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

¹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
e-mail: sergio.campana@brera.inaf.it
² INAF, IASF Milano, via E. Bassini 15, 20133 Milano, Italy
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁴ Kavli IPMU (WPI), Todai Institutes for Advanced Study, University of Tokyo, 277-8583, Chiba, Japan

Received: 30 September 2014
Accepted: 23 December 2014

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 ~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, 100–1000 times denser than the cosmic mean. An additional ~10% of cosmic metals could reside in this phase.