Search

Ongoing and future radio surveys aim to trace the evolution of black hole growth and feedback from active galactic nuclei (AGN) throughout cosmic time; however, there remain major uncertainties in translating radio luminosity functions into a reliable assessment of the energy input as a function of galaxy and/or dark matter halo mass. A crucial and long-standing problem is the composition of the radio-lobe plasma that traces AGN jet activity. In this paper, we carry out a systematic comparison of the plasma conditions in Fanaroff & Riley class I and II radio galaxies to demonstrate conclusively that their internal composition is systematically different. This difference is best explained by the presence of an energetically dominant proton population in the FRI, but not the FRII radio galaxies. We show that, as expected from this systematic difference in particle content, radio morphology also affects the jet-power/radio-luminosity relationship, with FRII radio galaxies having a significantly lower ratio of jet power to radio luminosity than the FRI cluster radio sources used to derive jet-power scaling relations via X-ray cavity measurements. Finally we also demonstrate conclusively that lobe composition is unconnected to accretion mode (optical excitation class): the internal conditions of low- and high-excitation FRII radio lobes are indistinguishable. We conclude that inferences of population-wide AGN impact require careful assessment of the contribution of different jet sub-classes, particularly given the increased diversity of jet evolutionary states expected to be present in deep, low-frequency radio surveys such as the LOFAR Two-Metre Sky Survey., 10 pages, 6 figures. Accepted for publication in MNRAS

The 2Jy sample is a survey of radio galaxies with flux densities above 2 Jy at 2.7 GHz. As part of our ongoing work on the southern subset of 2Jy sources, in paper I of this series we analysed the X-ray cores of the complete 2Jy sample with redshifts 0.05, Comment: 18 pages, 3 tables, 29 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS)

Due to their steep spectra, low-frequency observations of FR II radio galaxies potentially provide key insights in to the morphology, energetics and spectrum of these powerful radio sources. However, limitations imposed by the previous generation of radio interferometers at metre wavelengths has meant that this region of parameter space remains largely unexplored. In this paper, the first in a series examining FR IIs at low frequencies, we use LOFAR observations between 50 and 160 MHz, along with complementary archival radio and X-ray data, to explore the properties of two FR II sources, 3C452 and 3C223. We find that the morphology of 3C452 is that of a standard FR II rather than of a double-double radio galaxy as had previously been suggested, with no remnant emission being observed beyond the active lobes. We find that the low-frequency integrated spectra of both sources are much steeper than expected based on traditional assumptions and, using synchrotron/inverse-Compton model fitting, show that the total energy content of the lobes is greater than previous estimates by a factor of around 5 for 3C452 and 2 for 3C223. We go on to discuss possible causes of these steeper than expected spectra and provide revised estimates of the internal pressures and magnetic field strengths for the intrinsically steep case. We find that the ratio between the equipartition magnetic field strengths and those derived through synchrotron/inverse-Compton model fitting remains consistent with previous findings and show that the observed departure from equipartition may in some cases provide a solution to the spectral versus dynamical age disparity., 13 pages, 5 figures, 10 tables. Accepted for publication in MNRAS

We present high-sensitivity 1.51 GHz e-MERLIN (e-Multi-Element Radio-Linked Interferometer Network) radio images of the nearby galaxy NGC6217. We resolve the compact radio source at the centre of NGC6217 for the first time, finding a twin-lobed structure with a total linear size of ~4 arcsec (~400 pc). The radio source does not have a compact central core, but there is an unresolved hotspot near the outer edge of the southern lobe. Combining our e-MERLIN data with new VLA (Very Large Array) A-Array data and archival multiwavelength data, we explore possible scenarios which might explain this complex radio morphology. We conclude that this radio source is most likely powered by a low-luminosity active galactic nucleus but with a possible important contribution from nuclear star formation. We also investigate the origin of a putative X-ray jet in NGC6217, previously suggested in the literature. This 'jet' consists of three X-ray 'knots' in a line, pointed away from the nucleus, with a total size of ~3 arcmin (~15 kpc).We find no extended radio emission coincident with the 'jet'. An additional compact radio source, aligned with the knots but without an X-ray counterpart is detected. We detect infrared/optical sources falling within the X-ray extraction regions of the 'knots', but note that these sources could be chance associations due to high source density around the target field and we estimate the probability of three randomly aligned X-ray sources to be 0.3 per cent in this field.