MOND in galaxy groups: A superior sample

Intermediate-richness galaxy groups are an important testing ground for modified Newtonian dynamics (MOND). First, they constitute a distinct type of galactic systems, with their own evolution histories and underlying physical processes; second, they probe little-chartered regions of parameter space, as they have baryonic masses similar to massive galaxies, and similar velocity dispersions, but much larger sizes-similar to cluster cores (or even to clusters), but much lower dispersions. Importantly in the context of MOND, they have the lowest internal accelerations reachable inside galactic systems. Following my recent analysis of MOND in galaxy groups, I came across a much superior sample, which I analyze here. This extensive catalog permits strict quality cuts that still leave a large sample of 56 medium-richness groups, better suited for dynamical analysis-e. g., in having a large number (>= 15) of members with measured velocities. I find that these groups obey the deep-MOND relation between baryonic mass, M-M, and velocity dispersion, sigma: MMGa0 = (81/4)sigma(4), with individual, MOND, mass-to-light ratios, M-M/L-K of order 1 M-circle dot= L-K,L-circle dot, and a sample median value of (M-M/L-K)(med) = 0.7 M-circle dot= L-K,L-circle dot. These compare well with stellar values deduced for single galaxies, and with values deduced from population-synthesis analyses. In contrast, the dynamical, Newtonian Md= LK values are much larger-several tens solar units, and (M-d/L-K)(med) = 37 M-circle dot= L-K,L-circle dot. The same MOND relation describes (isolated) dwarf spheroidals-twothree orders smaller in size, and seven-eight orders lower in mass. The groups conformation to the MOND relation is equivalent to their lying on the deep-MOND branch of the "mass-discrepancy-acceleration relation," g approximate to (g(N)a(0))(1/2), for g as low as a few percents of a(0) (g(N) is the Newtonian, baryonic, gravitational acceleration, and g the actual one). This argues against systematic departure from MOND at extremely low accelerations (as has been suggested). This conformation also argues against the hypothesis that the remaining MOND conundrum in cluster cores bespeaks a breakdown of MOND on large-distance scales; our groups are as large as cluster cores, but do not show obvious disagreement with MOND. I also discuss the possible presence of the idiosyncratic, MOND external-field effect.