves are major tools for determining the dynamical mass distribution in the Milky Way and spiral galaxies. Discoveries of extended rotation curves have suggested the presence of dark matter in spiral galaxy haloes. It has led to many studies that estimated the galaxy total mass, mostly by using the Navarro Frenk and White (NFW) density profile.

Now, Jiao and colleagues aimed at verifying how the choice of the dark-matter profile may affect the predicted values of extrapolated total masses.

They have considered the recent Milky Way rotation curve (MW RC) for two reasons, firstly because of its unprecedented accuracy, and secondly because the Galactic disk is amongst the least affected by past major mergers having fully reshaped the initial disk.

They found that, for calculating the dark-matter contribution to the Milky Way rotation curve, the use of NFW profile (or its generalized form, gNFW) generates apparently inconsistent results, e.g., an increase of the baryonic mass leads to increase of the dark matter mass.

In addition, it has been found that NFW and gNFW profile narrow the total mass range, leading to a possible methodological bias particularly against small milky way (MW) masses.

Fig. 1. Top: Contribution to the rotation curve of different baryonic models and model components. Red points indicate the rotation curve of the Milky Way. The error-bars are estimated via bootstrapping and include the systematic uncertainties from the neglected term (see text). Bottom: Fit of the rotation curve by the best-fit model (solid blue curve, total mass of 2.6 × 10¹¹ M), and with the most massive MW model for which the χ² probability reaches P=0.05 (orange dash-dotted line, total mass of 18 × 10¹¹ M), both associated to the baryonic distribution from model I of Pouliasis et al. © Jiao et al.

Finally, they suggested, the use of Einasto profile is more appropriate to represent cold dark matter haloes and found that the Milky Way slightly decreasing rotation curve favors total mass that can be as small as 2.6 ×10¹¹ M, disregarding any other dynamical tracers further out in the Milky Way. It is inconsistent with values larger than 18 ×10¹¹ M for any kind of cold dark matter (CDM) halo profiles, under the assumption that stars and gas do not influence the predicted dark matter distribution in the Milky Way.

“Our study encourages the use of the Einasto profile for characterizing rotation curves with the aim of evaluating their total masses.”

— concluded authors of the study

Reference: Yongjun JIAO, Francois HAMMER, Jianling WANG, Yanbin YANG, “Which Milky Way masses are consistent with the slightly declining 5-25 kpc rotation curve?”, Arxiv, pp. 1-10, 2021. https://arxiv.org/abs/2107.00014


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Hubble Sees a Spiral in Good Company (Cosmology)

This image, taken with Hubble’s Wide Field Camera 3, features the spiral galaxy NGC 4680. Two other galaxies, at the far right and bottom center of the image, flank NGC 4680. NGC 4680 enjoyed a wave of attention in 1997, as it played host to a supernova explosion known as SN 1997bp. Australian amateur astronomer Robert Evans identified the supernova and has identified an extraordinary 42 supernova explosions. 

NGC 4680 is actually a rather tricky galaxy to classify. It is sometimes referred to as a spiral galaxy, but it is also sometimes classified as a lenticular galaxy. Lenticular galaxies fall somewhere in between spiral galaxies and elliptical galaxies. While NGC 4680 does have distinguishable spiral arms, they are not clearly defined, and the tip of one arm appears very diffuse. Galaxies are not static, and their morphologies (and therefore their classifications) vary throughout their lifetimes. Spiral galaxies are thought to evolve into elliptical galaxies, most likely by merging with one another, causing them to lose their distinctive spiral structures.

Text credit: European Space Agency (ESA)
Image credit: ESA/Hubble & NASA, A. Riess et al.


Provided by NASA

A “Missing” Galaxy Immortalized by the VST (Cosmology)

The photo of the week of the European Southern Observatory is an image obtained from Chile with the Vst telescope (made by INAF) of the ultra-diffuse galaxy Udg 4. The image was acquired as part of a program for the study of structures very faint in the clusters of galaxies, Vegas, led by Enrichetta Iodice of the INAF of Naples

If you look closely at the faint and blurred center of the image opposite, you can see a “ghost” galaxy – the Udg 4, with a not-so-creepy name – captured by ESO’s Vlt Survey Telescope ( Vst ).

The acronym Udg stands for ultra-diffuse galaxy : objects as large as the Milky Way but with one hundred to one thousand times fewer stars. These galaxies are extremely faint and lack gas to form stars, which makes them look almost like a soft cosmic cloud or patch in space. Their origins remain uncertain, but astronomers speculate they may be “missing” galaxies that lost gas early in their life.

This Udg 4 image was taken as part of a study by a much larger program, the Vst Early-type Galaxy Survey (Vegas), which aims to investigate very faint structures in galaxy clusters : large groups of many galaxies bound together. by gravity. The study – led by Vegas principal investigator Enrichetta Iodice of the National Institute of Astrophysics, and published last year in Astronomy & Astrophysics  – found several ultra-diffuse galaxies in the Hydra cluster .

Several INAF astronomers from Naples, Teramo and Padua participate in the study. In the photo the young Neapolitan researchers and students of the Vegas team. From left: Marco Mirabile, Marilena Spavone, Enrichetta Iodice (principal investigator of Vegas), Rossella Ragusa, Antonio La Marca and Giuseppe D’Ago. Credits: Inaf

“Further observations are necessary to clarify their true nature”, Iodice underlines to Media Inaf . «This is part of a large-scale project that aims to detect a large number of Udg in all images of Vegas. For the new Udg sample, we aim to obtain new spectroscopic data in order to study stellar populations and their dark matter content ».

Given their fragile appearance, UDGs can be difficult to spot. However, the VST, equipped with its OmegaCam camera , provides extraordinary sensitivity to light, allowing astronomers to study such elusive objects.

Featured image: The ultra-diffuse galaxy Udg 4 seen from the Vst. Credits: Eso / Iodice et al.


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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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