Testing gravity with motion of satellites around galaxies: Newtonian gravity against Modified Newtonian Dynamics

TL;DR

This study tests gravity theories using satellite galaxy motions around normal galaxies, finding that observations strongly support the standard cosmological model over Modified Newtonian Dynamics (MOND) at scales of 50-500 kpc.

Contribution

The paper provides empirical evidence that the standard cosmological model accurately predicts satellite galaxy dynamics, challenging MOND's predictions at these scales.

Findings

Standard model matches satellite density and velocity data.

MOND fails to fit observed velocity profiles without fine-tuning.

Satellite data favor the standard model over MOND at 50-500 kpc scales.

Abstract

The motion of satellite galaxies around normal galaxies at distances 50-500 kpc provides a sensitive test for the theories. We study the surface density and the velocities of satellites around isolated galaxies in the Sloan Digital Sky Survey. We find that the surface number-density of satellites declines with the projected distance as a power law with the slope -1.5-2. The rms velocities gradually decline: observations exclude constant velocities at a 10 sigma level. We show that observational data strongly favor the standard model: all three major statistics of satellites - the number-density profile, the line-of-sight velocity dispersion, and the distribution function of the velocities -- agree remarkably well with the predictions of the standard cosmological model. Thus, that the success of the standard model extends to scales (50-500) kpc, much lower than what was previously considered. MOND fails on these scales for models which assume any single power-law number-density profile of satellites and any constant velocity anisotropy by predicting nearly constant rms velocities of satellites. Satellite data can be fit by fine-tuned models, which require (1) specific non-power-law density profile, (2) very radial orbits at large distances (velocity anisotropy beta =0.6-0.7 at 200-300 kpc), and (3) 2-2.5 times more stellar mass than what is found in the galaxies. The external gravity force - a necessary component for MOND -- makes the situation even worse. We argue that a combination of satellite data and observational constraints on stellar masses make these models very problematic.