4MOST Begins its Journey of Cosmic Discovery

The sky around the Sculptor Galaxy NGC 253 and the globular cluster NGC 288 was the target of the first observations with 4MOST. The blue frame shows the boundary of 4MOST's field of view. Each circle symbolises one of the more than 2400 fibres. The embedded images show the spectrum of a star (right) and the spectrum of a globular cluster in the Sculptor Galaxy (left). © AIP/R. de Jong, Centre de Recherche Astrophysique de Lyon/J.-K. Krogager, Background: Harshwardhan Pathak/Telescope Live

ESO’s astronomical facilities in Chile are hives of activity — or oases! — in the otherwise barren and arid landscape of the Atacama Desert. This hostile and hard-to-reach location may seem like an odd choice for construction, but the Atacama is one of the best sites in the world for astronomy. It has practically no cloud cover, a distinct lack of light pollution, and is the driest non-polar location in the world, receiving under two centimetres of rainfall every year! Chile has hosted ESO’s telescopes since the 1960s, in observatories based at La Silla, Paranal, and Chajnantor Plateau. Shown here is the Visible and Infrared Survey Telescope for Astronomy (VISTA), situated at the Paranal Observatory. Perched atop a mountain adjacent to Cerro Paranal, the home of the flagship Very Large Telescope (VLT), VISTA is the largest telescope in the world designed to survey the sky in near-infrared light (just beyond that visible to humans). The spectacular sights of the cosmos — including the notable streak of our home galaxy, the Milky Way, stretching across the top of the frame here — are more than enough to keep VISTA and its telescopic siblings busy. © ESO/B. Tafreshi (twanight.org)

 

The components of the 4MOST instrument at the VISTA telescope.
© 4MOST Consortium

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© AIP/R. de Jong, AIP/K. Riebe, AIP/A. Saviauk, CRAL/J.-K. Krogager
Video MP4 (235 mb)

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First light marks the start of an ambitious mission to decode the physical and chemical fingerprints of thousands of celestial objects at once.

On October 18, the 4-metre Multi-Object Spectroscopic Telescope (4MOST) facility, installed on the VISTA telescope at the European Southern Observatory’s (ESO) Paranal Observatory in Chile, obtained its first light. This milestone is a crucial step in the life of any telescope marking the moment the instrument is deemed ready to begin its scientific journey. 4MOST does not simply take images of the sky; it records spectra, capturing the light of each object in every individual colour. With this capability, it can unravel the light of 2,400 celestial objects simultaneously into 18,000 colour components, allowing astronomers to study their detailed chemical composition and physical properties. And scientists from the Max Planck Institute for Extraterrestrial Physics (MPE) play a key role in this project.

MPE was part of the 4MOST Consortium since the very beginning, and contributed both resources towards the construction of the spectrographs as well as leadership in the development of the Operations System, the complex software system that ensures efficient planning and execution of the 4MOST observations. The planning of observations is done remotely from MPE. The main scientific focus for MPE scientists is the ability of 4MOST to provide spectra and distance measurements (redshifts) for millions of X-ray sources detected by the eROSITA all-sky survey.

Andrea Merloni, PI of one of the 4MOST surveys devoted to the study of growing Supermassive Black Holes, and Local Project Manager of the Operations System team at MPE, remarks: “With the start of 4MOST Operations, a long-term vision of our team gets a step closer to reality. Finally, we will be able to connect the X-ray emission from supermassive black holes and clusters of galaxies detected by eROSITA with the three-dimensional distribution of the Large-Scale Structure, probed by the 4MOST spectroscopic measurements. The combination of these datasets will have a long-lasting legacy impact on extra-galactic astrophysics and Cosmology.”

Jake Laas, who has been involved in the development of the Operations System software over the last five years, adds: “It’s exciting that all the hard work we’ve put in toward automating such a complex survey will soon be put to the true test. The operational concepts which have been designed and implemented for 4MOST as a joint effort between the Consortium and ESO resulted in many unique solutions.”

The 4MOST science team consists of more than 700 investigators from universities and research institutes around the world. The Leibniz-Institut für Astrophysik Potsdam (AIP) is the lead institute of the 4MOST Consortium that has built and will scientifically operate the facility. Next to overall management, AIP has been involved in many aspects of the facility, like its wide field camera with six lenses that are up to 90 cm in diameter, its guiding and focussing system, and its fibre system that contain more than 2500 glass fibres, each with a diameter of a human hair. AIP is also strongly involved in determining 4MOST’s operations scheme, including observing planning and data archiving.

The Principal Investigator for 4MOST, Roelof de Jong from the AIP, remarks: “It is incredible to see the first spectra from our new instrument. The data looks fantastic from the start and bodes well for all the different science projects we want to execute. That we can catch the light that has travelled sometimes for billions of light years into a glass fibre the size of a hair is mindboggling.”

4MOST_First-Observation

From ESO's VISTA telescope in Chile to the First Light sky region. Here, 4MOST used its 2400 fibres to capture the light of many different objects for further spectral analysis, including the centre of the Sculptor Galaxy, stars in the globular cluster NGC288 and the active core of a distant galaxy.

Once fully operational, 4MOST will investigate the formation and evolution processes of stars and planets, the Milky Way and other galaxies, black holes and other exotic objects, and of the Universe as a whole. By analysing the detailed rainbow-like colours of thousands of objects every 10–20 minutes, 4MOST will build a catalogue of distances, temperatures, chemical compositions, velocities and many more physical parameters of tens of millions of objects spread across the entire Southern sky.

The First Light observations exemplify the unique capabilities of 4MOST: its ability to observe a very large field of view and its capability to investigate a large number of very different objects and science cases simultaneously in great detail. One of the objects dominating the First Light observation of 4MOST is the elongated galaxy NGC253, also called the Sculptor or Silver Coin galaxy, which was discovered by Caroline Herschel in 1783 and is at a distance of about 11.5 million lightyears.

The other large object seen in the field is the Globular Cluster NGC288, a very dense group of about 100,000 very old stars in the outskirts of the Milky Way. It formed about 13.5 billion years ago in the very earliest phases of the formation of the Milky Way. Its stars contain very small amounts of most chemical elements heavier than hydrogen and helium, reflecting it’s pristine composition.

Source: Max Planck Institute for Extraterrestrial Physics/News

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About 4MOST

4MOST is the largest multi-object spectroscopic survey facility in the southern hemisphere and is unique in its combination of large field of view, number of simultaneous observed objects, and number of spectral colours simultaneously registered. Development started in 2010 and the facility has been designed to operate for at least the next 15 years.

The 4MOST Consortium

The 4MOST facility is designed, built, and scientifically operated by a Consortium of 30 universities and research institutes in Europe and Australia under leadership of the Leibniz Institute for Astrophysics Potsdam (AIP). The main institutes involved in building and operating of the facility are:

• Leibniz Institute for Astrophysics Potsdam (AIP): consortium lead, telescope corrector and guiding system, metrology, control software, fibre system, and archive system,


• Macquarie University / Australian Astronomical Optics (AAO): fibre positioner,


• Centre de Recherche Astrophysique de Lyon (CRAL): low-resolution spectrographs,


• European Southern Observatory (ESO): detector systems


• Max Planck Institute for Astronomy (MPIA): instrument control hardware


• Max Planck Institute for Extraterrestrial Physics (MPE): observation planning and remote operations,


• Nederlandse Onderzoekschool Voor Astronomie (NOVA): calibration system,


• University of Cambridge, Institute of Astronomy (IoA): data management,


• Universität Hamburg (UHH), Hamburger Sternwarte: archive and user management,


• Universität Heidelberg, Zentrum für Astronomie (ZAH): high-resolution spectrograph and instrument control software.

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Contacts:

Dr. Andrea Merloni
Senior Scientist Highenergy Group; PI eROSITA
Tel: +49 89 30000-3893
Email: am@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics

Dr. Jake Laas
Tel: +49 89 30000-3812
Email: jclaas@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics

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Further Information:

4MOST Project website

4MOST to follow up on eROSITA sources

On 6 March, a series of White Papers was published to introduce the 4MOST survey program to the scientific community. The 4-metre Multi-Object Spectroscopic Telescope 4MOST will be the largest spectroscopic survey facility of its kind in the Southern hemisphere and will address today’s most pressing astronomical questions in the fields of Galactic archaeology, high-energy astrophysics, galaxy evolu­tion and cosmology, starting its public survey program in 2022. MPE has contributed two of the White Papers with the aim of using 4MOST to the followup of eROSITA sources in two major surveys dedicated to Active Galactic Nuclei and Clusters of Galaxies, respectively.

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The X-ray sky opens to the world

January 31, 2024

First eROSITA sky-survey data release makes public the largest ever catalogue of high-energy cosmic sources 

 

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  eROSITA relaxes cosmological tension

February 14, 2024

Results from the first X-ray sky survey resolve the previous inconsistency between competing measurements of the structure of the Universe

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Unveiling the 'Ghost' Baryonic Matter

November 19, 2024

A team of scientists from the Max Planck Institute for Extraterrestrial Physics (MPE) has shed light on one of the most elusive components of the universe: the warm-hot intergalactic medium (WHIM).

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Cosmic dance of the ‘Space Clover’

April 30, 2024

A group led by MPE has, for the first time, detected X-ray gas at the location of the cloverleaf ORC, an odd radio circle (ORC). The origin of ORCs is unknown; in the case of the cloverleaf ORC, the combined data from different wavelengths indicate that the emission is due to a merger of two small galaxy groups.

more

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An improved quantification of the intergalactic medium and cosmic filaments

Smoothed 0.3–1.2 keV eROSITA count rate map of the analysis footprint. The missing pixels show source-masked regions. Credit: Astronomy & Astrophysics (2024). DOI: 10.1051/0004-6361/202450933

Much of the mass in the universe lies not in stars or galaxies, but in the space between them, known as the intergalactic medium. It is warm and even hot, and is called the "warm-hot intergalactic medium," or WHIM. It holds about 50% of the normal mass (viz. baryonic, not including dark matter) of the universe but with a density of hydrogen ions less than 100 per cubic meter.

At temperatures between 100,000 and 10 million Kelvin, it is a web of "cosmic filaments" that are regions of hot, diffuse gas stretching between galaxies. These cosmic filaments, also called "galactic filaments," are the largest structures known in the universe, commonly 150 to 250 megaparsecs long (500 to 800 million light-years), the latter 8,000 times the width of the Milky Way galaxy.

Together they form the cosmic web, and they form the boundaries between cosmic voids, enormous regions of empty space containing almost no galaxies.

"The properties of the warm-hot intergalactic medium in cosmic filaments are among the least quantified units in modern astrophysics," writes a team of scientists from Europe, mostly Germany.

Using an instrument on a satellite that started surveying the universe in late 2019, they examined the X-ray emissions from almost 8,000 cosmic filaments and used a model to determine the temperature and baryon density contrast of the detected WHIM. Their work was published in the journal Astronomy & Astrophysics.

Cosmic filaments span almost the entire universe. Between them are voids with atom densities around one per cubic meter. (That is an extremely intense vacuum—by comparison, the density in interstellar space inside our own galaxy is a million to a trillion atoms per cubic meter, and the best vacuums that can be created on Earth is on the order of 1016 atoms per cubic meter.)

The void closest to us is the "Local Void." The cosmic filaments connect galaxies in a vast web; they are mostly full of gas, dust, stars, and a lot of dark matter. They are very hot, in a plasma state, but not as hot or as dense as the sun, consisting of ionized hydrogen atoms (a proton), and are detected by the absorption of light given off by quasars.

To study these structures, the group used data from eROSITA, an X-ray instrument that was part of the Russian-German Spectrum Roentgen Gamma space observatory. (Launched in July 2019, eROSITA was to image the entire sky for seven years, but the instrument stopped collecting data in February 2022, two days after Russia invaded Ukraine and institutional relations broke down.)

Distribution of the selected filaments in the redshift and physical length space. Credit: Astronomy & Astrophysics (2024); DOI: 10.1051/0004-6361/202450933

"Stacked" scans—the same images taken multiple times, a common way to deal with weak single scan intensities—were collected between December 12 to 19, 2021 in the X-ray spectrum of about 1 kilo-electronvolt (wavelengths of about 1 nm), utilizing four stacks. They then used a catalog of optical filaments, compiled in 2011 from the Sloan Digital Sky Survey, which contains over 63,000 filaments.

Assuming the standard cosmological parameters for the canonical ΛCDM model—the Hubble constant, the matter density, the baryon density and dark matter energy density, they calculated the physical length of the filaments.

Lengthy data analysis followed. First, they obtained the surface brightness profile of all filaments at discrete distances along each, carefully accounting for a host of effects such as projection effects, overlapped filaments and subtracting out the local background near each filament.

Next, they estimated the fraction of each signal due to unmasked galactic sources such as X-ray detected point sources, galaxy clusters and groups and other complicating factors. Finally, detailed astrophysical models (some from established libraries), corrections for instrument bias and statistical reasoning gave the best-fit temperature and density profiles of the gas in the weak hot intergalactic medium (WHIM).

Their best-fit temperature was 106.84 Kelvin, which is about 7 million K. For the baryon density contrast—the difference between the density of baryons and the average density of baryons—they found 101.88, which is 76. The density of ordinary matter, which is mostly baryons, in the WHIM was 76 times greater than the background baryon density of space.

Their average density contrast agrees with numerical simulations, but the relatively simple temperature they calculated was near the upper boundary of the X-ray emitting WHIM. This was not unexpected, they write, as the simple temperature was expected to be "biased to the high end of the temperature distribution when fitting a spectrum with a multi-temperature nature."

Understanding the X-ray emitting cosmic filaments and WHIM through studies such as this is expected to significantly improve in the coming decade, as improved filament finders are completed and a better understanding develops of the X-ray properties of galaxy groups, active galaxy nuclei and fast radio bursts allows better subtraction from the total WHIM signal.

X-ray missions such as the Hot Universe Baryon Surveyor and Line Emission Mapper "will be able to explore a wider parameter space of the WHIM properties," throwing more metaphorical light on the mysterious intergalactic medium.

by David Appell , Phys.org

Source: Phys.org/Astronomy and Space

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More information: X. Zhang et al, The SRG/eROSITA all-sky survey, Astronomy & Astrophysics (2024). DOI: 10.1051/0004-6361/202450933

Journal information: Astronomy & Astrophysics

© 2024 Science X Network

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Explore further

• A technique for estimating the mass of gigantic filaments of dark matter

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The First Bubble in the Intergalactic Stew

A supercomputer simulation of a galaxy protocluster similar to costco-i that is surrounded by hot gas (yellow) boiling amid an intergalactic medium filled with much cooler gas (blue). Credit: The Three Hundred Collaboration

Maunakea, Hawaiʻi – Astrophysicists using W. M. Keck Observatory on Maunakea in Hawaiʻi have discovered a galaxy protocluster in the early universe surrounded by gas that is surprisingly hot.

This scorching gas hugs a region that consists of a giant collection of galaxies called COSTCO-I. Observed when the universe was 11 billion years younger, COSTCO-I dates back to a time when the gas that filled most of the space outside of visible galaxies, called the intergalactic medium, was significantly cooler. During this era, known as ‘Cosmic Noon,’ galaxies in the universe were at the peak of forming stars; their stable environment was full of the cold gas they needed to form and grow, with temperatures measuring around 10,000 degrees Celsius.

In contrast, the cauldron of gas associated with COSTCO-I seems ahead of its time, roasting in a hot, complex state; its temperatures resemble the present-day intergalactic medium, which sear from 100,000 to over 10 million degrees Celsius, often called the ‘Warm-Hot Intergalactic Medium’ (WHIM).

This discovery marks the first time astrophysicists have identified a patch of ancient gas showing characteristics of the modern-day intergalactic medium; it is by far the earliest known part of the universe that’s boiled up to temperatures of today’s WHIM.

The research, which is led by a team from the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, part of the University of Tokyo), is published in today’s issue of The Astrophysical Journal Letters.

A simulated visualization depicts the scenario of large-scale heating around a galaxy protocluster, using data from supercomputer simulations. This is believed to be a similar scenario to that observed in the COSTCO-I protocluster. The yellow area in the center of the picture represents a huge, hot gas blob spanning several million light years. The blue color indicates cooler gas located in the outer regions of the protocluster and the filaments connecting the hot gas with other structures. The white points embedded within the gas distribution is light emitted from stars. Simulation Credit: The THREE HUNDRED Collaboration

“If we think about the present-day intergalactic medium as a gigantic cosmic stew that is boiling and frothing, then COSTCO-I is probably the first bubble that astronomers have observed, during an era in the distant past when most of the pot was still cold,” said Khee-Gan Lee, an assistant professor at Kavli IPMU and co-author of the paper.

The team observed COSTCO-I when the universe was only a quarter of its present age. The galaxy protocluster has a total mass of over 400 trillion times the mass of our Sun and spans several million light years.

While astronomers are now regularly discovering such distant galaxy protoclusters, the team found something strange when they checked the ultraviolet spectra covering COSTCO-I’s region using Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS). Normally, the large mass and size of galaxy protoclusters would cast a shadow when viewed in the wavelengths specific to neutral hydrogen associated with the protocluster gas. 

No such absorption shadow was found at the location of COSTCO-I. 

“We were surprised because hydrogen absorption is one of the common ways to search for galaxy protoclusters, and other protoclusters near COSTCO-I do show this absorption signal,” said Chenze Dong, a Master’s degree student at the University of Tokyo and lead author of the study. “The sensitive ultraviolet capabilities of LRIS on the Keck I Telescope allowed us to make hydrogen gas maps with high confidence, and the signature of COSTCO-I simply wasn’t there.”

The absence of neutral hydrogen tracing the protocluster implies the gas in the protocluster must be heated to possibly million-degree temperatures, far above the cool state expected for the intergalactic medium at that distant epoch. 

This figure compares observed hydrogen absorption in vicinity of the COSTCO-I galaxy protocluster (top panel), compared with the expected absorption given the presence of the protocluster as computed from computer simulations. Strong hydrogen absorption is shown in red, lower while weak absorption is shown in blue, and intermediate absorption is denoted as green or yellow colors. The black dots in the figure show where astronomers have detected galaxies in that area. At the position of COSTCO-I (with its center marked as a star in both panels), astronomers found that the observed hydrogen absorption is not of much different from the mean value of the universe at that epoch. This is surprising because one would expect to find extended hydrogen absorption spanning millions of light years in that region corresponding to the high observed concentration of galaxies. This figure is adapted from the Dong et al. 2023 Astrophysical Journal Letters article. Credit: Dong et al.

“The properties and origin of the WHIM remains one of the biggest questions in astrophysics right now. To be able to glimpse at one of the early heating sites of the WHIM will help reveal the mechanisms that caused the intergalactic gas to boil up into the present-day froth,” said Lee. “There are a few possibilities for how this can happen, but it might be either from gas heating up as they collide with each other during gravitational collapse, or giant radio jets might be pumping energy from supermassive black holes within the protocluster.”

The intergalactic medium serves as the gas reservoir that feeds raw material into galaxies. Hot gas behaves differently from cold gas, which determines how easily they can stream into galaxies to form stars. As such, having the ability to directly study the growth of the WHIM in the early universe enables astronomers to build up a coherent picture of galaxy formation and the lifecycle of gas that fuels it.

Source: W. M. Keck Observatory

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About LRIS

The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the addition of a second, blue arm optimized for shorter wavelengths of light and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion.

About W. M. Keck Observatory

The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain..

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X-ray emission from Warm-Hot Intergalactic Medium

Figure 1. Distribution of the heavy element oxygen across the density-temperature parameter space in the intergalactic medium, extracted from a snapshot of the Magneticum cosmological hydro-simulation by Klaus Dolag. The color-coding is on a logarithmic scale (arbitrary units) with corresponding white contours spaced by a factor of two. Black solid contours depict the ionization fraction of He-like oxygen taking into account photoionization by the cosmic X-ray background. © MPA

The Warm-Hot Intergalactic Medium contributes substantially to the matter budget in the Universe – but it is only poorly studied, as it is very difficult to observe. Researchers at MPA have now predicted how it can be explored using heavier elements as tracers. Due to scattering of the cosmic X-ray background some of this line emission can be boosted substantially and should be accessible by the upcoming X-ray survey missions.

Half of the baryonic budget in the present-day Universe is very well hidden – astronomers believe it can be found in the Warm-Hot Intergalactic Medium, which is as abundant and imperceptible as the nitrogen in the air we breathe. Being produced naturally by the ongoing formation of the largest structures in the Universe, this gas has a temperature between 100,000 and 1 million Kelvin and its density exceeds the mean baryonic density by less than a factor of 100. The high temperature of this gas implies that hydrogen and helium should be almost fully ionized and, as a consequence, it cannot be revealed via the Lyman alpha absorption features in the spectra of background quasars (contrary to the high-redshift intergalactic medium, which is readily detected in this way). It is also difficult to observe this gas directly, since its thermal emission is very faint (due to its low density) and also happens to peak in the observationally-challenging extreme UV/soft X-ray energy range.

Figure 2. Three main signatures (middle) of a layer of the Warm-Hot Intergalactic Medium (left) in X-rays: it is seen in intrinsically produced X-ray emission (E, top), resonant absorption in the spectra of bright background sources (A, middle), and resonant scattering of the isotropic cosmic X-ray background emission (S, bottom).

The spectrum of the intrinsic emission possesses both significant continuum and line emission, with comparable amplitude of the resonant (r), intercombination (i) and forbidden (f) lines. The absorption spectrum is essentially the spectrum of the background source (black dashed line) with imposed resonant absorption lines. The scattered component is dominated by resonantly scattered emission lines of the most abundant ions, with no contribution from forbidden lines.
The right panel illustrates an observing strategy for detecting the scattered component: emission of bright background sources (marked with crossed red circles) should be removed from the aperture. The residual signal will then contain both the unresolved fraction of the cosmic X-ray background and emitted plus scattered radiation from the Warm-Hot Intergalactic Medium. © MPA

Fortunately, the Warm-Hot Intergalactic Medium is enriched by heavier elements (such as carbon, nitrogen, oxygen, neon and iron) expelled from the star-forming galaxies by powerful galactic-scale outflows (as hinted e.g. by cosmological hydro-simulations, see Fig.1). Having escaped full ionization, atoms of heavy elements produce numerous emission lines and resonant absorption features. For a low density gas, such as the Warm-Hot Intergalactic Medium, the absorption features are particularly important, since their amplitude is proportional to the total number of ions on the line-of-sight, so it scales linearly with the gas number density. While a large amount of observing time has already been invested in searches for the Warm-Hot Intergalactic Medium by this technique (taking advantage of high resolution grating spectrometers on board the Chandra and XMM-Newton X-ray observatories), only marginal detections have been reported so far.

In fact, these absorption features are a result of resonant scattering, which is not a true absorption process by itself. Indeed, the intensity lost in the direction of the bright background sources is compensated by increased intensity in all other directions (see Fig. 2). The net effect of course cancels out after integrating over all directions in the case of an isotropic radiation field, such as the Cosmic X-ray Background. Nonetheless, a large portion of this background is contributed by bright individual sources (mainly Active Galactic Nuclei), which can be resolved and excluded from a given aperture. The remaining signal will then contain both the unresolved part of the background radiation (with similar absorption features as in resolved part) plus the spatially-extended resonantly-scattered background radiation. This emission is heavily dominated by the brightest resonance lines and supplements the intrinsic thermal emission from a slab of Warm-Hot Intergalactic Medium, boosting its overall X-ray emissivity and changing important spectral characteristics such as the equivalent widths of the lines and their respective ratios.

Recently, MPA scientists performed calculations of the X-ray emission from a layer of Warm-Hot Intergalactic Medium that take into account photoionization by the Cosmic X-ray Background and allow self-consistent inclusion of the resonantly scattered line emission (see Fig.3). The overall boost of emission in the most prominent resonant lines (O VII, O VIII and Ne IX) was found to equal ~30, and this boost is pretty much uniform across almost the whole region of the density-temperature diagram relevant for the Warm-Hot Intergalactic Medium. Even after averaging over broader spectral bands, the boost factor remains very significant (~5) but declines steeply at temperatures above T~1 million K (for all considered densities) and at over-densities > 100, as demonstrated in Fig.4 for the 0.5-1 keV band. The predicted total emission in this band is predicted to be dominated by the resonant lines of the helium- and hydrogen-like oxygen, which have comparable intensity for the major part of the explored parameter space.

Figure 4. Ratio of scattered to intrinsic X-ray emission integrated over 0.5-1 keV energy band as a function of number density and temperature of the Warm-Hot Intergalactic Medium. The black dashed contours indicate the ionization fraction of He-like oxygen weighted with the mass and mean metallicity of the corresponding gas portion extracted from the Magneticum simulation snapshot at z~0. The black solid triangle and square connected by a dotted line mark the parameters of typical sheet-like and filament-like structures. © MPA

A significant detection of a layer of Warm-Hot Intergalactic Medium (at a redshift ~0.1) in emission might be achieved by an X-ray instrument with an effective area of about 1000 cm^2 (at 0.5-1 keV) with a exposure on the order of 1 million seconds over one square degree of the sky – taking into account contamination by the unresolved cosmic X-ray background and the Galactic diffuse soft X-ray foreground. These requirements might already be met with a single observation by the eROSITA telescope onboard of the forthcoming SRG mission.

Future X-ray missions will indeed provide great opportunities to study the Warm-Hot Intergalactic Medium, both with large-area X-ray surveys and with deep small-area observations with X-ray calorimeters. For the former, the signal can be detected by a cross-correlation of the stacked (absorption and emission) X-ray signal with certain tracers of overdensities in the large-scale structure (e.g. 2MASS galaxies), while for the latter detection (and potentially diagnostics) of prominent individual filaments at z~0.1 is the primary goal.

Source: Max Planck Institute for Astrophysics

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Author

Khabibullin, Ildar
Postdoc
Phone: 2236
Email: ildar@mpa-garching.mpg.de
Room: 109

Churazov, Eugene
Scientific Staff
Phone: 2219
Email: echurazov@mpa-garching.mpg.de
Room: 225

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Original publication

1. Khabibullin, I.; Churazov, E.

X-ray emission from warm-hot intergalactic medium: the role of resonantly scattered cosmic X-ray background 2019, MNRAS, Volume 482, Issue 4, p. 4972-4984

Source / DOI

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More Information

Spectral model

The calculated spectral model suitable for use in numerical simulations and data analysis (along with the extracted scattered-to-intrinsic emissivity ratios and other data) is available here.

Magneticum simulation

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Where is the Universe Hiding its Missing Mass?

 H1821+643

 Credit  Illustration: Springel et al. (2005); Spectrum: NASA/CXC/CfA/Kovács et al. 

JPEG (641.3 kb) - Large JPEG (3.6 MB) - Tiff (24.1 MB) - More Images

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New results from NASA's Chandra X-ray Observatory may have helped solve the Universe's "missing mass" problem, as reported in our latest press release. Astronomers cannot account for about a third of the normal matter — that is, hydrogen, helium, and other elements — that were created in the first billion years or so after the Big Bang.

Scientists have proposed that the missing mass could be hidden in gigantic strands or filaments of warm (temperature less than 100,000 Kelvin) and hot (temperature greater than 100,000 K) gas in intergalactic space. These filaments are known by astronomers as the "warm-hot intergalactic medium" or WHIM. They are invisible to optical light telescopes, but some of the warm gas in filaments has been detected in ultraviolet light. The main part of this graphic is from the Millenium simulation, which uses supercomputers to formulate how the key components of the Universe, including the WHIM, would have evolved over cosmic time.

If these filaments exist, they could absorb certain types of light such as X-rays that pass through them. The inset in this graphic represents some of the X-ray data collected by Chandra from a distant, rapidly-growing supermassive black hole known as a quasar. The plot is a spectrum — the amount of X-rays over a range of wavelengths — from a new study of the quasar H1821+643 that is located about 3.4 billion light years from Earth. 

The latest result uses a new technique that both hones the search for the WHIM carefully and boosts the relatively weak absorption signature by combining different parts of the spectrum to find a valid signal. With this technique, researchers identified 17 possible filaments lying between the quasar and Earth, and obtained their distances.

Light Path 

Credit: NASA/CXC/K. Williamson, Springel et al.

For each filament the spectrum was shifted in wavelength to remove the effects of cosmic expansion, and then the spectra of all the filaments were added together so that the resulting spectrum has a much stronger signal from absorption by the WHIM than in the individual spectra.

Indeed, the team did not find absorption in the individual spectra. But by adding them together, they turned a 5.5-day-long observation into the equivalent of almost 100 days' worth (about 8 million seconds) of data. This revealed an absorption line from oxygen expected to be present in a gas with a temperature of about one million Kelvin.

By extrapolating from these observations of oxygen to the full set of elements, and from the observed region to the local Universe, the researchers report they can account for the complete amount of missing matter. 

A paper describing these results was published in The Astrophysical Journal on February 13, 2019, and is available online at https://arxiv.org/abs/1812.04625. The authors of the paper are Orsolya Kovács, Akos Bogdan, Randall Smith, Ralph Kraft, and William Forman all from the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Mass.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

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Fast Facts for WHIM: H1821+643:

Category: Quasars & Active Galaxies, Cosmology/Deep Fields/X-ray Background
Constellation: Draco
Observation Date: 4 observations during Jan 17-24, 2001
Observation Time: 130 hours 33 minutes (5 days 10 hours 33 minutes )
Obs. ID: 2186, 2310, 2311, 2418
Instrument: ACIS
References: Kovács O. et al., 2019, ApJ (in press); arXiv:1812.04625
Distance Estimate: About 3.4 billion light years

Source: NASA’s Chandra X-ray Observatory

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