Starbursts in barred spiral galaxies. IV. On young bars and the formation of abundance gradients

The oxygen (O/H) and N/O abundance ratios along the bar of 16 barred spiral starburst galaxies are determined using long–slit spectroscopy. The abundance gradients and the spatial distribution of the ionized gas along the bar are used to understand the role of bars in starburst galaxies. The oxygen abundance gradients are steeper than in normal barred galaxies, with a mean of –0.15 dex/kpc, while the intersects are low. This excludes the possibility that starburst galaxies in this sample are chemically evolved galaxies rejuvenated by the effect of a bar. The nitrogen-to-oxygen abundance gradients are flatter than the oxygen ones with a mean of –0.05 dex/kpc. But N/O intersects are high, which rules out the possibility that a large quantity of gas was recently funneled by a bar toward the center of a young galaxy. There is no correlation between the abundance gradients and the bar properties, which suggests that bars did not influence the chemical evolution of these galaxies. Therefore, bars cannot be at the origin of the bursts in the nuclei of our sample galaxies. The oxygen and N/O abundance gradients are generally stronger in the bar than in the disk and are linked together by a linear relation consistent with a primary + secondary origin for the production of nitrogen. This can be fully explained in terms of star formation history in these galaxies. The gradients build up from the inside out, becoming stronger as the oxygen and N/O abundances increase in the bulge while staying low in the disk. This behavior is consistent with a simple Schmidt law relating the density of star formation to that of gas. In many of the sample galaxies, star formation occurs at one or both ends of the bar. The low level of chemical enrichment in these regions suggests that they recently experienced bartriggered star formation: this is the only visible effect of bars. Our analysis shows that bars probably appeared very recently (a few 107 years) in the starburst galaxies, which are relatively “young” galaxies still in the process of formation.