Annual Meeting of the Deep Sky Section, 2011 March 12


Active Galactic Nuclei, and why amateurs should observe them

Dr Hewitt explained that he would be arguing in his talk that active galactic nuclei (AGNs) deserved greater attention from amateur deep sky observers, even though they posed quite different observational challenges to more familiar deep sky objects. Their study was, he added, a comparatively recent development. Modern cosmology, and the vast distance scales it implied, had now become so well rehearsed that it seemed surprising that, as recently as 1920, Harlow Shapley and Heber Curtis had been able to debate whether spiral nebulae lay within or outside the Milky Way without clear victor.

The decisive evidence in that debate had been published in 1925: Edwin Hubble's measurements of the comparative faintness of Cepheid variable stars within these nebulae, made using the 100-inch Hooker Telescope on Mount Wilson, implied that they must be galaxies external to our own. The identification of active galaxies as a class distinct from others had begun 15 years later in 1940–2 with the work of Carl Seyfert, also working at the Mount Wilson Observatory. He had noticed that some galaxies, now known as Seyfert galaxies, had unusually bright point-like features at their centres; the clearest example was M77 in Cetus.

From here onwards, the study of AGNs had followed a more complicated path. It had gradually transpired that they could appear in a number of different observational guises depending upon the angle from which they were viewed. Many of these guises had been assigned separate names, and treated as entirely separate classes of object, until later theoretical work had shown them to be equivalent. For example, the rapid development of radio astronomy after the Second World War had given rise to the designations radio galaxy (1940s) – galaxies which produced unusually large amounts of radio radiation – quasar (early 1960s) – mysterious point-like sources of radio radiation in the sky – and blazar (mid-1970s) – quasars which showed dramatic variations in brightness. All of these objects would later be understood to be active galaxies viewed in different orientations.

The recognition that these represented some of the most powerful objects in the Universe had emerged from observations of quasars. In 1960, the position of the radio source 3C48 had been pinned down with sufficient accuracy to identify an optical counterpart whose spectrum indicated a redshift of 0.37, corresponding to a distance of several billion lightyears. That it could appear so bright in the radio sky, despite lying at such a great distance, implied that it must have a phenomenal luminosity, and the claim was not widely believed at first. The clinching evidence had come in 1963, when the radio source 3C273 had been occulted by the Moon. Precise timings of its disappearance and reappearance using the Parkes Radio Telescope had allowed an optical counterpart to be found for it also, and once again a spectrum was found that indicated a very great distance.

The speaker explained that it was now understood that black holes lay at the centres of active galaxies, and that their intensely bright nuclei were the result of gas accreting onto these black holes. As the gas approached the black hole's event horizon, it became heavily compressed and incredibly hot, forming an accretion disc which glowed at all wavelengths from radio waves through to X-rays. Though such accretion discs appeared bright from any direction, they produced two especially intense beams of radiation, which were highly collimated in either direction perpendicular to the disc. Objects observed as quasars represented those rare AGNs which happened to be viewed exactly along the directions of these narrow beams, which accounted both for their rarity and also for their astonishing brightnesses, even at very great distances.

Dr Hewitt went on to explain that amateurs could contribute scientifically useful observations of AGNs by monitoring their brightness changes. This was rather different from most deep-sky imaging: the nebulosity associated with the point-like nuclei of these objects was often beyond the reach of professional, let alone amateur, telescopes, and so there was no structure to resolve. He personally worked with a 115mm TMB apochromatic refractor and performed photometry on his images using the software package AIP4WIN.

A list of a few potential targets could be found in the BAA Handbook. Of these, one of the easiest to find was Mrk 421 as it lay a mere 2' south of the star 51 Ursa Majoris. Though the AAVSO quoted its magnitude as varying between 11.6 and 16, in practice it was usually close to mag 13. A more difficult object, of historical significance previously mentioned, was 3C273. Typically a little fainter than mag 13, it could be found in the Virgo cluster, but not within easy hopping distance of any readily accessible way-marker stars. One of the personal favourites of the speaker was 3C66A, a rather distant quasar lying at a distance of 4.5 billion lightyears, which varied between mag 13.5 and 15.5. He remarked that it was awe-inspiring to think that the light being detected from this quasar had set out on its journey at about the same time that the Solar System had been forming. Finally, more distant and more challenging objects still included the double quasar, Q0957+561, at around mag 16.5, and the Einstein cross, Q2237+030 at closer to mag 18.

After the applause, the Director invited Prof. Derek Ward-Thompson of the University of Cardiff to close the afternoon, as was traditional, with a professional perspective on the deep sky.




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