A density-gradient model for the H II galaxy I ZW 18

Abstract

view Abstract Citations (15) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Density-Gradient Model for the H II Galaxy I ZW 18 Campbell, Alison Abstract A model of the low-abundance H II galaxy I Zw 18 is presented in which the electron density decreases with radius, and the central density is high enough to cause partial collisional deexcitation of [O III] λ5007. The model predicts emission-line ratios F_line_/F_Hβ_ that are in good agreement those observed and simultaneously reproduce the electron temperature T_e_(O^2+^) estimated from the ratio Flambda4363_/Flambda4959,5007_, a problem unsolved by constant-density models. Under such conditions, suppression of λ5007 makes Flambda4363_/Flambda4959,5007_ an inaccurate estimator of T_e_(O^2+^), and hence of oxygen abundance, O/H. The best-fit density-gradient model has log O/H = -4.68, ~70% higher than the value inferred from the observed line ratios. The best-fit constant-density model, constructed for comparison, fits most line ratios well, but underestimates the strength of Flambda4363_ and hence produces a T_e_(O^2+^) which is 2500 K too low. Relative to that of oxygen, the nitrogen abundance of both density- gradient and constant-density models is ~50% higher than in most low- metallity galaxies, while the abundances of carbon, sulphur, and neon are approximately normal. Both models overestimate the strength of He I λ5876 for reasonable helium abundances. The Hα surface brightness--radius relation of the density- gradient model is similar to that of a constant-density model at radii greater than the limit imposed by seeing. The observed surface brightness profile changes with position angle and is somewhat different from the profiles of both models, indicating that the true density distribution of I Zw 18 is more complicated than either a constant or a simple power law with radius. The density-gradient model provides a slightly better fit to the data. I Zw l8 is one of a small class of low-abundance H II galaxies and giant H II regions whose emission-line ratios are not well fitted by a simple Stromgren sphere model; the success of the density-gradient model suggests that it may be possible to explain the properties of this small class of objects in the same way. If so, their abundances are higher than suggested by their observed Flambda4383_/Flambda4959.5007. The inclusion of such galaxies would lead to an underestimate of the true slope of the oxygen abundance---helium mass fraction relation, and hence to an overestimate of the primordial helium abundance. Publication: The Astrophysical Journal Pub Date: October 1990 DOI: 10.1086/169246 Bibcode: 1990ApJ...362..100C Keywords: Electron Density (Concentration); Electron Energy; Galactic Structure; H Ii Regions; Oxygen Ions; Astronomical Models; Density Distribution; Emission Spectra; H Alpha Line; Line Spectra; Astrophysics; GALAXIES: ABUNDANCES; GALAXIES: INDIVIDUAL ALPHANUMERIC: I ZW 18; GALAXIES: STRUCTURE; NEBULAE: H II REGIONS full text sources ADS | data products SIMBAD (5) NED (1)