GMRT Observations of the Group Holmberg 124: Evolution by Tidal Forces   and Ram Pressure ?
  We report radio continuum and 21cm HI observations using the Giant Metrewave Radio Telescope of the group Ho124 comprising four late-type galaxies, NGC2820, Mrk108, NGC2814 and NGC2805. Three of the galaxies have clearly undergone tidal interactions as seen from the various morphological tidal signatures whereas other features in the group members appear to be due to ram pressure. Here, we describe four interesting results:(a) detection of the tidal radio continuum bridge at 330 MHz connecting the galaxies NGC2820+Mrk108 with NGC2814 with a spectral index of -1.8. The radio bridge was discovered by van der Hulst & Hummel (1985) who reported a spectral index of -0.8.(b) Detection of various tidal features including a possible tidal dwarf galaxy.(c) Sharp truncation in the HI distribution in the south of NGC2820 and in the HI & radio continuum in the north of NGC2814. The optical disks in both the cases look undisturbed. (d) Detection of a large one-sided HI loop to the north of NGC 2820. No radio continuum emission or Halpha emission is detected. We discuss various scenarios for the origin of this loop. The central starburst scenario is not supported due to lack of ionized gas in the loop. For an electron density of 4x10^(-4) cm^-3 we estimate the ram pressure force of the IGrM to be comparable to the gravitational pull of the disk of NGC2820. We suggest that the loop could have formed by ram pressure stripping if tidal effects had reduced the surface density of HI in NGC2820. Finally we suggest that the evolution of the Ho124 group may be governed by both tidal forces due to the interaction and ram pressure due to motion of the member galaxies in the IGrM and that the IGrM densities might not be too low (>~4x10^(-4)cm^-3). This should be verified by other observations. 
N. G. Kantharia, S. Ananthakrishnan, R. Nityananda, Ananda Hota