The Cen A galaxy group: Dynamical mass and missing baryons^⋆

¹ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
² Institute of Physics, Laboratory of Astrophysics, Ècole Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
e-mail: oliver@oliver-mueller.ch
³ INAF, Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, 50125 Florence, Italy
⁴ Leibniz-Institut fur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁵ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁶ European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
⁷ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia

Received: 1 October 2021
Accepted: 25 November 2021

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 v_rot and velocity dispersion σ_int of the satellite system. We found v_rot/σ_int ≃ 0.7, indicating that the satellite system has non-negligible rotational support. Using Jeans’ equations, we measured a circular velocity of 258 km s⁻¹ and a dynamical mass of 1.2 × 10¹³ M_⊙ within 800 kpc. In a Λ cold dark matter cosmological context, we found that the Cen A group has a baryon fraction M_b/M₂₀₀ ≃ 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 × 10¹¹ M_⊙, 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.