Death in the 15th century

29 June 2015

Joe Callingham, an astronomy student co-supervised by the University of Sydney and CSIRO (and affiliated with CAASTRO), has re-examined a 'boring' radio source and shown it could hold the key to a long-standing problem in astronomy.

For his PhD, Joe has been studying a radio source called PKS B0008-421. The source was discovered with CSIRO's Parkes radio telescope in the 1960s. Just a dot in the radio sky, this source appears to have done nothing since it was discovered - not become stronger or weaker, or grown bigger.

Radio astronomers need some of these boring, unchanging sources to use as calibrators, sources whose strength is known and with which they can check the behaviour of their telescopes. PKS B0008-421 has been used as a calibrator for most of the radio telescopes in the southern hemisphere.

Joe Callingham
Joe Callingham

Calibrators have to be observed from time to time, to make sure that they are not changing. As a result, there are lots of observations of PKS B0008-421, made at several wavelengths and dating back to the 1960s. Many of these come from CSIRO's Compact Array radio telescope; others were made with the Parkes telescope, the University of Sydney's Molonglo Observatory Synthesis Telescope, and the Giant Meterwave Radio Telescope in India.

Joe has taken this historical data and combined it with more recent observations made using the Compact Array (which observes high-frequency radio waves) and the Murchison Widefield Array (which observes low-frequency radio waves).

PKS B0008-421 looks like just a dot in the radio sky, but what sort of a thing is it? Is it a source in our Galaxy, is it another galaxy, or what? The historical observations suggest that PKS B0008-421 is something called a 'gigahertz-peaked spectrum source', or GPS source for short. That just means that its radio spectrum peaks (puts out most energy) at a frequency of about a gigahertz.

These GPS sources are thought to be the earliest stage of radio galaxies - their embryos, if you like. Radio galaxies have a central power source - a black hole - and produce giant jets and lobes of radio-emitting particles. The largest of them, the giant radio galaxies, are far bigger than 'regular' galaxies such as our own Milky Way.

The GPS sources, though, are very small by comparison, and we can not make out much detail in their internal structure. However, since 1966 astronomers have had a model to explain the pattern of radio frequencies that GPS sources emit.

When Joe plotted all his data for PKS B0008-421, two things were apparent. First, PKS B0008-421 is very different from a typical GPS source: it peaks at a lower frequency, and the peak is very sharp. Second, it does not fit the conventional model.

Joe tried tweaking the conventional model (called synchrotron self-absorption), and then tried fitting the data to another model (free-free absorption). The free-free absorption model works (slightly) better. This could shake up astronomers' ideas about the age of GPS sources and the environments they live in.

The Murchison Wide-Field Array. Photo: Curtin University
The Murchison Wide-Field Array. Photo: Curtin University

Importantly, Joe could make the models fit the data only by assuming that the source's central black hole had stopped injecting high-energy particles into the body of PKS B0008-421 about 550 years ago. In other words, the source 'turned off' about the time the Wars of the Roses were raging in England, and hasn't 'turned on' again. It is now slowly dying.

We know of some radio galaxies where the black hole seems to have turned off and then later turned back on. But we don't know what proportion of them do this.

What we do know is that there are far more GPS sources (and a similar, related kind of source) than there are giant radio galaxies, which the GPS sources and their relatives are thought to evolve into. Perhaps many of the GPS sources 'die young' and simply fade away rather than evolving into 'proper' radio galaxies and then giants.

Once a source like this has 'turned off', it will fade rapidly - that is, unless it is surrounded by relatively dense gas. We think PKS B0008-421 is cocooned in such gas. Dying sources that are swaddled like this will be distinguishable by new low-frequency radio telescopes such as the Murchison Widefield Array and the LOFAR telescope in the northern hemisphere. We may be able to find a large number of GPS sources that have 'died young', and so reconcile the numbers of GPS sources and giant radio galaxies.

To get a handle on what is happening inside the GPS sources - what mechanism is producing the observed spectrum - we need to know how dense the hydrogen gas inside them is. This problem should be licked by a survey such as FLASH (the First Large Absorption Survey in HI), which will run on CSIRO's new Australian SKA Pathfinder telescope.

Joe recently presented his work at a meeting in Italy.