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

Abstract

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 HII regions, to elucidate the conditions under which HII regions form and their spatial relation to the principal dynamical features of the bar. Methods: We obtained BVRKs and Halpha photometry of the HII regions of the bar in NGC 1530. Broad-band integrated colours and Halpha 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 Halpha equivalent widths of the HII 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 [NII] 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 HII 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 ~1.5-2.5 Myr. In addition, a clear spatial correlation has been found between: location of HII 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.