Missing cosmic metals revealed by X-ray absorption towards distant sources

TL;DR

This paper demonstrates that the missing cosmic metals can be detected through their X-ray absorption signatures in distant sources, revealing their distribution and properties in the intergalactic medium.

Contribution

It introduces a novel method to trace dispersed cosmic metals via X-ray absorption, supported by cosmological simulations, highlighting their presence outside galaxies.

Findings

Approximately 10% of cosmic metals are in the intergalactic medium.

Most X-ray absorption is caused by discrete, dense, metal-enriched structures.

An additional ~10% of metals may reside in hot, dense gas structures.

Abstract

The census of heavy elements (metals) produced by all stars through cosmic times up to present-day is limited to ~50%; of these only half are still found within their parent galaxy. The majority of metals is expelled from galaxies into the circumgalactic (or even more distant, intergalactic) space by powerful galactic winds, leaving unpleasant uncertainty on the amount, thermal properties and distribution of these key chemical species. These dispersed metals unavoidably absorb soft X-ray photons from distant sources. We show that their integrated contribution can be detected in the form of increasing X-ray absorption with distance, for all kinds of high-energy cosmic sources. Based on extensive cosmological simulations, we assess that $\sim$ 10\% of all cosmic metals reside in the intergalactic medium. Most of the X-ray absorption arises instead from a few discrete structures along the line of sight. These extended structures, possibly pin-pointing galaxy groups, contain million degree, metal-enriched gas, 100-1,000 times denser than the cosmic mean. An additional ~10% of cosmic metals could reside in this phase.