Chandra X-Ray Observatory Helps Astronomers Weigh Most Massive Galaxy Cluster Discovered to Date

A composite, multi-wavelength image of the young galaxy cluster XDCP J0044.0-2033, also known as the Gioiello Cluster, which is located 9.6 billion light-years away. In the image, X-ray wavelengths appear purple, infrared appear as large red halos around some galaxies, and visible light appears as are red, green, and blue. Image Credit: X-ray: NASA/CXC/INAF/P.Tozzi, et al; Optical: NAOJ/Subaru and ESO/VLT; Infrared: ESA/Herschel

One thing the science of astronomy isn’t short on are superlatives: The larger, the most massive, and the most distant are some of the adjectives most often used when describing the properties of various celestial objects. In keeping with this long-standing tradition, a European team of astronomers using data from NASA’s Chandra X-ray Observatory has recently announced the results of its study of the distant galaxy cluster XDCP J0044.0-2033, which was found to be the most massive one detected to date, allowing scientists to shed more light on the formation and distribution of these large-scale cosmic structures in the early Universe.

Due to the fleeting nature of our existence against the passage of cosmic time, it is very easy to consider the Universe as a static, largely inactive, and unchanging place. Yet the scientific study of the physical reality during the last couple of centuries has slowly awakened us to the true nature of the Cosmos, which from the smallest to the largest of scales conforms to what the ancient Greek philosopher Heraclitus had described around 500 BCE as ‘panta rhei’ or “everything flows.” Our views of a largely immobile Universe, as captured by our ground- and space-based telescopes, in reality constitute just single, individual frames from the greater, ongoing dance of cosmic evolution and change, whose processes greatly exceeds the comparatively very short timescales of our lives. Luckily, astronomers are aided in their efforts to better understand the Universe’s evolution as a whole by the fact that the speed of light is finite. Since it takes light a certain amount of time to travel a specified distance, the deeper scientists look into space, the further they probe back in time. This way, by observing as many objects as possible at varying cosmic distances, astronomers can study the early Universe and shed more light on the way it has evolved over time.

A plot of sky coordinates vs. distance for galaxies from the Sloan Digital Sky Survey. The earth is at the center, and each point represents a galaxy, typically containing about 100 billion stars. Galaxies are colored according to the ages of their stars, with the redder, more strongly clustered points showing galaxies that are made of older stars. The outer circle is at a distance of two billion light years. The region between the wedges was not mapped by the SDSS because dust in our own Galaxy obscures the view of the distant universe in these directions. Image Credit: M. Blanton and the SDSS

One of the biggest questions in astrophysics and cosmology today regarding the evolution of the Universe concerns the distribution of matter and its structure on the largest of scales. Very precise observations of the Universe’s overall energy-mass distribution, which had been conducted by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck space observatories during the last decade, have established that all the ordinary baryonic matter that can be directly observed—like planets, stars, and galaxies—constitutes just a little less than 5 percent of the Universe’s total. Approximately 25 percent is consisted of the hypothetical and as to date undetected so-called “dark matter,” with the equally mysterious “dark energy,” which is believed to be the cause of the observed accelerating expansion of the Universe, making up for the remaining 70 percent.

Furthermore, several galaxy redshift surveys that have taken place during the last three decades, like the Sloan Digital Sky Survey and the 2MASS Redshift Survey, have managed to make very detailed 3-D maps of the positions and motions of millions of galaxies over large areas of the sky. The results of these observations have been shown to be largely in accordance with the currently accepted Standard Cosmological Model, also known as the Lambda Cold Dark Matter or Lambda-CDM model, which posits that the Universe’s structure is hierarchical in nature. Stars are grouped together to form galaxies, which in turn form clusters and superclusters, and those superclusters are grouped together to create filaments—long threads of matter running through intergalactic space, interspersed with gigantic spaces of empty void. According to theoretical predictions, all the galaxies and galactic clusters that we observe today are just the luminous nodes of a vast interconnected network of cosmic filaments, which spawns the entire Universe. All the primordial dark matter that existed after the Big Bang has in time formed huge, invisible halos whose gravity attracted most but not all of the ordinary matter, making it to coalesce on top, like the icing on a cake. The leftovers of ordinary matter that weren’t pulled around these dark matter halos were dispersed as diffuse gas in the vast dark matter filaments that connect every single galaxy together. Thus, all the matter and energy in the Universe can be traced inside these gigantic cosmic filaments, which when observed from afar can be described as a cosmic “web.”

Based on the observational results from deep-sky surveys and the predictions of currently accepted theoretical models, scientists have constructed various computer simulations of the Universe’s evolution of unprecedented detail, allowing them to trace the structure and distribution of matter from the time of the Big Bang 13.8 billion years ago up to the present day. Yet, even though many dozens of galaxy clusters had been observed at distances of more than 10 billion light-years away, astronomers until recently lacked a more detailed characterisation of their thermal and chemical properties, due to the fact that the gas of hydrogen and helium plasma, which fills the intracluster medium between galaxy clusters and is heated to tens of millions of degrees, mostly emits in X-ray wavelengths which are obscured by the Earth’s atmosphere.

An image frame from the 3-dimensional visualization of the Millennium Simulation, showing the web-like structure of the Universe in the largest of scales. Image Credit: Springel et al. (Virgo Consortium)/The Millennium Simulation Project — An image frame from the 3-D visualization of the Millennium Simulation, showing the web-like structure of the Universe in the largest of scales. Image Credit: Springel et al. (Virgo Consortium)/The Millennium Simulation Project

The launch of next-generation space X-ray telescopes during the last two decades, like NASA’s Swift and Chandra X-ray observatories and the European Space Agency’s XMM-Newton, have ushered in a new era of X-ray astronomy, allowing scientists to observe the high-energy Universe in great detail and conduct comprehensive wide-field surveys of distant galaxy clusters, which are difficult to observe from the ground in optical wavelengths. While the results of these surveys have revealed the presence of many proto-clusters of galaxies in the distant Universe which appear to be still in their formation process, they have also led to the discovery of very large structures of immense galaxy superclusters like the Hercules–Corona Borealis Great Wall, which was discovered in November 2013 at a distance of approximately 10 billion light-years away. Spanning 45 degrees of the sky in the northern hemisphere, while covering no less than 16 constellations and measuring a whopping 10 billion by 7 billion light-years across, the Hercules–Corona Borealis Great Wall is the largest cosmic structure ever to be discovered, by far surpassing the size of the Sloan Great Wall of galaxies, which had been discovered 10 years earlier by the Sloan Digital Sky Survey project. Other notable additions to the list of surprisingly large and distant galaxy clusters are CL J1449+0856, which is located approximately 11 billion light-years away and has been found to be consisted of mature galaxies whose stars were at least one billion years old, and XMMUJ2235.3 2557, which lies 9 billion light-years away and whose mass was found to be greater than originally thought.

Now, in a new paper which was recently published on The Astrophysical Journal, a team of astronomers led by Dr. Paolo Tozzi from the National Institute for Astrophysics in Florence, Italy, have presented the results of their study of the galaxy cluster XDCP J0044.0-2033, also known as the Gioiello Cluster, which means “jewel” in Italian. Located at a distance of 9.6 billion light-years away at the direction of the northern constellation Cetus, it spans approximately 6.2 million light-years across, and since its discovery in 2011 by the XMM-Newton space telescope it has been found to have at least a dozen galaxy members. The researchers used the Chandra X-ray Observatory to conduct a deep-sky survey of the Gioiello Cluster between October and December 2011, in order to better characterise its dynamical properties and measure its absolute mass in the hopes of putting more constrains to the theoretical parameters of cosmological models. Chandra’s long exposure time of a total of 380,000 seconds, or 4.5 days-worth of observations, allowed scientists to study the X-ray emission of the Gioiello Cluster’s intracluster medium and determine that, despite its relatively young age of approximately 800 million years, the cluster’s overall mass was a staggering 400 trillion times the mass of the Sun, making it the most massive galaxy cluster discovered to date, surpassing the mass of such previous record-holders like the El Gordo Cluster.

A composite image of the El Gordo galaxy cluster, which until recently was the largest in the distant Universe to have been discovered. X-ray data from Chandra appear in blue, while optical data from the European Southern Observatory's Very Large Telescope in red, green, and blue, and infrared emission from the NASA's Spitzer Space Telescope in red and orange. Image Credit: X-ray: NASA/CXC/Rutgers/J.Hughes et al, Optical: ESO/VLT/Pontificia Universidad. Catolica de Chile/L.Infante & SOAR (MSU/NOAO/UNC/CNPq-Brazil)/Rutgers/F.Menanteau, IR: NASA/JPL/Rutgers/F.Menanteau — A composite image of the El Gordo galaxy cluster, which until recently was the largest in the distant Universe to have been discovered. X-ray data from Chandra appear in blue, while optical data from the European Southern Observatory’s Very Large Telescope in red, green, and blue, and infrared emission from the NASA’s Spitzer Space Telescope in red and orange. Image Credit: X-ray: NASA/CXC/Rutgers/J.Hughes et al, Optical: ESO/VLT/Pontificia Universidad. Catolica de Chile/L.Infante & SOAR (MSU/NOAO/UNC/CNPq-Brazil)/Rutgers/F.Menanteau, IR: NASA/JPL/Rutgers/F.Menanteau

Another interesting finding from the recent Chandra observations is the detection of two uneven clumps in the intracluster medium of the Gioiello Cluster, which exhibited a significant difference in their surface brightness, hinting at the Cluster’s dynamic nature and ongoing evolution. Nevertheless, the resolution needed to better characterize these features in the intracluster medium, exceeded that of Chandra. “A detailed analysis of the surface brightness distribution reveals the presence of two clumps, with marginally different temperatures, which may be due to a recent merger or to a young dynamical status,” write the researchers. “We also find some evidence for clumping in the surface brightness distribution on scales of approximately 40 kpc [130,000 light-years], which may be interpreted as the signature of a not fully relaxed dynamical status, possibly due to the young age of the cluster … However, deeper data would be needed before reaching firm conclusions. In fact, it is impossible to decide whether the elongation and the two-region structure are due to an ongoing merger between comparable mass halos or simply to a young dynamical status.”

The Gioiello Cluster is also the latest in the growing list of galaxy clusters, whose properties seem to challenge several predictions of the Standard Cosmological Model regarding the evolution of galaxies in the early Universe. More particularly, according to the Lambda Cold Dark Matter model, galaxy clusters in the early Universe should be much less massive and evolved than the ones detected by deep-sky surveys like the XMM-Newton Distant Cluster Project. Yet the presence of clusters like XMMUJ2235.3 2557 and the Gioiello Cluster indicate, according to several researchers in the scientific community, the possibility that several aspects of the Standard Cosmological Model might be flawed. “The hint that there might be problems with the standard model of cosmology is interesting,” says Dr. James Jee, a project physicist at the University of California in Davis and co-author of the recent study. “But we need bigger and deeper samples of clusters before we can tell if there’s a real problem.”

“It is premature to translate the rarity of XDCP0044 by itself into any tension with the current Lambda-CDM model paradigm,” comments Tozzi’s team. “However, it is still interesting to note that the relatively small XMM-Newton Distant Cluster Project survey have already discovered two extreme clusters: XDCP0044 and XMMUJ2235 at z=1.58 and 1.4 [9.6 and 9 billion light-years away] respectively … To summarize, there are hints that the detection of sparse, massive galaxy clusters at high redshift points toward inconsistencies of the standard CDM model or a significantly higher normalization parameter. However, more robust studies based on deep and complete samples of galaxy clusters are necessary to quantify the actual tension with the CDM.”

These latest observations by Chandra showcase the continuing importance of the X-ray observatory in the study of the distribution and large-scale structure of matter in the Universe, allowing scientists to answer fundamental questions regarding the latter’s overall evolution. “Finding this enormous galaxy cluster at this early epoch [in the age of the Universe] means that there could be more out there,” comments Tozzi. “This kind of information could have an impact on our understanding of how the large-scale structure of the Universe formed and evolved.”

Contrary to what we can perceive with our senses, the electronic eyes of our dedicated space-based telescopes are constantly observing the skies across the entire electromagnetic spectrum, revealing in the process all the surprising, dynamic processes of a wondrous and ever-changing Cosmos.