The Cen A galaxy group: dynamical mass and missing baryons

Abstract

The nearby elliptical galaxy Cen A is surrounded by a flattened system of dwarf satellite galaxies with coherent motions. Using a novel Bayesian approach, we measure the mean rotation velocity $v_{\mathrm{rot}}$ and velocity dispersion $\ddot{I}{ƒ}_{\mathrm{int}}$ of the satellite system. We find $v_{\mathrm{rot}}/\ddot{I}{ƒ}_{\mathrm{int}}\simeq 0.7$ indicating that the satellite system has non-negligible rotational support. Using Jeans' equations, we measure a circular velocity of 258 km s${}^{-1}$ and a dynamical mass of $1.2\times 10^{13}$ M${}_{\odot }$ within 800 kpc. In a $\hat{I}›$CDM cosmological context, we find that the Cen A group has a baryon fraction $M_{\mathrm{b}}/M_{200}\simeq 0.035$ and is missing $\sim$77$\%$ of the cosmologically available baryons. Consequently, Cen A should have a hot intergalactic medium with a mass of $\sim$8$\times$10${}^{11}$ M${}_{\odot }$, which is more than $\sim$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.

Accepted for publication in A&A, 7 pages, 3 figures, 1 table