A multiwavelength study of the starburst galaxy NGC7771

Abstract

We present a multiwavelength study of the interacting starburst galaxy NGC7771, including new optical and ultraviolet spectra and a previously unpublished soft X-ray ROSAT image and spectrum. The far-infrared, radio and X-ray fluxes suggest that a massive burst of star formation is currently in progress, but the small equivalent width of the Balmer emission lines (equivalent width Hα~100A), the weak UV flux, the low abundance of ionized oxygen, and the shape of the optical spectrum lead us to conclude that there are few O stars. This might normally suggest that star formation has ceased, but the barred gravitational potential and large gas reserves of the galaxy imply that this should not be so, and we therefore consider other explanations. We argue that the observations cannot be due to effects of geometry, density-bounded nebulae, or dust within the nebulae, and conclude that a truncated IMF is required. The dwarf galaxy NGC7770 appears to be in the initial stages of a merger with NGC7771, and the resulting tidal perturbations may have induced the apparent two-armed spiral pattern, and driven a substantial fraction of the disc gas inwards. The presence of a bulge in NGC7771 may be moderating the starburst so that, while still occurring on a large scale with a supernova rate of 0.8-1 yr^-1, it is less violent and the IMF has a relatively low upper mass limit. We find that there is a cluster of stars obscuring part of the starburst region, and we offer an explanation of its origin.