Calibration of the H II region abundance sequence and abundances in Seyfert galaxies

Abstract

An extensive, homogeneous grid of theoretical photoionization models with conditions appropriate to observed H n regions is computed. This grid is used to develop a comprehensive set of theoretical diagnostic diagram s for H ii regions which employ ratios of prom inentemission lines to determine the ionization parameter within the nebula, and the ionization temperature of its exciting star(s). It is also possible to estimate the element-averaged m etallicity in the nebula and stars from these diagnostics. Theoretical loci for H ii regions on the principal excitation diagrams of Baldwin, Phillips, and Terlevich (1981) are constructed and confirm th at the theoretical models overlie the observed distribution of H u regions on these diagrams. Comparison of current observational data with the theoretical models on these and other diagrams implies the existence of a correlation between elementaveraged metallicity and mean ionization parameter. The data also suggest that the ionization temperature of the exciting star(s) in the ionizing OB associations of H ii regions is approximately constant and independent of metallicity and ionization parameter for the range of conditions considered here. A multiline fit to the observed emission-line spectral sequence of extragalactic H u regions is generated. It is possible to produce an excellent fit between the (rather narrow) emission-line ratio sequences that are observed and the theoretical sequences, provided that the average ionization parameter is strongly coupled w ith metallicity, and provided that nitrogen is a product of secondary nucleosynthesis. Using this theoretical sequence of H ii region models, the semiempirical abundance diagnostic ratios used by Al-loin et al. (1979), Pagel et al. (1979), and McCall, Rybski, and Shields (1985) are recalibrated and new abundance diagnostic ratios formulated. The correlation between ionization parameter and abundance is found to have the effect of changing the previous abundance calibration toward lower abundances, particularly for the most metal-rich H ii regions. A new technique to derive abundances from H n region spectra is suggested. Oxygen and other heavy element abundances in a selection of well observed H n regions in M101 are determined from the theoretical abundance sequence calibration. Comparison of abundances derived from the sequence with abundances derived from “exact” photoionization modeling demonstrate that the accuracy with which H ii region abundances may be derived from the sequence is ~ 0.15 dex. The data are employed to measure radial abundance gradients and heavy element yields for oxygen, nitrogen, and sulfur in M101. The combined abundance gradient and heavy element yield data suggest that the production of elements by massive stars (oxygen, neon) is enhanced in the inner regions of the disk of M101, whilst the production of heavy elements (S, Ar, Ca, Fe) by low and intermediate mass stars is enhanced in the outer disk. A simple model for galaxy disk formation is suggested which is at least qualitatively capable of explaining these observations. The theoretical abundance sequence calibration is also applied to optical spectrophotometry of 23 H ii regions located in the inner disk regions of two Seyfert 1 and two Seyfert 2 galaxies, including the prototype Seyfert 2, NGC 1068, in order to determine oxygen, nitrogen, and sulfur abundances. The mean oxygen abundance derived for each galaxy is shown to range between solar abundance and twice solar abundance. There is no evidence for abnormal N /O or S/O abundance ratios in any of the H ii regions observed. The observations suggest that the abundances derived for the H n regions may be adopted as nuclear abundances and employed to constrain theoretical models of the Seyfert nucleus. The observations then place limits on the influence which the active nucleus can have on chemical enrichment of the local interstellar medium. Observations of the candidate near-nuclear H n regions in the disk of NGC 1068 also indicate the presence of high-excitation extra-nuclear emission. The optical spectra appear to be very similar to ordinary H n region spectra with the addition of strong lines of [Ne v] AA3346,3426 and He n A4686. This is interpreted as the superposition of an H u region spectrum with the high-excitation emission, and theoretical nebular models are presented which suggest that the high-excitation gas is most likely photoionized by a power-law spectrum , probably from the nearby Seyfert nucleus. Show more