General Meeting Report – April
Active Galactic Nuclei
A Historical Perspective
Centre for Space & Planetary Astrophysics
We live in a Universe that is full of bright objects. On a clear night you can see thousands of stars with the naked eye. In fact if, as the paradox named after Heinrich Olbers (1758-1840) already states, if the Universe was infinite and eternal the whole sky would be one continues sea of stars, as bright as the surface of the sun, day and night. Well, there would not be any night and we would not be here to marvel about it. No one would be here to enjoy the sun-drenched beaches, for without a temperature gradient, without an energy source and sink to drive evolutionary processes, there would be what is called “heat-death” in the world and nothing, probably not even stars would have evolved. But that is another story. What is our story for today is man’s gradual awareness of the nature of what we loosely call stars and galaxies and their exotic relatives.
Increase in our understanding of astronomy, like biological evolution, seems to go in punctuated equilibriums. Just when we started to feel at home in our MilkyWay Universe, nebulas were discovered that seemed to be outside our Galaxy. ‘Island universes’ Hubble called them in 1926. In time the analysis of spectra lines improved and in 1943 K. Seyfert was able to classify a series of galaxies with unusual behaviour that still carry his name. By this time nuclear power had been identified as the source driving the luminosity of stars, and their life-cycle seemed to be understood. In 1960 the limits of known physics were again stretched when a known radio source, object 3C 48, was identified by Alan Sandage with a 16th magnitude star. A point-like source with broad emission lines and high red-shift, “the thing was exceedingly weird” said Sandage. First discovered as radio signal emitters and given the descriptive category name quasi-stellar-radio-sources, or quasars for short, these objects were considered too powerful to be outside our Galaxy. Its spectra could not be recognised, until Maarten Schmidt in 1963 realised that the light of quasar 3C 273 was red-shifted to 3.5, that is, the object is moving away from us at 90% of the speed of light, billions of light-years away. Even weirder object, the BL Lacertae, were discovered in 1968. They show no emission lines, a non-thermal signature and have rapid variability (down to hours and less) in radio, IR and visual. We are looking here to a range of celestial objects with such unimaginable power output that they have for years been shrouded in controversy over their true distance from us until modern technology allowed precise measurements of position and spectra.
With a colourful PowerPoint presentation Stuart Muir lead us neatly through the above to his deduction that many of the anomalies of these different ‘weird’ astronomical objects can be explained by considering a unified model of an inferred structure for an accretion disk of a black hole seen from different angles. ‘Gravitational energy conversion from infalling matter is the most powerful energy process WE know’. In our present understanding of physics only a massive, billion-solar-masses black hole would have the capacity to generate the fantastic energy output displayed by these objects. Seen edge-on the model represents the various Quasar sources; when seen directly from above or below the magnetically focussed axial jets could explain the unrealistically high power and variability of the BL Lac objects, and in-between angles would give the range of broad and narrow line emission/absorption regions of the Seyfert galaxies. Observational evidence in the form of Hubble Space Telescope pictures of NGC4151 seems to support this theory of Active Galactic Nuclei, and we wish Stuart all the best with his PhD project.