Infant mortality in space
CAASTRO PhD student Joe Callingham (University of Sydney) has re-examined a ‘boring’ radio source and shown it could hold the key to a long-standing problem in astronomy: why there seem to be more ‘baby’ radio galaxies than mature ‘giant’ ones.
The source is called PKS B0008-421. Just a dot in the radio sky, it doesn’t seem to have changed at all since it was discovered in the 1960s.
Radio astronomers use such constant sources to check the behaviour of their telescopes. PKS B0008-421 has been used for this purpose for most of the radio telescopes in the southern hemisphere. As a result, there are many observations of it, made at several wavelengths and dating back to the 1960s.
Joe combined these historical data with more recent observations made using CSIRO’s Australia Telescope Compact Array and the Murchison Widefield Array, and found a few surprises.
PKS B0008-421 puts out most of its radio energy in the gigahertz range of the spectrum, so it’s called a ‘gigahertz-peaked spectrum’ or GPS source.
GPS sources are thought to be the earliest stage of radio galaxies—galaxies with a central black hole that produces giant jets and lobes of radio-emitting particles.
The largest of the radio galaxies, the giant radio galaxies, are far bigger than ‘regular’ galaxies such as our own Milky Way. But the GPS sources are a lot smaller, and we can’t make out much of their internal structure.
When Joe plotted his data on PKS B0008-421, two things stood out. First, PKS B0008-421 is very different from a typical GPS source: its radio spectrum peaks at a lower frequency (at 0.6 GHz, rather than a more typical 3 GHz), and the peak is very sharp. Second, it doesn’t fit the model traditionally used to explain the spectra of GPS sources.
Joe tried tweaking the conventional model (called synchrotron self-absorption) but found that another model (free-free absorption) fitted the data (slightly) better. This could shake up astronomers’ ideas about the age of GPS sources and the environments they live in.
Interestingly, 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’ then and is now fading away. But the fading is very slow, suggesting that PKS B0008-421 is cocooned in relatively dense gas.
The Murchison Widefield Array will be excellent at finding similar dying, gas-swaddled sources through the characteristics of their low-frequency radio emission. By finding these sources, which haven’t been obvious to other telescopes, it may be able to tell us the ages at which radio galaxies die, and why there are so many more baby radio galaxies than giants.
J. R. Callingham and 42 co-authors in MNRAS (2015, accepted). “Broadband Spectral Modeling of the Extreme Gigahertz-Peaked Spectrum Radio Source PKS B0008-421”.