An intergalactic medium temperature from a giant radio galaxy

Monthly Notices of the Royal Astronomical Society, vol. 518, pp. 240–256

Abstract

ABSTRACT The warm–hot intergalactic medium (warm–hot IGM, or WHIM) pervades the filaments of the Cosmic Web and harbours half of the Universe’s baryons. The WHIM’s thermodynamic properties are notoriously hard to measure. Here we estimate a galaxy group–WHIM boundary temperature using a new method. In particular, we use a radio image of the giant radio galaxy (giant RG, or GRG) created by NGC 6185, a massive nearby spiral. We analyse this extraordinary object with a Bayesian 3D lobe model and deduce an equipartition pressure Peq = 6 × 10−16 Pa – among the lowest found in RGs yet. Using an X-ray-based statistical conversion for Fanaroff–Riley II RGs, we find a true lobe pressure $P = 1.5\substack{+1.7\\ -0.4}\times 10^{-15}\ \mathrm{Pa}$. Cosmic Web reconstructions, group catalogues, and MHD simulations furthermore imply an Mpc–scale IGM density $1 + \delta _\mathrm{IGM} = 40\substack{+30\\ -10}$. The buoyantly rising lobes are crushed by the IGM at their inner side, where an approximate balance between IGM and lobe pressure occurs: PIGM ≈ P. The ideal gas law then suggests an IGM temperature $T_\mathrm{IGM} = 11\substack{+12\\-5} \times 10^6\ \mathrm{K}$, or $k_\mathrm{B}T_\mathrm{IGM} = 0.9\substack{+1.0\\-0.4}\ \mathrm{keV}$, at the virial radius – consistent with X-ray-derived temperatures of similarly massive groups. Interestingly, the method is not performing at its limit: in principle, estimates TIGM ∼ 4 × 106 K are already possible – rivalling the lowest X-ray measurements available. The technique’s future scope extends from galaxy group outskirts to the WHIM. In conclusion, we demonstrate that observations of GRGs in Cosmic Web filaments are finally sensitive enough to probe the thermodynamics of galaxy groups and beyond.

Authors 8

1. Martijn S. S. L. Oei corresponding Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University , Niels Bohrweg 2, NL-2300 RA Leiden, the Netherlands

2. Reinout J. van Weeren corresponding Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University , Niels Bohrweg 2, NL-2300 RA Leiden, the Netherlands

3. M. J. Hardcastle

   University of Hertfordshire

   Affiliation as printed

   Centre for Astrophysics Research, University of Hertfordshire , College Lane, Hatfield AL10 9AB, UK

4. F. Vazza

   Universität Hamburg · Istituto di Radioastronomia di Bologna · University of Bologna

   Affiliation as printed

   Department of Physics and Astronomy, University of Bologna , Via Gobetti 93/2, I-40129 Bologna, Italy

   Hamburg Observatory, Hamburg University , Gojenbergsweg 112, D-21029 Hamburg, Germany

   INAF, Istituto di Radioastronomia , Via Gobetti 101, I-40129 Bologna, Italy

5. T. W. Shimwell Aachen Leiden Observatory

   Leiden University · Leiden Observatory · Netherlands Institute for Radio Astronomy

   Affiliation as printed

   ASTRON, the Netherlands Institute for Radio Astronomy , Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, the Netherlands

   Leiden Observatory, Leiden University , Niels Bohrweg 2, NL-2300 RA Leiden, the Netherlands

6. Florent Leclercq

   Centre National de la Recherche Scientifique · Institut d'Astrophysique de Paris · Sorbonne Université

   Affiliation as printed

   CNRS and Sorbonne Université, UMR 7095, Institut d’Astrophysique de Paris , 98 bis Boulevard Arago, F-75014 Paris, France

7. M. Brüggen

   Universität Hamburg

   Affiliation as printed

   Hamburg Observatory, Hamburg University , Gojenbergsweg 112, D-21029 Hamburg, Germany

8. Huub J. A. Röttgering Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University , Niels Bohrweg 2, NL-2300 RA Leiden, the Netherlands