Insight into the baryon-gravity relation in galaxies

Abstract

Observations of spiral galaxies strongly support a one-to-one analytical relation between the inferred gravity of dark matter at any radius and the enclosed baryonic mass. It is baffling that baryons manage to settle the dark matter gravitational potential in such a precise way, leaving no "messy" fingerprints of the merging events and "gastrophysical" feedbacks expected in the history of a galaxy in a concordance Universe. This correlation of gravity with baryonic mass can be interpreted from several non-standard angles, especially as a modification of gravity called TeVeS, in which no galactic dark matter is needed. In this theory, the baryon-gravity relation is captured by the dieletric-like function µ of Modified Newtonian Dynamics (MOND), controlling the transition from 1/r 2 attraction in the strong gravity regime to 1/r attraction in the weak regime. Here, we study this µ-function in detail. We investigate the observational constraints upon it from fitting galaxy rotation curves, unveiling the degeneracy between the stellar mass-to-light ratio and the µ-function as well as the importance of the sharpness of transition from the strong to weak gravity regimes. We also numerically address the effects of non-spherical baryon geometry in the framework of non-linear TeVeS, and exhaustively examine how the µ-function connects with the free function of that theory. In that regard, we exhibit the subtle effects and wide implications of renormalizing the gravitational constant. We finally present a discontinuity-free transition between quasi-static galaxies and the evolving Universe for the free function of TeVeS, inevitably leading to a return to 1/r 2 attraction at very low accelerations in isolated galaxies.

Galactic kinematics with modified Newtonian dynamics

“…Nowadays, the dominant paradigm is that dark matter is made of non-baryonic weakly-interacting massive particles, called "cold dark matter" (CDM). However, on galactic scales, the observations appear to be at variance with a sizeable list of CDM predictions (see, e.g., Famaey et al 2007b). The observed conspiracy between the mass profiles of baryonic matter and dark matter at all radii in spiral galaxies (e.g., Famaey et al 2007b) rather lends support to modified Newtonian dynamics (MOND, Milgrom 1983), a paradigm postulating that for accelerations below a 0 ≈ 10 −10 m s −2 the true gravitational attraction approaches (g N a 0 ) 1/2 , where g N is the usual Newtonian gravitational field.…”

“…However, on galactic scales, the observations appear to be at variance with a sizeable list of CDM predictions (see, e.g., Famaey et al 2007b). The observed conspiracy between the mass profiles of baryonic matter and dark matter at all radii in spiral galaxies (e.g., Famaey et al 2007b) rather lends support to modified Newtonian dynamics (MOND, Milgrom 1983), a paradigm postulating that for accelerations below a 0 ≈ 10 −10 m s −2 the true gravitational attraction approaches (g N a 0 ) 1/2 , where g N is the usual Newtonian gravitational field. Without resorting to CDM, this paradigm is known to reproduce galaxy scaling relations, as well as the rotation curves of individual galaxies ranging over five orders of magnitude in mass (e.g., Sanders & McGaugh 2002).…”

THINGS about MOND

“…Although Brada & Milgrom (1995), Famaey et al (2007) and Zhao & Famaey (2010) have shown that the expected differences in the predictions of the various formulations for rotation curves are not very large, they can be of the same order of magnitude as the differences produced by different choices for the μ-function. To constrain μ within the modified gravity framework, one should calculate predictions of the modified Poisson formulations of Bekenstein & Milgrom (1984) or Milgrom (2010) numerically for each galaxy model and for each choice of parameters.…”

Dynamics of the NGC 4636 globular cluster system

“…M being the total mass, G the gravitational constant, and a 0 the MOND constant, which has the value 1.35 +0.28 −0.42 × 10 −8 cm s −2 (Famaey et al 2007). The factor of 50 in the above relation (Eq.…”