| Abstract | The Stephan's Quintet group of galaxies has been observed at both low (̃750 km s-1) and high (̃6700 km s-1) redshift in the 21-cm H I line. Westerbork synthesis maps of the low redshift H I with a resolution of 25 × 45" reveal that the gas is entirely associated with the low redshift spiral NGC 7320. The distribution and motion of the H I show no peculiarities which cannot reasonably be related to known instrumental deficiencies. These results therefore do not support the hypothesis of a strong interaction of NGC 7320 with the other members of the Quintet. The high redshift synthesis maps detect H I only in the range 6570 to 6630 km s-1 This cloud, previously detected with large reflectors, is not at the position of the high redshift galaxies, but ̃3' to the east. Its angular extent is ̃2;5 × 4' and total H I mass ̃1 1010 Msun (if at a distance of ̃100 Mpc). The velocity profile width of this cloud is only ̃100 km s-1 The peculiar mature of the high redshift H I means that it cannot in any way be used for distance determinations. The following possible scenario to explain the salient features of the high redshift H I is proposed: The isolated cloud of gas at 6600 km s-1 is the remnant of a collision 108-109 yr ago (perhaps involving NGC 7318a and 7319) which stripped the galaxies, heated the gas, and caused the gas to be separated from the galaxies in the manner proposed by Spitzer and Baade. Over the past 108-109 yr, the gas comprising the cloud has recombined, cooled, and expanded to ̃104 times its original volume. In this scenario, NGC 7318b would be a modern-day interloper in the Quartet and responsible for the observed radio continuum emission. |
