Citizen scientists bag a bunch of 'two-faced' galaxies
27 November 2017
A team of professional and citizen scientists from the international Radio Galaxy Zoo project has doubled the known number of a rare type of galaxy. This work, led by CAASTRO’s Dr Anna Kapínska (International Centre for Radio Astronomy Research, University of Western Australia) is published today in the Astronomical Journal; Ivan Terentev, a citizen scientist, is the second author.
Kapínska’s team has been looking for rare galaxies called Hybrid Morphology Radio Sources or HyMoRS. These combine the characteristics of two classes of galaxy that were first thought to be distinct. HyMoRS are the astronomical equivalent of a centaur, the mythical man-horse hybrid.
Finding more HyMoRS helps us understand what kind of galaxy can turn out this way, and what gives them their unusual properties. Knowing that, in turn, helps us better understand how all galaxies evolve.
Large galaxies have massive black holes at their centres. While consuming matter, these black holes often produce large jets of radio-emitting material that blast millions of light years out into space.
Galaxies with jets are often divided into two classes, Fanaroff-Riley I and Fanaroff-Riley II (or FR I and II). FR I galaxies have jets that fade away as they extend outwards, while FR II galaxies have jets that end in a bright, strongly-emitting region (a ‘hotspot’).
The two galaxy classes were first described by astronomers Bernie Fanaroff and Julie Riley in 1974. For the next quarter-century astronomers thought they were quite distinct. Then, in 2002, a rare hybrid form – the HyMoRS – was discovered. But fewer than 30 HyMoRS had been found, until the Radio Galaxy Zoo team identified 25 more.
L-R: An FR I galaxy (radio jets in blue, overlaid on an infrared image); a HyMoRS galaxy; and an FR II galaxy. The HyMoRS galaxy shows both FR I and FR II characteristics. Image: A. Kapínska et al.
Finding more HyMoRS is giving us clues as to how they form.
Some may simply be an illusion. The jets may be physically the same on both sides, but because one is pointed towards us and the other away from us, they look different.
But we may also be seeing the central black hole ‘switching off’ (ceasing to actively swallow material), or switching off and then on again. This seems to have happened in one of the new HyMoRS from Radio Galaxy Zoo.
Yet other HyMoRS may be caused by environmental effects, such as the jets travelling out into regions of space that contain different densities of material. Modelling shows that the environment can affect the jets’ size and brightness.
Radio Galaxy Zoo demonstrates how effective citizen science can be at discovering rare objects. The project is led by two CAASTRO scientists, Dr Julie Banfield (Australian National University) and Dr Ivy Wong (International Centre for Radio Astronomy Research, University of Western Australia).
Environment can alter the appearance of jets from a black hole, as this simulation shows. The black hole is situated 144 kpc from the centre of a galaxy cluster. The jet extending to the left is directed out of the cluster, while the jet extending to the right is directed towards the cluster's centre, where the intergalactic medium is denser. This denser medium constrains the expansion of the right-hand jet. Credit: Katie Vandorou, Patrick Yates and Stas Shabala (University of Tasmania).
A.D. Kapínska et al., "Radio Galaxy Zoo: A search for hybrid morphology radio galaxies", 2017, Astronomical Journal, 154