May 2000 General Meeting
Guest Speaker Dr. Matthew O’Dowd
Space Telescope Science Institute, Melbourne University
Host Galaxies of Active Galactic Nuclei
In shirtsleeves and tussled hair, Matthew started his presentation with a crash course in radio astronomy. How in its infancy exact positioning of radio sources in the sky seemed beyond the reach of technology. How gradually, through clever use of reflected signals, interferometry and occultations by the moon the first sources were pin-pointing in the early 1950s.
The history of Active Galactic Nuclei (AGN) started to fall into place with optical identification of radio source 3C 48 in 1960 by Allan Sandage. The term AGN now covers a variety of energetic galaxy types discovered at different times and given different names, including Seyfert galaxies, N galaxies, quasars and finally BL Lac objects. When in 1968 the connection with the variable BL Lacertae objects was made, the full implications of the power source driving these monsters in the sky struck home. From a point smaller than the size of our solar system they radiate more energy than 1000 Milky Way galaxies combined. No process known at that time could do this.
In the end the ubiquitous black hole was found to be capable to act as just such a prodigious matter-to-energy converter. Almost half of the mass falling into it can, under ideal conditions, be converted into energy. Current models favour an accretion disc of disrupted stellar material spiralling into the black hole. This system can also account for the jets often associated with AGNs, as the rapid rotational acceleration squeezes high energy charged particles out along the axis of rotation at high velocity.
The picture on quasars is becoming clearer. But what sort of galaxy does it take to produce an AGN? And what is the evolutionary significance of the limited band, from redshift 1 to redshift 5, in which the majority of these objects have been found? One possible clue to this puzzle is that most of the quasars found so far seem to have elliptical, or none-spiral, host galaxies. Matthew and his team devised a computer program of two colliding spiral galaxies to see how the resulting mass of stars would mix and re-assemble. With millions of points of matter (stars) and millions of points of dust, and lots of primeval hydrogen, the program clearly shows how the disruptive forces of the merger turn two spiral galaxies into one giant elliptical galaxy. The simulated process condensed a million years of evolution into about 60 seconds of film.
It is reasonable to assume this same collision process would create the necessary density in the centre to form a black hole and thus start the active nuclei process. Furthermore, galaxy density in the early Universe would have been high and collisions consequently much more frequent tha now. Closer scrutiny of older (closer) galaxies with active nuclei such as M87 shows remnants of an accretion disk and hints of an axial jet.
This, Dr. Matthew O’Dowd concluded, is were AGN research stands today: The same object can appear to be a quasar, a laser, or a radio galaxy, depending from what angle it is seen. The questions to follow up are: What sort of galaxy does it take to make a quasar? Is the size of the AGN related to the size of the galaxy? And is there a pattern of evolution from quasar to standard elliptical galaxy? I wonder. AK