In the local Universe, large galaxy clusters are immersed in gigantic, dense “atmospheres” of extremely hot gas, with temperatures of tens of millions of kelvin, known as the intracluster medium (ICM). By contrast, when observing the more distant and ancient Universe, telescopes have so far almost always detected only the cold-gas component. A study published today in the journal Astronomy & Astrophysics reveals, for the first time and with clarity, the precise moment when this vast hot atmosphere begins to form, when the Universe was only a fraction of its current age.
The discovery was made by analysing 634,000 seconds, approximately 180 hours, of data from NASA’s Chandra X-ray Space Telescope. The research, led by Andrea Travascio, a researcher at the Italian National Institute for Astrophysics (INAF), and Sebastiano Cantalupo, Professor of Astrophysics at the University of Milano-Bicocca, identified a structure of extremely hot and dense gas extending for approximately 100,000 light-years around a quasar, the extremely bright and active nucleus of a very distant galaxy. The quasar lies at the centre of the enormous MQN01 protocluster, an aggregation of galaxies still in the process of formation dating back to when the Universe was approximately two billion years old.
The researcher, who conducted the research as a postdoctoral fellow at the University of Milano-Bicocca, explains: «The central scientific question is to understand how this hot phase forms: what the physical conditions of the gas are during its formation and which processes contribute to heating it.» The data «show extraordinary gas properties that could provide us with the first information on how this hot phase of the circumgalactic medium formed, which today we observe in the form of the intracluster medium.»
Until now, the extremely rare extended X-ray emissions detected at such distances had been associated with radio-loud active galaxies, or active galactic nuclei (AGN), in which X-ray radiation is produced by jets of particles travelling at speeds close to the speed of light. The quasar at the centre of MQN01, however, is radio-quiet, which rules out contamination caused by jets.
«We are looking at one of the most distant detections of extended thermal X-ray emission associated with the formation of hot gas in dense regions of the Universe, which will probably evolve into the familiar ICM of the local Universe,» adds Cantalupo, who leads the “Cosmic Web” research group that produced the study. «We believe we have identified a phase in which cold gas falls towards the gravitational potential of this massive halo and is heated by gravitational shocks, reaching temperatures of approximately 20 million kelvin. The densities and pressures we measured are high: between one and two orders of magnitude greater than those of clusters in our local Universe.»
The result was made possible by a methodological insight from Travascio. «I became curious about applying to these extremely distant and hyperluminous quasars of the early Universe an analytical method usually reserved for Seyfert galaxies, which host the active black holes of the local Universe,» he explains. The challenge was therefore to isolate the faint, diffuse light of the gas from the dazzling glow of the central black hole.
«At first, we ourselves were extremely sceptical,» Travascio admits. «Given the exceptional nature of the data, we examined every alternative explanation: from artificial outflows to instrumental contamination and other unexpected effects. But every alternative scenario encountered insurmountable theoretical limitations. The thermal explanation is the only one consistent with the physical data.» He adds: «Considering how unusual the result was, and also taking into account Chandra’s technical difficulties during the final stages of its mission, the satellite has once again proved to be an extraordinary instrument, capable of producing scientifically significant results even after decades of operation.»
Travascio and his colleagues are already analysing numerous archival datasets relating to hundreds of other quasars to determine whether this heating phase is common among protoclusters or whether MQN01 represents a rare case. The future direction of the investigation has already been established: the team has been granted observing time with ESO’s ALMA telescope in Chile to seek independent confirmation, while definitive answers regarding the distribution of this hot gas will come from next-generation X-ray space telescopes such as AXIS and NewAthena.
FURTHER INFORMATION
The article “X-ray view of a massive node of the Cosmic Web at z ∼ 3 II. Discovery of extended X-ray emission around a hyperluminous QSO”, by A. Travascio, S. Cantalupo, G. Pezzulli, P. Tozzi, L. Di Mascolo, M. Esposito, T. Lazeyras, M. Lepore, S. Borgani, M. Elvis, G. Fabbiano, F. Fiore, M. Galbiati, N. Ledos, R. Middei, A. Pensabene, E. Piconcelli, G. Quadri, F. Vito, W. Wang and L. Zappacosta, has been accepted for publication in the journal Astronomy & Astrophysics.