ABSTRACT

We present a study of the O/H gradients derived from large numbers of HII regions in a sample of barred galaxies. It is shown that the slopes of the O/H gradients in galaxies with a barred structure are related to the bar strength: the stronger is the bar, the flatter the abundance gradient becomes. This result is in agreement with a scenario of mixing of the interstellar gas by radial flows across the disks of barred galaxies. Thus we concluded that a bar acts as a homogenizer on the chemical composition in a spiral galaxy.

INTRODUCTION: RADIAL FLOWS

Observations and numerical simulations have shown that strong radial flows are present in barred spiral galaxies (e.g. Sellwood and Wilkinson 1993). Although the exact origin of such flows is unclear, their amplitudes seem to be related to the bar properties and to the presence of radiative shocks induced when the gas passes through the bar. A possible important consequence of radial flows in a spiral galaxy is a radial redistribution of the chemical composition across the disk. Such mechanism was suggested to explain why the O/H gradients in few barred galaxies (e.g. NGC 1365) are shallower than gradients observed in normal spirals.

Review works by Vila-Costas and Edmunds (1992, VE) and Zaritsky et al. (1993, ZKH) have confirmed this difference. However, their large samples of galaxies included very few barred spirals where the O/H gradient is derived from a large number of HII regions.