  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.
