About a third of the normal matter — meaning hydrogen, helium and other elements — created shortly after the Big Bang is not seen in the present-day Universe. One idea is that this missing mass resides in large-scale filaments in the form of warm-hot intergalactic medium (WHIM). Using a new technique, an international team of astrophysicists from the Harvard Smithsonian Center for Astrophysics, Konkoly Observatory and Eotvos University has found strong evidence for the hot component of the WHIM based on data from NASA’s Chandra X-ray Observatory and other telescopes. The results will be published in the Astrophysical Journal.

“If we find this missing mass, we can solve one of the biggest conundrums in astrophysics,” said Dr. Orsolya Kovacs, lead author of the study.

“Where did the Universe stash so much of its matter that makes up stuff like stars and planets and us?”

Dr. Kovacs and colleagues used Chandra to look for and study filaments of warm gas lying along the path to a quasar, a bright source of X-rays powered by a rapidly growing supermassive black hole.

This quasar, called H1821+643, is located about 3.4 billion light-years from Earth.

If the WHIM’s hot gas component is associated with these filaments, some of the X-rays from H1821+643 would be absorbed by that hot gas.

Therefore, the astrophysicists looked for a signature of hot gas imprinted in the quasar’s X-ray light detected by Chandra.

One of the challenges of this method is that the signal of absorption by the WHIM is weak compared to the total amount of X-rays coming from the quasar. When searching the entire spectrum of X-rays at different wavelengths, it is difficult to distinguish such weak absorption features from random fluctuations.

Dr. Kovacs and co-authors overcame this problem by focusing their search only on certain parts of the X-ray light spectrum, reducing the likelihood of false positives.

They did this by first identifying galaxies near the line of sight to H1821+643 that are located at the same distance from Earth as regions of warm gas detected from ultraviolet data. With this technique they identified 17 possible filaments between the quasar and us, and obtained their distances.

Because of the expansion of the Universe, which stretches out light as it travels, any absorption of X-rays by matter in these filaments will be shifted to redder wavelengths. The amounts of the shifts depend on the known distances to the filament, so the team knew where to search in the spectrum for absorption from the WHIM.

While narrowing their search helped, the scientists also had to overcome the problem of the faintness of the X-ray absorption. So, they boosted the signal by adding spectra together from 17 filaments, turning a 5.5-day-long observation into the equivalent of almost 100 days’ worth of data.

With this technique they detected oxygen with characteristics suggesting it was 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, we can account for the complete amount of missing matter. At least in this particular case, the missing matter had been hiding in the WHIM after all,” the researchers said.

“We were thrilled that we were able to track down some of this missing matter,” said Dr. Randall Smith, also from the Harvard Smithsonian Center for Astrophysics.

“In the future we can apply this same method to other quasar data to confirm that this long-standing mystery has at last been cracked.”

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Orsolya E. Kovacs et al. 2019. Detection of the Missing Baryons toward the Sightline of H1821+643. ApJ, in press; arXiv: 1812.04625