Star Formation and Tidal Encounters with the Low Surface Brightness Galaxy UGC 12695 and Companions

We present VLA H I observations of the low surface brightness galaxy UGC12695 and its two companions, UGC 12687 and a newly discovered dwarf galaxy2333+1234. UGC 12695 shows solid body rotation but has a very lopsidedmorphology of the H I disk, with the majority of the H I lying in the southernarm of the galaxy. The H I column density distribution of this very blue, LSBgalaxy coincides in detail with its light distribution. Comparing the H Icolumn density of UGC 12695 with the empirical (but not well understood) valueof Sigma_c = 10E21 atoms/cm^2 found in, i.e., Skillman's 1986 paper shows thestar formation to be a local affair, occurring only in those regions where thecolumn density is above this star formation threshold. The low surfacebrightness nature of this galaxy could thus be attributed to an insufficientgas surface density, inhibiting star formation on a more global scale.Significantly, though, the Toomre criterion places a much lower criticaldensity on the galaxy (+/-10E20 atoms/cm^2), which is shown by the galaxy's lowSFR to not be applicable. Within a projected distance of 300kpc/30kms of UGC 12695 lie two companiongalaxies - UGC 12687, a high surface brightness barred spiral galaxy, and2333+1234, a dwarf galaxy discovered during this investigation. The closeproximity of the three galaxies, combined with UGC 12695's extremely blue colorand regions of localized starburst and UGC 12687's UV excess bring to mindmutually induced star formation through tidal activity.Comment: 14 pages, 8 figures (2 color), To be published in A.J., May 2000