Recent observations have discovered pervasive, low-ionization gas with unusual excitation in the halos of nearby spiral galaxies. This has led to speculation concerning the excitation mechanisms operating at high galactic latitudes, including exotic processes involving nonequilibrium physics, local grain destruction and decaying dark matter. We demonstrate that the excitation conditions of the diffuse ionized media (DIMs) in several such objects, NGC 891, the Galaxy and NGC 1068, can be understood in terms of photoionization by the dominant radiation fields in their halos, if proper consideration is given to the spatial distributions of their stellar populations, the relative importance of nuclear continuum sources and subsequent radiative transfer.
Estimation of the stellar populations in NGC 891 and the Galaxy indicates that OB stars, and the central stars of planetary nebulae dominate the excitation of their DIMs. Photoionization models require local DIM electron densities of order unity to reproduce the observed emission characteristics of the high-$\vert z\vert$ gas. The DIM filling factor in NGC 891 is therefore restricted to the range 10$\sp{-3.5}\ \sbsp{\sim}{<}\ \phi\ \sbsp{\sim}{<}\ 10\sp{-1.5}$, while in the Galaxy $\phi\ \sbsp{\sim}{<}\ 10\sp{-1.2}$ is suggested. Ionization equilibrium indicates that only $\sim$5% of the total number of ionizing photons from OB stars escape the neutral disk, highlighting the importance of radiative transfer through the multiphase ISM.
The DIM in NGC 1068 demonstrates a bipolar asymmetry common among active galaxies, with high-excitation emission confined to two diametric sectors aligned along the radio jet axis and low-excitation emission uniformly distributed over the disk. We show that this bipolar morphology can be understood using the developing picture of NGC 1068, in which its burried Type 1 Seyfert nucleus is visible only through scattered polarized light. The DIM excitation is therefore anisotropic, with high-excitation gas along the jet axis photoionized by direct nuclear continuum and low-excitation gas away from the axis, illuminated by indirect nuclear emission scattered into it. Photoionization models using power-law continua, indicate that the filling factor of this DIM is restricted to the range $10\sp{-5} \ \sbsp{\sim}{<}\ \phi\ \sbsp{\sim}{<}\ 10\sp{-4}$ in order to reproduce the observed DIM emission characteristics.
| Identifer | oai:union.ndltd.org:RICE/oai:scholarship.rice.edu:1911/16590 |
| Date | January 1992 |
| Creators | Sokolowski, James Kurt |
| Contributors | Bland-Hawthorn, Jonathan |
| Source Sets | Rice University |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | 225 p., application/pdf |
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