MOND in galaxy groups
  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).
