An intergalactic medium temperature from a giant radio galaxy

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