The bimodal spiral galaxy surface brightness distribution

We have assessed the significance of Tully & Verheijen’s (1997) bimodal Ursa Major Cluster spiral galaxy near-infrared surface brightness distribution, focussing on whether this bimodality is simply an artifact of small number statistics. A KolmogorovSmirnov style of significance test shows that the total distribution is fairly represented by a single-peaked distribution, but that their isolated galaxy subsample (with no significant neighbours within a projected distance of ∼ 80 kpc) is bimodal at the 96 per cent level. We have also investigated the assumptions underlying the isolated galaxy surface brightness distribution, finding that the (often large) inclination corrections used in the construction of this distribution reduce the significance of the bimodality. We conclude that the Ursa Major Cluster dataset is insufficient to establish the presence of a bimodal near-infrared surface brightness distribution: an independent sample of ∼ 100 isolated, low inclination galaxies is required to establish bimodality at the 99 per cent level.