FM03: Radio Galaxies — Resolving the AGN Phenomenon

3C 84

Radio image of the central parsec in 3C 84 obtained with the space-VLBI array. (Giovannini et al. 2018)

Can A

Image by NASA’s Chandra X-ray observatory of the radio galaxy Centaurus A based on 719 000 seconds of observation time. The image is color coded using red (0.5–1.0 keV), green (1.0–1.5 keV) and blue (1.5–2.0 keV). (NASA / CXC / CfA / R. Kraft et al., Hardcastle et al. (2007))

AGN Schematic

Schematic and simplified representation of our understanding of the AGN phenomenon in the unified scheme. The type of object we see depends on the viewing angle, whether or not the AGN produces significant jet emission and how powerful the central engine is. Note that radio loud objects are generally thought to display symmetric jet emission. Radio galaxies occupy the top left corner in this schema. (Beckmann & Shrader (2012))

Radio galaxies provide excellent laboratories for investigating the physical aspects, unification and cosmic evolution of active galactic nuclei (AGN). Thanks to recent multi-wavelength observations, we are now able to separate many different physical components of radio galaxies through imaging and spectroscopy. Observations from radio through X-ray wavelengths can probe the ejection of matter into jets and monitor decades of jet evolution. Gamma-ray observations have shown that radio galaxies are detectable up to the very-high-energy range despite unfavorable jet alignment.

We observe radio galaxies out to redshifts greater than z = 5, which makes them important cosmological probes. Planck maps have provided us with new insights into the populations of radio galaxies and their distributions in space in the 30–900 GHz range. NuSTAR provides high-quality spectra in the hard X-ray range. The Event Horizon Telescope (EHT) has begun mapping close to the event horizon of the Milky Way’s central black hole, and the Square Kilometre Array (SKA), the next generation of extremely large telescopes (ELTs) and other future telescopes will open up a new and vast discovery space.

Focus Meeting 3 will bring together multiwavelength observers and theorists to synthesize progress made over the last three years and define future directions. In order to channel the presentations and discussions, the meeting is organized into five sessions:

  • During the first session, starting on August 22nd, we will discuss the radio-galaxy structures that are produced on kiloparsec scales and beyond, with an emphasis on plasma composition, and seek to address their underlying causes. Daniel Schwartz (Harvard-Smithsonian Center for Astrophysics, USA) will talk about high-resolution studies of 100 kpc jets based on data from the Chandra X-ray Observatory. For example, a new Chandra survey for jets in radio quasars at z > 3 has revealed X-ray jets and lobes extending beyond the region of detected radio emission.
  • The second session will focus on the central engine and production of the jet(s). Alexander Tchekhovskoy (University of California, Berkeley, USA) will introduce the topic with a presentation about how numerical simulations incorporating general relativity and magnetism allow us to use black hole accretion phenomena to quantitatively probe strong-field gravity and constrain black hole physics in various astrophysical contexts.
  • Thursday morning we will first discuss populations and statistics of radio galaxies, motivated by a review from Elaine Sadler (University of Sydney, Australia) focusing on results from multi-wavelength radio surveys.
  • This will be followed by a session about future prospects. Lindy Blackburn (Harvard-Smithsonian Center for Astrophysics, USA) will start the discussion with a presentation on the goals and status of the EHT, which has already revealed structure on the scale of the Schwarzschild radius in Sagittarius A*, the supermassive black hole at the centre of our galaxy, and in Messier 87 in the Virgo Cluster.
  • A fifth session will deal with the interaction of radio galaxies with their environments. Andy Fabian (Institute of Astronomy, Cambridge University, UK) will discuss AGN feedback in clusters of galaxies and will explain how energy can be transported and dissipated throughout the cluster core.

In addition to the invited reviews, 20 high-profile contributed talks will review the state of the art within the field, and 20 posters have been selected for one-minute/one-slide presentations. The sessions will be accompanied by dedicated discussions, giving the audience ample opportunities to help us obtain a wide and complete view of what’s driving radio galaxies and what we can learn from them about physical processes in the universe.

Finally, Annalisa Celotti (International School for Advanced Studies (SISSA), Italy) will wrap up FM3 by giving a summary of the results presented during the 1.5 days of the meeting.

VOLKER BECKMANN is an expert on active galactic nuclei and data analysis for space missions. He is scientific director for computing and data science at the CNRS National Institute of Nuclear and Particle Physics (IN2P3).