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About a third of the normal material – namely hydrogen, helium and other elements – created shortly after the Big Bang is not seen in the current universe. One idea is that this missing mass lies in large-scale filaments in the form of warm-hot intergalactic medium (WHIM). Thanks to a new technique, an international team of astrophysicists from the Harvard Smithsonian Astrophysical Center, Konkoly Observatory and Eotvos University has found solid evidence of the warm component of WHIM, based on observatory data from Chandra X-rays from NASA and other telescopes. The results will be published in Astrophysical Journal.
The main part of this graph comes from the Millenium simulation, which uses supercomputers to formulate how 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 the X-rays that pass through them. The box in this graph represents some of the radiological data collected by NASA's Chandra Observatory from a rapidly growing supermassive black hole known as quasar. The plot is a spectrum – the amount of x-rays over a range of wavelengths – a result of a new quasar study H1821 + 643 located at around 3.4 billion years ago. light of the Earth. Image credit: Springel et al / NASA / CXC / CfA / Kovacs et al.
"If we find this mass missing, we will be able to solve one of the biggest problems of astrophysics," said Dr. Orsolya Kovacs, lead author of the study.
"Where did the Universe hide so much of its matter that makes up things like stars and planets and us?"
Dr. Kovacs and his colleagues used Chandra to research and study hot gas filaments on the way to a quasar, an X-ray light source powered by a rapidly growing supermassive black hole.
This quasar, called H1821 + 643, is located about 3.4 billion light-years away from Earth.
If the WHIM hot gas component is associated with these filaments, part of the X-rays of H1821 + 643 would be absorbed by this hot gas.
As a result, astrophysicists looked for a hot gas signature etched in the X-ray light of the quasar detected by Chandra.
One of the challenges of this method is that the signal of absorption by the WHIM is small compared to the total amount of X-rays from the quasar. When searching for the full spectrum of X-rays at different wavelengths, it is difficult to distinguish these absorption characteristics as low as random fluctuations.
Dr. Kovacs and his co-authors solved this problem by concentrating their research on only parts of the X-ray spectrum, reducing the risk of false positives.
To do this, they first identified the galaxies near H1821 + 643 located at the same distance from the Earth as the regions of hot gas detected from ultraviolet data. With this technique, they identified 17 possible filaments between the quasar and us and got their distances.
Due to the expansion of the universe, which stretches the light during its displacement, any absorption of X-rays by the material in these filaments will be shifted to redder wavelengths. The importance of offsets depends on the known distances to the filament. The team knew where to look in the spectrum for WHIM absorption.
Although research was reduced, scientists also had to overcome the problem of weak X-ray absorption. Thus, they amplified the signal by adding 17-filament spectra, transforming a 5.5-day observation. in a quantity of data equivalent to nearly 100 days.
With this technique, they detected oxygen with features suggesting that it was in a gas with a temperature of about one million Kelvin.
"By extrapolating from these observations of oxygen to the complete set of elements and the region observed at the local universe, we can count the total amount of missing material. At least in this particular case, the missing question was hidden in the WHIM, after all, "the researchers said.
"We were delighted to have been able to find some of this missing question," said Dr. Randall Smith, also of the Harvard Smithsonian Center for Astrophysics.
"In the future, we will be able to apply this same method to other quasar data to confirm that this long-standing mystery has finally been solved."
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Orsolya E. Kovacs et al. 2019. Detection of missing baryons towards line of sight of H1821 + 643. ApJ, in the press; arXiv: 1812.04625
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