Where are the stars of the bar of NGC 1530 forming?

Aims. NGC 1530 has one of the strongest bars ever observed and recent star formation sites are distributed across its bar. Our aim is to study the photometric properties of the bar and its H II regions, to elucidate the conditions under which H II regions form and their spatial relation to the principal dynamical features of the bar.

Methods. We obtained BVRKs and H alpha photometry of the H II regions of the bar in NGC 1530. Broad-band integrated colours and H alpha equivalent widths were carefully measured and analysed as a function of position with respect to the main dust-lanes of the bar.

Results. We measured differences in the H alpha equivalent widths of the H II regions that are located in the trailing and leading sides of the bar dust-lane. The possible factors that could produce this difference, such as [N II] contamination, Lyman-continuum-photon dust-extinction, escape of ionizing radiation, metallicity, IMF and age, were carefully analysed. Age differences were confirmed to be the most plausible explanation. This implies that H II regions that are located further away from the bar dust-lane in its leading side, downstream from the main bar dust-lane, are older than the rest by similar to 1.5-2.5 Myr. In addition, a clear spatial correlation has been found between: location of H II regions, dust spurs on the trailing side of the bar dust-lane, and the loci of maximum velocity gradients parallel to the bar major axis (possibly tracing gas flow towards the main bar dust-lane). These results support the hypothesis that massive stars are forming on the trailing side of the bar dust-lane, and age as they cross the bar, on a timescale that is compatible with the bar dynamics timescale.