MOND in galaxy groups

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 $⟨M_d/L_K⟩=(56,25,30)M_{\odot }/L_{\odot }$, 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 $1M_{\odot }/L_{\odot }$, with $⟨M_M/L_K⟩=(0.8,0.56,1.0)M_{\odot }/L_{\odot }$, 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: $g\approx (g_N{a}_0{)}^{1/2}$, down to these very low accelerations ($g$ is the measured, and $g_N$ the baryonic, Newtonian acceleration).

5 text pages and 3 Tables. Added Figures and references, and expanded discussion. Version accepted for publication in Phys. Rev. D

References in corpus (8)

• Fitting the radial acceleration relation to individual SPARC galaxies
• X-ray Group and cluster mass profiles in MOND: Unexplained mass on the group scale
• Andromeda Dwarfs in Light of MOND
• Marriage à-la-MOND: Baryonic dark matter in galaxy clusters and the cooling flow puzzle
• Large-scale structure and galaxy motions in the Leo/Cancer constellations
• Consequences of the external field effect for MOND disk galaxies in galaxy clusters
• Galaxy motions in the Bootes strip
• MOND from a brane-world picture

Cited by in corpus (7)

• From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity
• MOND in galaxy groups: a superior sample
• Predicted MOND velocity dispersions for a catalog of ultra-diffuse galaxies in group environments
• The prediction of rotation curves in gas-dominated dwarf galaxies with modified dynamics
• Peculiar dark matter halos inferred from gravitational lensing as a manifestation of modified gravity
• Examining Baryonic Faber-Jackson Relation in Galaxy Groups
• Are disks of satellites comprised of tidal dwarf galaxies?