Abundance gradients in a sample of barred spiral galaxies

Laboratório de Análise Numérica e Astrofísica, Departamento de Matemática, Universidade Federal de Santa Maria, 97119-900 Santa Maria, RS, Brazil e-mail: oli@lana.ccne.ufsm.br

Received: 6 February 2004
Accepted: 14 March 2005

Abstract

We used photoionization models in order to reproduce the observed gradients of emission-line ratios for HII regions located in the normal spiral galaxy M 101 and in three barred spiral galaxies, namely NGC 1365, NGC 925, and NGC 1073. The behavior of the following nebular parameters across the disk of these galaxies was determined: temperature of the ionizing star (T_eff), ionization parameter (U), and the abundance ratios O/H, N/O, and S/O. Our O/H predictions were found to be consistent with some empirical abundance determinations, but are overestimated by a factor of 0.1-0.4 dex when compared to the direct abundance determinations. NGC 1073 seems to be overabundant in nitrogen compared to other spiral galaxies. No gradient of S/O was found in the studied galaxies, and we derived a positive T_eff gradient of K and U ranging from -3.0 to -2.3. The N/O vs. O/H diagram in general is consistent with chemical evolution models that assume that the nitrogen synthesis has both a primary and a secondary component compared to oxygen. However, a very strong N/O dependence on the O/H at high abundance was found.