Between the galaxies of large clusters lies a thin plasma of many million kelvin. When those hot atmospheres formed has barely been observed. A team at the Università degli Studi di Milano-Bicocca has now detected an early form of one.
The researchers studied the protocluster MQN01, a dense gathering of young galaxies at a redshift of 3.25. Its light comes from a time about 2.1 billion years after the Big Bang. At its centre sits the extremely bright quasar CTS G18.01. The analysis shows extended soft X-ray emission around it. The emission reaches at least 30 kiloparsecs outward, close to 100,000 light years. This is reported by scinexx. According to space.com, the study appeared on 24 July in the journal Astronomy & Astrophysics.
The signal is tiny. After the background and the point-like quasar emission were subtracted, about 66 net counts remained in the 0.5 to 2 kiloelectronvolt band. The basis was 634,000 seconds of observing time with the Chandra X-ray telescope, roughly 176 hours. From the shape and spectrum of the emission the team calculates a gas temperature of about 20 million kelvin. It puts the hot gas mass in the halo at some 2.6 trillion solar masses.
Where the heat comes from
The authors see gravity as the main heat source. Cooler gas falls into the deep gravitational field of the growing halo, and shock waves heat it sharply on the way in. "We believe we have identified a phase in its life in which cold gas falls towards the gravitational potential of this massive halo and is heated by gravitational shocks, reaching temperatures of about 20 million Kelvin," says Sebastiano Cantalupo of the University of Milano-Bicocca. The measured densities and pressures are one to two orders of magnitude above those of present-day clusters.
The type of quasar matters for the interpretation. CTS G18.01 is radio-quiet. In earlier detections at comparable distances the X-rays usually came from powerful radio jets. That explanation does not apply here. The quasar's light alone is also too weak, by the team's calculations, to make the gas shine this brightly.
At first the team did not trust its own data. "We were initially extremely skeptical ourselves," says Andrea Travascio of the Istituto Nazionale di Astrofisica, who led the work as a postdoc in Milan. He told space.com how the group tested and discarded artificial outflows and instrumental effects. Whether MQN01 is a special case remains open. Travascio and his colleagues are now searching archival data from hundreds of other quasars for the same hot phase.