2007A&A...474...43V

Star formation depends strongly on both the local environment of galaxies and the internal dynamics of the interstellar medium. To disentangle the two effects, we obtained, in the framework of the AMIGA project, H_α and Gunn r photometric data for more than 200 spiral galaxies lying in very low-density regions of the local Universe. We characterise the H_α emission, tracing current star formation, of the 45 largest and least inclined galaxies observed for which we estimate the torques between the gas and the bulk of the optical matter. We subsequently study the H_α morphological aspect of these isolated spiral galaxies. Using Fourier analysis, we focus on the modes of the spiral arms and also on the strength of the bars, computing the torques between the gas and newly formed stars (H_α), and the bulk of the optical matter (Gunn r). We interpret the various bar/spiral morphologies observed in terms of the secular evolution experienced by galaxies in isolation. We also classify the different spatial distributions of star forming regions in barred galaxies. The observed frequency of particular patterns brings constraints on the lifetime of the various evolution phases. We propose an evolutive sequence accounting for the transitions between the different phases we observed. Isolated galaxies do not appear to be preferentially barred or unbarred. Fitting the H_α distributions using numerical simulations yields constraints on the star formation law, which is likely to differ from a genuine Schmidt law. In particular, it is probable that the relative velocity of the gas in the bar also needs to be taken into account.