Echo mapping of Active Galactic Nuclei

Keith Horne

AGN

The figures are simulations of an Active Galactic Nucleus (AGN) disc with a spiral density wave, as seen in the light of certain important emission lines. The first frame shows how the disc actually looks, a view we have never seen because it is too compact - a mere 50 light-days across - to be resolved even with the largest optical telescopes. Spiral density waves such as these are seen in the accretion discs of close binary star systems. They are also expected to arise in the accretion disc of an AGN when its host galaxy merges with a second galaxy possessing a nuclear black hole. Such mergers are thought to happen repeatedly during the life of a galaxy.

The second frame is a velocity (x-axis) - delay (y-axis) map of the same disc showing the speed and distance from the ionising source of the gas seen in the Ly alpha 1215 A (red), CIV 1550 A (green) and HeII 1640 A (blue) lines respectively. The HeII 1640 A emission is coming from material orbiting the black hole at ~6000 km/sec - a full width of 12,000 km/sec in this image. Such maps can be recovered by the technique of echo (or reverberation) mapping , which was pioneered at St Andrews and which we continue to develop. The method works by recording small time-delayed "echoes" in the velocity profiles of photoionised emission lines. These emission line changes are driven by ionising radiation from the hot inner regions of the accretion disc. Observational material for echo mapping is currently obtained via major campaigns with HST's ultraviolet spectrographs in conjunction with various X-ray satellites and ground-based optical telescopes.

The multi-wavelength monitoring satellite Kronos (a proposed NASA Midex mission) is designed to deliver a revolutionary improvement in the quality of the spectrophotometric monitoring data, sharpening the resolving power of echo maps to measure accurate black hole masses and to reveal structures like those illustrated in the above simulations. More details about Kronos work at St Andrews.