New light and shadows on Stephan's Quintet

Abstract

We present deep broad-band R and narrow-band Halpha images of Stephan's Quintet. The observations in the R band show that the diffuse halo of the Stephan's Quintet is larger than previously thought and extends out to NGC 7320C. However, we have not found emission connecting NGC 7331 and NGC 7320 to R ~ 26.7 mag/arcsec^2 (at more than 3-sigma level), so there is no direct evidence up to this limiting magnitude of a relation between the peculiar kinematical structure found in NGC 7331 and an ongoing or past interaction between this galaxy and NGC 7320. The Halpha emission at high-velocity (6000-7000 km/s) is distributed in a diffuse structure running NS between NGC 7319 and NGC 7318B and in some other more concentrated features. Some of these are located in the tidal tails produced by the interaction between the galaxies of the group. With the Halpha images we have made a two-dimensional velocity map which helps to identify the origin of each structure detected. This map does not show features at intermediate velocities between the high- and low-redshift members of the group. This is in agreement with the standard scenario in which the apparent proximity of NGC 7320 to the rest of the galaxies of the Quintet is merely a projection effect. The only point which is unclear in this interpretation, is an Halpha filament which is seen extending throughout NGC 7320 with velocity at 6500 km/s instead of the 800 km/s expected for this galaxy.