The baryonic Faber-Jackson relation and fundamental plane of galaxy groups, elliptical galaxies, and dwarf galaxies

The baryonic Faber-Jackson relation (BFJR) links the baryonic mass of pressure-supported systems to their mean velocity dispersion. For elliptical galaxies, the BFJR is thought to be a projection of the fundamental plane (FP), which includes the stellar half-mass radius as a third variable. We studied the BFJR and FP across eight orders of magnitude in baryonic mass, encompassing galaxy groups, ellipticals, dwarf ellipticals, and dwarf spheroidals. We compiled and homogenized data for 1400 pressure-supported systems and measured their mean internal baryonic acceleration, ⟨gbar⟩. We find that the properties of the BFJR and FP systematically depend on the internal acceleration of the sampled systems, with a transition around the acceleration scale a0 ≃ 1.2 × 10−10 m s−2. For low-acceleration systems with ⟨gbar⟩< 0.6 a0 (dwarf galaxies and galaxy groups), the BFJR takes the form log10(Mbar/M⊙) = (4.19 ± 0.10) log10(σlos/km s−1) + (2.55−0.16+0.16. The FP expected from the Newtonian virial theorem is followed by high-acceleration systems (massive ellipticals with ⟨gbar⟩≳6 a0), whereas low-acceleration systems deviate from the FP at both low masses (dwarf galaxies) and high masses (galaxy groups). Our results generally agree with the expectations of modified Newtonian dynamics (MOND): high-acceleration systems follow the Newtonian virial theorem in which a radial variable explicitly appears (the FP), while low-acceleration systems follow the MOND virial theorem in which the radial dependence disappears (the BFJR). On average, the MOND external field effect seems to play a secondary role in dwarf galaxies in galaxy groups and clusters.