A slow bar in a dark matter dominated galaxy

Aims. We report on an estimate of the bar pattern speed Omega(p) for the low surface brightness spiral galaxy UGC 628. Methods. We applied the Tremaine-Weinberg method to high resolution H alpha velocity and integrated emission maps of this dark matter dominated galaxy. Observations were made at the CFHT using the optical Fabry-Perot interferometer, FANTOMM. Results. The Tremaine-Weinberg method estimates a bar pattern speed of (11.3 +/- 2.0) km s(-1) kpc(-1) for UGC 628, which is among the lowest values found for a spiral galaxy. The corotation radius R-c of the bar and the gaseous disc is R-c = 9.8(-2.0)(+2.9) kpc, implying a ratio R = R-c/a(B) of 2.0(-0.3)(+0.5), where a(B) is the bar radius. The ratio is well beyond the usual range of values, 1.0 <= R <= 1.4, found for fast bars of high surface brightness barred galaxies. This implies that the bar in UGC 628 is slow. Conclusions. As shown through the use of numerical simulations, fast bars survive when the inner mass distribution of galaxies is dominated by the baryons over the dark matter. Our result suggests that the presence of slow bars in galaxies is likely related to the dominance of dark matter over the mass distribution.