Heating of blue compact dwarf galaxies: gas distribution and photoionization by stars in I Zw 18

Abstract

Photoionization models so far are unable to account for the high electron temperature Te([O III]) implied by the line ratio [O III]4363A/[O III]5007A in low-metallicity blue compact dwarf galaxies, casting doubts on the assumption of photoionization by hot stars as the dominant source of heating of the gas in these objects. Combinations of runs of the 1-D photoionization code NEBU are used to explore alternative models for the giant H II region shell I Zw 18 NW. Acceptable models are obtained, which represent schematically an incomplete shell comprising radiation-bounded condensations embedded in a low-density matter-bounded diffuse medium. The thermal pressure contrast between gas components is about a factor 7. The diffuse phase can be in pressure balance with the hot superbubble fed by mechanical energy from the inner massive star cluster. The failure of previous modellings is ascribed to (1) the adoption of an inadequate small-scale gas density distribution, which proves critical when the collisional excitation of hydrogen contributes significantly to the cooling of the gas, and possibly (2) a too restrictive implementation of Wolf-Rayet stars in synthetic stellar cluster spectral energy distributions. A neutral gas component heated by soft X-rays, whose power is less than 1% of the star cluster luminosity and consistent with CHANDRA data, can explain the low-ionization fine-structure lines detected by SPITZER. [O/Fe] is slightly smaller in I Zw 18 NW than in Galactic Halo stars of similar metallicity and [C/O] is correlatively large. Extra heating by, e.g., dissipation of mechanical energy is not required to explain Te([O III]) in I Zw 18. Important astrophysical developments are at stakes in the 5% uncertainty attached to [O III] collision strengths.

20 pages, 8 figures, to be published in A&A

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Cited by in corpus (12)

• The Herschel Dwarf Galaxy Survey: I. Properties of the low-metallicity ISM from PACS spectroscopy
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• The Te[N II]-Te[O III] temperature relation in H II regions and the reliability of strong-line methods
• Inferring the HII region escape fraction of ionizing photons from infrared emission lines in metal-poor star-forming dwarf galaxies
• Cosmic metal density evolution in neutral gas: insights from observations and cosmological simulations
• Metal-poor dwarf galaxies in the SIGRID galaxy sample. II. The electron temperature-abundance calibration and the parameters that affect it
• Keck Cosmic Web Imager Observations of He II Emission in I Zw 18
• Low-metallicity massive single stars with rotation. III. Source of ionization and C-IV emission in I Zw 18
• Probing the heating of the neutral atomic interstellar medium in the Dwarf Galaxy Survey through infrared cooling lines
• Recovering the properties of the interstellar medium through integrated spectroscopy: application to the z~0 ECO volume-limited star-forming galaxy sample
• Infrared view of the multiphase ISM in NGC 253 II. Modelling the ionised and neutral atomic gas
• Topological models to infer multiphase interstellar medium properties