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 vrot and velocity dispersion σint of the satellite system. We found vrot/σint ≃ 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 Mb/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.
Highlights
Galaxy groups are a key testbed for the Λ cold dark matter (ΛCDM) cosmological model (Kroupa et al 2010; Bullock & Boylan-Kolchin 2017; Oppenheimer et al 2021) as well as for alternative theories (Banik & Zhao 2018; Milgrom 2019)
In a Λ cold dark matter cosmological context, we found that the Centaurus A (Cen A) group has a baryon fraction Mb/M200 0.035 and is missing ∼77% of the cosmologically available baryons
We studied the dynamics of the Cen A galaxy group using accurate 3D distances and line-of-sight velocities of its member galaxies
Summary
Galaxy groups are a key testbed for the Λ cold dark matter (ΛCDM) cosmological model (Kroupa et al 2010; Bullock & Boylan-Kolchin 2017; Oppenheimer et al 2021) as well as for alternative theories (Banik & Zhao 2018; Milgrom 2019). Both methods assume that the member galaxies are isotropically distributed (spherical symmetry) and follow random orbits These assumptions appear unreasonable for our own Local Group: most dwarf satellites of the Milky Way and M31 are distributed in narrow planar structures with significant angular momentum (Pawlowski et al 2012; Ibata et al 2013; Pawlowski & Kroupa 2020; Santos-Santos et al 2020; Pawlowski & Tony Sohn 2021). Satellite systems with a similar kinematic coherence are extremely rare in ΛCDM simulations, leading to the so-called planes of satellites problem (Kroupa et al 2010; Libeskind et al 2015; Pawlowski 2018) These observations suggest that dynamical mass estimates of Cen A should consider a flattened (non-spherical) system with both rotation and pressure support. We compare our new mass estimate with previous determinations in the literature and discuss the implications for ΛCDM cosmology and alternative theories (Sect. 4)
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