Star formation and tidal encounters with the low surface brightness galaxyUGC 12695 and companions

We present VLA H I observations of the low surface brightness (LSB) galaxy UGC 12695 and its two companions, UGC 12687 and a newly discovered dwarf ga laxy 2333 + 1234. UGC 12695 shows solid-body rotation but has a very lopsid ed morphology of the H I disk, with the majority of the H I lying in the so uthern arm of the galaxy. The H I column density distribution of this very blue LSB galaxy coincides in detail with its light distribution. Comparing the H I column density of UGC 12695 with the empirical (but not well-unders tood) value of Sigma(c) = 10(21) atoms cm(-2) found in, e.g., Skillman's 19 87 paper shows the star formation to be a local affair, occurring only in t hose regions where the column density is above this star formation threshol d. The low surface brightness nature of this galaxy could thus be attribute d to an insufficient gas surface density, inhibiting star formation on a mo re global scale. Significantly, however, the Toomre criterion places a much lower critical density on the galaxy (similar to 10(20) atoms cm(-2)), whi ch is shown by the galaxy's low star formation rate not to be applicable. W ithin a projected distance of 300 kpc/30 km s(-1) of UGC 12695 lie two comp anion galaxies-UGC 12687, a high surface brightness barred spiral galaxy, a nd 2333 + 1234, a dwarf galaxy discovered during this investigation. The cl ose proximity of the three galaxies, combined with UGC 12695's extremely bl ue color and regions of localized starburst and UGC 12687's UV, excess brin g to mind mutually induced star formation through tidal activity.