Optical studies of H I-rich southern galaxies. I. The supergiant spiral NGC 6902.

NGC 6902, originally classified as an early-type galaxy, is shown to be a high-luminosity (bolometric magnitude-21.8) Sb I galaxy. Integral properties are derived from photometric studies which include surface photometry from CTIO 4-m prime focus plates. In most respects NGC 6902 is a normal, luminous spiral, but there are two exceptions: (1) its H I content exceeds the mean for Sb galaxies by about a factor of 3; (2) the spiral pattern is disorganized and the arms have low brightness amplitude as compared with typical Sb I systems, a feature which seems characteristic of H I-rich spirals. NGC 6902 therefore is difficult to recognize as a high luminosity galaxy from its morphology, which might account for an apparent bias against the inclusion of gas-rich galaxies in samples of luminous spirals. Mechanisms which could produce a correlation between high gas content and low amplitude spiral arms are considered.