Clusters of galaxies: hot gas in the deep gravitational field

Perseus clusters of Galaxies
Perseus clusters of Galaxies (Fabian et al. 2011)

Perseus cluster is an enormous structure of the local Universe which aggregates thousands of galaxies (see picture at right) immersed in a vast cloud of multimillion degree gas (see picture at left). The gas reach such high temperature because heated by collisions in the huge gravitational field produced by the galaxies of the cluster.

At these temperature the gas strongly emits in the X-ray band and, actually, Perseus is the brightest cluster of galaxies in this energy band. In 2003 a team of astronomers led by Prof. Andrew Fabian at Cambridge University discovered one of the deepest notes ever detected, a Bb, after 53 hours of Chandra observations. No human will actually hear the note, because its time period between oscillations is 9.6 million years, which is 57 below the keys in the middle of a piano.[wiki-link] The radio waves appear to be generated by the inflation of bubbles of relativistic plasma by the central active galactic nucleus in NGC 1275. The bubbles are visible as ripples in the X-ray band since the X-ray brightness of the interstellar medium that fills the cluster is strongly dependent on the density of the plasma.

Kirkpatrick et al.

Active galactic nuclei (AGN) harbor super-massive black holes of above million solar masses, which accrete matter from the surrounding medium in the center of the galaxies and produce powerful radio jets (red color in figure at right) which inflate bubbles and create cavities in the surrounding X-ray emitting gas (blue color in the figure).

Pinto et al. - Centaurus clusters of galaxies as seen with Chandra and ASTRO-H
Pinto et al. – Centaurus clusters of galaxies as seen with Chandra and ASTRO-H

Centaurus clusters of galaxies is another example where AGN “Feedback” inflate bubbles and cavities in the surrounding medium (see Chandra map at right). The new X-ray mission ASTRO-H, meant to take off next year, will reveal a huge amount of emission lines from the gas in the core of the cluster that will allow measurements of dynamics and chemical abundances in the gas with an incomparable accuracy. This will help to understand the role on the central AGN (which occupies a very tiny central region) in the evolution of the cluster (on a megaparsec scale).