Abstract
In this work we study QSO absorption systems associated
with intervenient galaxies. The physical conditions in these systems are
modelled through a photoionization code coupled to a statistical balance code of
atomic levels.
The latter was designed with a very flexible structure in
order to allow it to be used in different applications, such as the calculation
of emissivity ratios of collisionally excited lines, or also radiative cooling
functions. As a first step, we have obtained population ratios of the
fine-structure levels of O0 and Fe+, complementing the
work carried out earlier with the ions C0, C+ and Si+.
The photoionization code employed was Aangaba, for which it
was made a revision of the atomic data in order to incorporate recent advances
in the atomic database available in the literature. In order to enable the
calculation of the ionization equilibrium of all elements from H to Zn, we have
made a compilation of charge exchange (with hydrogen and helium) and collisional
ionization rates. We have also attempted to obtain a self-consistent dataset of
photoionization cross sections and recombination rates. We have found that such
a dataset is not available in the literature, and, even for the ions of the most
abundant elements, for which there are sofisticated R-matrix calculations
available, the data suffer from a serious shortcoming related to the resolution
of the ressonances in the cross sections.
In order to facilitate future upgrades, and allow its
widespread adoption by other authors in different astrophysical applications, we
have designed a simple and flexible structure to this new atomic database. To
design its software infrastructure, we have made use of modern programming
tools: object-oriented programming and UML.
As most astronomical applications are written in Fortran,
inclusive Aangaba, this was the language of choice to implement the atomic
database. At this point we have spotted some difficulties related to the
implementation of object-oriented concepts in Fortran. We also point out that
the new language standard currently being voted will not incorporate direct
support to object-oriented programming in a fully satisfactory way. In this work
we were able to overcome these difficulties.
Finally, both codes above were incorporated to the
photoionization code Aangaba, and as a first application we present a grid of
models for the analysis of column density ratios of fine-structure levels
derived in Lyman-Limit systems. These line ratios may provide fundamental clues
to pin down the nature of the ionization source in these systems.