Can Giant Radio Galaxies teach us about cosmology?

18 September 2013

In a recent publication by Jurek Malarecki, our Deputy Director Lister Staveley-Smith’s PhD student at ICRAR-UWA, CAASTRO partner Ravi Subrahmanyan (Raman Research Institute in India), CAASTRO Affiliate Dr Alan Duffy (University of Melbourne), and their co-authors tested whether the morphology of giant radio galaxies can give insights into Warm-Hot Intergalactic Medium (WHIM) gas. The WHIM has been predicted to contain half the baryons in the Universe but has not yet been conclusively detected. Giant galaxies might be a suitable tool to study WHIM gas due to their large extent and assumed older age, implying more interaction with the gas and potential disruptions in galaxy evolution and perturbations to their morphology.

The team limited their radio observations at 2.1 GHz with the Australia Telescope Compact Array to galaxies of redshift up to 0.15 to also allow for optical spectroscopic observations. In their sample of twelve giant radio galaxies, they found varied morphologies, some of which indicated restarted nuclear activity and some of which showed faint extended emission beyond the previously known source, possibly revealing relic emission from a previous active phase. The team also observed three of the galaxies with the AAOmega spectrograph on the Anglo-
Australian Telescope and identified two as Type II Seyfert galaxies and one as an early-type galaxy.

While the Lorentz factor, a measure of electron energy, is usually calculated from X-ray data, the co-authors used their sample of giant radio galaxies to calculate the pressure in galaxy lobes, assuming these galaxies were old enough to have reached equilibrium. Their calculations produced pressures that extend below the X-ray inferred values from observations of galaxy filaments and temperatures in excess of 106.5 Kelvin. The conclusion from this is that giant radio galaxies in equilibrium are likely a minority in Universe. Surveys with the Australian SKA Pathfinder will be able to detect fainter giant radio galaxies and address their relevance for cosmology.

Publication details:

J. M. Malarecki, L. Staveley-Smith, L. Saripalli, R. Subrahmmanyan, D. H. Jones, A. R. Duffy, M. Rioja in MNRAS 432 "Giant Radio Galaxies: I. Intergalactic Barometers"