MOND in galaxy groups

TL;DR

This paper investigates galaxy groups using MOND, showing that MOND can explain their dynamics without dark matter, even at very low accelerations, extending MOND's applicability beyond individual galaxies.

Contribution

The study applies MOND to 53 galaxy groups, demonstrating that MOND accounts for their dynamics with stellar mass-to-light ratios consistent with stellar populations, extending MOND analysis to larger, low-acceleration systems.

Findings

MOND explains galaxy group dynamics without dark matter.

Dynamical M/L ratios align with stellar population estimates.

Groups exhibit accelerations much lower than previous tests, confirming deep-MOND predictions.

Abstract

Galaxy groups, which have hardly been looked at in MOND, afford probing the acceleration discrepancies in regions of system-parameter space that are not accessible in well-studied galactic systems, such as galaxies, galaxy clusters, and dwarf-spheroidal satellites of galaxies. Groups are typically the size of galaxy-cluster cores, but have masses typically only a few times that of a single galaxy. Accelerations in groups get far below those in galaxies, and far below the MOND acceleration. So much so, that many groups might be affected by the external-field effect, which is unique to MOND, due to background accelerations. Here, I analyze the MOND dynamics of 53 galaxy groups, recently catalogued in 3 lists. Their Newtonian, K-band, dynamical $M/L$ ratios are a few tens to several hundreds solar units, with $\langle{M_d/L_K}\rangle= (56, 25, 30)~M_{\odot}/L_{\odot}$, respectively for the 3 lists; thus evincing very large acceleration discrepancies. I find here that MOND requires dynamical $M_M/L_K$ values of order $1~M_{\odot}/L_{\odot}$, with $\langle{M_M/L_K}\rangle=(0.8, 0.56, 1.0) ~M_{\odot}/L_{\odot}$, for the 3 lists, which are in good agreement with population-synthesis stellar values, and with those found in individual galaxies. MOND thus accounts for the observed dynamics in those groups with baryons alone, and no need for dark matter -- an important extension of MOND analysis from galaxies to galactic systems, which, to boot, have characteristic sizes of several hundred kiloparsecs, and accelerations much lower than probed before -- only a few percent of MOND's $a_0$. The acceleration discrepancies evinced by these groups thus conform to the deep-MOND prediction: $g\approx (g_Na_0)^{1/2}$, down to these very low accelerations ($g$ is the measured, and $g_N$ the baryonic, Newtonian acceleration).