A huge telescope looked closely at the smallest radio galaxies
Radio galaxies are twin-lobed structures visible in the radio portion of the electromagnetic spectrum which typically extend 100-1,000 kilo-parsecs end to end. The lobes are the result of two opposing jets coming out of the central black hole of these galaxies. These jets ‘inflate’ lobes as they slam into the surrounding material and recoil back towards the centre. A powerful instrument to locate, identify and categorise such structures is the Very Long Baseline Array (VLBA), a telescope array of ten identical antennas that spans 8,000 km across the US, from Hawaii in the Pacific Ocean to the Virgin Islands at the border between the Caribbean Sea and the Atlantic Ocean. These long baselines allow researchers to precisely pinpoint – or triangulate – the location of objects. As part of a full polarisation survey of radio sources, the VLBA Imaging and Polarimetry Survey, data were also suitable to search for rare compact symmetric objects (CSOs). CSOs are smaller than 1 kilo-parsec and generally thought to be young, generally only a few hundreds of years old. They give astronomers an opportunity to observe the early evolutionary phases of these radio galaxies.
Using the VLBA data, CAASTRO postdoc Dr Steven Tremblay (Curtin University) and his research team have recently published their identification of 24 radio galaxies as CSOs , the smallest in their sample measuring only 1.5 parsecs across. Within their sample, the researchers were able to confirm 15 new CSOs, effectively doubling the number of these sources known to date. They also uncovered a possible new sub-class of low-powered CSOs which is similar to a distinction that also occurs in normal radio galaxies. It also allows insight into the local environments around such sources.
The multi-frequency observations (at 5, 8 and 15 GHz) presented in their study also enable understanding of the structure within the galaxies, due to the strong frequency dependence from different stages of the jets. New emission streaming out from the central engine changes slowly with frequency while the surface where the jets are pushing against the surrounding media vary greatly across the observed frequencies. These data can be combined into spectral index maps, highlighting the dynamics happening within these sources in a single snapshot. Overlaying the contour lines for one frequency (e.g. 5 GHz) with a colour scale that depicts the brightness change to another frequency (5 and 8 GHz), the researchers were able to produce 120 of such multi-frequency snapshots that were the primary basis for classification.
Steven Tremblay et al. in the Monthly Notices of the Royal Astronomical Society (2016): “Compact Symmetric Objects and Supermassive Binary Black Holes in the VLBA Imaging and Polarimetry Survey”