Inner and outer star forming regions over the disks of spiral galaxies

Abstract

Context.The H IIregions are all studied employing the same general prescriptions and models, independently of the regions location in the galaxy disk. However, observed discrepancies between physical properties of inner and outer regions may indicate systematic differences in their star formation processes due to the influence of their environments.Aims.Through the analysis of inner and outer H IIregion observed spectra, we aim to explore possible systematic differences between the physical properties (metallicity, mass, and age) of their ionising clusters in order to study how star formation proceeds in different environments.Methods.We analysed two samples of 725 inner and 671 outer regions, characterised in the first paper of this series. Their functional parameters (oxygen abundances, ionisation parameters, and effective temperatures) were estimated and this parameter grid is employed as input for the computation of 540 Cloudy photoionisation models. Observed regions are compared with model predictions using diagnostic and evolutionary diagrams.Results.Higher metallicities are confirmed for the inner regions, although there are important discrepancies between the diagnostic diagrams. Calibrations based on the N2 index may underestimate inner regions oxygen abundances due to the [N II] saturation at solar metallicities. The degeneracy between the age and ionisation parameter affects oxygen abundance calibrations based on the O3N2 index. Innermost regions seem to have enhanced N/O ratios with respect to the expected values considering secondary production of nitrogen, which indicate an increase in the slope of the relation between N/O and O/H. Ionisation parameter calibrations based on the [S II]/Hαratio are not valid for inner regions due to the observed bivalued behaviour of this ratio with O/H. Innermost regions have lower [O II]/[O III] ratio values than expected, indicating a possible non-linear relation betweenuandZ. Composite stellar populations (ionising and non-ionising) are present in both inner and outer regions, with an ionising contribution of around 1%. In considering the effects of evolution and underlying populations, inner regions show larger ionising cluster masses that possibly compose star-forming complexes. The most conservative lower limit for ionising cluster masses of outer regions indicate that they might be affected by stochastic effects. Equivalent widths indicate younger ages for outer regions, but degeneracy between evolution and underlying population effects prevent a quantitative determination. Nebular properties of the H IIregions are also derived: inner regions have larger angular sizes, lower filling factors, and larger ionised hydrogen masses.Conclusions.Systematic physical differences are confirmed between ionising clusters of inner and outer H IIregions. These differences condition the validity and range of reliability of oxygen abundance and ionisation parameter calibrations commonly applied to the study of H IIregions.