Fig. 5.1: Line ratios measured in NGC5775 plotted as a function of z. The data outside the blue lines is not fitted by standard photoionization models. However, this does not necessarily imply that additional ionization mechanisms have to be taken into account. An alternative explanation would be that standard physics is too simple in order to fully explain the observed ionization structure of the DIG.
Another way to account for elevated line ratios has been suggested by Haffner, Reynolds & Tufte (1999) for the Milky Way. They showed that if the DIG temperature increases towards the halo line ratios would increase, too. The thin dashed lines in the right panel of Fig. 5.1 represent theoretical line ratios depending on temperature and ionization fraction. As one can easily see these ratios are in good agreement with observations if the ionization fraction is varied, correspondingly. However, some aspects remain puzzling. First, the unusual increase of [OIII]/Ha. From a theoretical point of view, this ratio should be high in the disk and decrease towards the halo, because the energetic photons that produce [OIII] are assumed to be emitted from star forming regions in the disk. The expected decrease would be a consequence of the dilute radiation field and the lack of ionizing sources in the halo. Hence, an increasing temperature would be inconsistent with decreasing [OIII]/Ha ratios.
Secondly, the observed [OII]/Ha ratio requires to fit unreasonably high ionization fractions for the halo, implying oxygen to be almost completely ionized although energetic ionizing photons (> 13.6 eV) are most likely lacking.
All this indicates that the ionization mechanism of DIG in galaxy halos is still not well understood. In order to overcome these problems, the commonly accepted (and perhaps misleading) strategy is to look for additional heating or ionization mechanisms of the eDIG.
However, before complicating things and applying additional heat terms, we decided to analyze these models critically according to radiative transfer and geometry. The main aspects of criticism are:
(1) CLOUDY as well as the Mathis code solve the problem of radiation transport analytically which requires assumptions that are not realistic. As an initial guess of the radiation field, the OTS approximation is used which states that all emitted photons are absorbed in the immediate environment of their creation. This simplification is only applicable to high density regions, but no longer valid for the low density eDIG. In addition, both codes make use the OWO or IO technique which significantly underestimates the diffuse radiation field.
(2) the geometry assumed by standard model codes is spherical symmetric and is not appropriate to model galaxies with two dimensional stellar disks and extended gaseous halos. This change in geometry is expected to substancially influence ionization conditions (Fig. 5.2).
Fig. 5.2: Comparison between the geometry of a standard photionization
model (left) and SOAP (right).
A volume element dV in the standard model receives ionizing radiation only
from a point source, whereas the same element in SOAP gains stellar photons from an extended 2D source.
Our motivation to investigate, whether the alleged failure of standard
photoionization codes is simply caused by geometrical effects and insufficiently treated radiative transfer, resulted in a
new self-consistent 3D Monte-Carlo photoionization code, which is
called SOAP. This work is based on
the standard photoionization MC-model of Och, Lucy and Rosa (1998) and is a collaboration with Michael Rosa and Ralf-Jürgen Dettmar.
Main ingredients of our model-galaxy are a disk containing O-stars and gas with a certain metal abundance plus an extended gaseous halo with an arbitrary density profile. The disk size is 9 by 9 kpc with an z-extent of 1.5 kpc. We further assume a spherical halo of 15 kpc in radius. Due to the symmetry of the problem we can describe the model galaxy simply by an octant. Cartesian coordinates are used for the present geometry. The whole volume is subdivided into 1400 discrete cells with sizes of 1.5 kpc by 1.5 kpc by z kpc, where z is a variable that changes from 0.1 kpc at the galactic plane to 1.5 kpc in the outer halo.
Stellar photon frequencies are determined by a probability distribution function. These photons are emitted within a defined region (disk) under randomly chosen angles theta and phi. For every single photon the radiative transfer is calculated in each cell precisely as well as the corresponding photon histories. After all photons have been processed (traced until they leave the halo) the physical conditions within each cell are determined and updated. The updated values, mainly the ionization fractions of the individual ions and electron temperatures, are the new input parameters for a new iteration cycle of the model. Just as an observer at the telescope can choose a slit position, we can define the "read out plane" along which the artifical spectrum is created. Flowcharts of SOAP and the photon tracker TRACKPHOT are accessible via their hyperlinks.
The following results have been obtained by adopting initial guesses for the free parameters (elemental abundance, gas density/distribution, stellar input spectrum, stellar temperature, and stellar photon luminosity). One of the most important results concerns the ionization structures for H, He, and O, as well as those for N and S. Corresponding line ratios can be found here.
In order to model the ionization structure and the observed line ratios of individual galaxies, the complex parameter space of SOAP requires further detailed examination. This, together with an evaluation to what extent a more realistic geometry of the model can influence the radiation field and reproduce observational results, is subject of future work. For detailed information I refer to my PhD thesis which can be downloaded as a gzipped postscript file here.
Although SOAP was developed to simulate the ionization conditions of the planar and extraplanar ISM in normal spiral galaxies, it also provides an elegant way to investigate the ionization of the IGM. This is done by means of the so called photon statistics which is just a "by-product" of the code (Fig. 5.3).
Fig. 5.3: The photon statistics provides valuable insights into the radiation field which leaves the disk and escapes the halo, thus contributing to the ionization of the IGM.
From this figure it can be seen that a significant fraction of diffuse ionizing radiation escapes the halo. As the gas density in the disk increases, this fraction becomes smaller and smaller as most photons are absorbed "on the spot". Obviously, ionizing stellar photons can be main contributers to the ionizing background radiation field (mind that dust is neglected in this model).
As Fig. 5.3 suggest, normal spiral galaxies could be an important, if
not the most important, ionizing source of the IGM. If
5-7% ionizing photons escape the galaxy, they would
dominate clearly over AGN, given the number density of
normal spirals. This preliminary finding is in good
agreement with estimates by Bland-Hawthorne & Maloney (2001).
However, detailed investigations on the influence of dust and varying gas densities need to be carried out in order to establish late type spiral as important contributors to the ionization of the IGM.