An improved quantification of the intergalactic medium and cosmic filaments

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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.)

"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.

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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 10^6.84 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 10^1.88, 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.

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

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