Alka Mishra,1 N. G. Kantharia2 and D. C. Srivastava1
1D.D.U. Gorakhpur University, Gorakhpur, India
2National Centre for Radio Astrophysics, TIFR, Post Bag 3, Ganeshkhind, Pune, India
Abstract. In this paper we present new H 21cm spectral line images of the poor group of late type galaxies, Holmberg 124 made using archival data from the Giant Metrewave Radio Telescope (GMRT). Holmberg 124 is a group of four late type galax- ies: NGC 2805, NGC 2814, NGC 2820 and Mrk 108. We detect spectral line emis- sion from all the four galaxies and note several signatures of tidal interactions among the member galaxies. Our results for the triplet (namely NGC 2820, NGC 2814 and Mrk 108) confirm the earlier results of Kantharia et al. (2005) notably the detection of a possible tidal dwarf galaxy to the north-east of NGC 2820. Further, in these images where the pointing center of the observations was changed, we have recovered most of the H emission in NGC 2805 as compared to Kantharia et al. (2005). We also report possible detection of small discrete clouds between NGC 2820 and NGC 2805 which might be stripped H in the intragroup medium (IGrM). However, these need confirmation. The H distribution of NGC 2805 is asymmetric with peak H column densities seen along the southern spiral arm and along a northern arc. Diffuse H is seen from the entire optical galaxy and extends much further in the southern parts, especially in the south-west. An abrupt fall in H column densities is observed to the south of NGC 2820 which is a high inclination galaxy and to the north of NGC 2805 which is a low inclination galaxy. Vigorous star formation has been observed along the southern spiral arm of NGC 2805. Based on these new H images, we support the scenario given by Kantharia et al. (2005) that both tidal interactions and ram pressure are currently playing a role in the evolution of the triplet galaxies. From the observed northern H arc and extensive star formation in the southern spiral arm of NGC 2805 and additionally the systemic velocities of the four galaxies, we suggest ram pressure effects are also playing a role in the evolution of NGC 2805. We believe that the H in the north of NGC 2805 is being compressed as it moves in the IGrM and that it is moving along a direction in the north-east close to the line-of-sight so that the entire disk is encountering the IGrM. This interaction with the IGrM would have triggered star formation in the southern spiral arm. This model for NGC 2805 succeeds in ex- plaining the compressed H in the north, widespread star formation and the diffuse H detected in the south/south-west of the optical galaxy. Deep X-ray observations of the IGrM and deeper H observations sensitive to larger angular scales will be very useful in furthering the understanding of this interesting group of galaxies.
Keywords: galaxies: interactions – galaxies: individual: NGC 2805 – galaxies: kinematics and dynamics – radio lines: galaxies – radio continuum: galaxies