The Cen A galaxy group: Dynamical mass and missing baryons
Oliver Müller, Federico Lelli, Benoît Famaey, Marcel S. Pawlowski, Katja Fahrion, M. Rejkuba, M. Hilker, Helmut Jerjen
Abstract
The nearby elliptical galaxy Centaurus A (Cen A) is surrounded by a flattened system of dwarf satellite galaxies with coherent motions. Using a novel Bayesian approach, we measured the mean rotation velocity <i>v<i/><sub>rot<sub/> and velocity dispersion <i>σ<i/><sub>int<sub/> of the satellite system. We found <i>v<i/><sub>rot<sub/>/<i>σ<i/><sub>int<sub/> ≃ 0.7, indicating that the satellite system has non-negligible rotational support. Using Jeans’ equations, we measured a circular velocity of 258 km s<sup>−1<sup/> and a dynamical mass of 1.2 × 10<sup>13<sup/> <i>M<i/><sub>⊙<sub/> within 800 kpc. In a Λ cold dark matter cosmological context, we found that the Cen A group has a baryon fraction <i>M<i/><sub>b<sub/>/<i>M<i/><sub>200<sub/> ≃ 0.035 and is missing ∼77% of the cosmologically available baryons. Consequently, Cen A should have a hot intergalactic medium with a mass of ∼8 × 10<sup>11<sup/> <i>M<i/><sub>⊙<sub/>, which is more than ∼20 times larger than current X-ray estimates. Intriguingly, the whole Cen A group lies on the baryonic Tully-Fisher relation defined by individual rotationally supported galaxies, as expected in Milgromian dynamics (MOND) with no need of missing baryons.