Star formation rate based on the FUV and 24    μm emission as a function of the molecular gas surface density. The crosses are based on X = 10²⁰ cm^-2 (K km s^-1)^-1, which is half of the Galactic value. The bridge region outside the giant Hii region near UGC 12915 are marked as squares with crosses. Squares without crosses correspond to a 2.5 times lower X factor as suggested by Zhu et al. (2007). Data of NGC 4438 are shown as triangles. The dashed lines correspond to constant star formation timescales of 2 and 4 Gyr.