Inner and outer star forming regions over the disks of spiral galaxies. II. A comparative of physical properties and evolutionary stages

TL;DR

This study compares the physical properties and evolutionary stages of inner and outer disk HII regions in spiral galaxies, revealing systematic differences in metallicity, ionisation, and cluster characteristics through spectroscopic analysis.

Contribution

It provides a comprehensive comparison of inner and outer HII regions, highlighting differences in metallicity, ionising cluster mass, and evolutionary indicators, and evaluates the reliability of common calibration methods.

Findings

Inner regions have higher metallicities and N/O ratios.

Outer regions show younger ages based on equivalent widths.

Calibration methods have limitations depending on region location.

Abstract

The HII regions are all studied employing the same general prescriptions, despite the possible influence of their environment in their star formation processes. Through the analysis of two samples of 725 inner and 671 outer disk HII region observed spectra, we explore possible systematic differences between their ionising clusters physical properties (metallicity, mass, and age), comparing observations and predictions by photoionisation models. Higher metallicities are confirmed for inner regions, although there are important discrepancies between the diagnostic diagrams. Calibrations based on the N2 index may underestimate inner regions O/H due to the [NII] saturation at solar metallicities. The degeneracy between the age and ionisation parameter affects O/H calibrations based on the O3N2 index. Innermost regions have enhanced N/O ratios, indicating an increase in the slope of the relation between N/O and O/H. Ionisation parameter calibrations based on the [SII]/H{\alpha} ratio are not valid for inner regions due to the bivalued behaviour of this ratio with O/H. Innermost regions have lower [OII]/[OIII] ratios than expected, indicating a possible non-linear relation between u and Z. Ionising and non-ionising populations are present in both inner and outer regions. Inner regions show larger ionising cluster masses that possibly compose star-forming complexes. Outer regions 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. Inner regions have larger angular sizes, lower filling factors, and larger ionised hydrogen masses. The confirmed systematic differences between ionising clusters of inner and outer HII regions condition the validity and range of reliability of O/H and u calibrations commonly applied to the study of HII regions.