Chemical Abundance Determinations in Extragalactic HII Regions
with FUSE: the Status
Vianney Lebouteiller
The Far Ultraviolet Spectroscopic Explorer (FUSE) offers the possibility
to study the extragalactic interstellar medium (ISM) along the lines of
sight of UV-bright HII regions where stars are forming. From the
absorption lines of species such as HI, OI, NI, and FeII, we are able
to derive the chemical composition of the neutral gas and compare it
to the HII region composition. A major aim is to know whether HII regions
truly reflect the ISM abundances of a galaxy or are self-polluted with
metals ejected by massive stars during the present starburst episode.
Our team has studied 4 Blue Compact Dwarfs galaxies (BCDs). The situation,
however, remains still unclear: oxygen abundance is either equal in the
neutral and ionized ISM (in IZW18, IZW36 and SBS0335-052) or 10 times lower
in the neutral ISM (in Mrk 59). In addition, nitrogen (using NI as a tracer
in the neutral gas) and argon (using ArI) are always lower in the neutral
phase.
Conscious of the potential problems due to the complexity of the lines of
sight and the ionization structure in BCDs, we have obtained high quality
spectra of individual HII regions in spiral galaxies. The situation is
much easier to understand, allowing us to compare the abundances derived
with FUSE with the abundances computed with PDR models and photoionization
models (CLOUDY). A pilot study on NGC604, an HII region in M33, allows us
to validate the method used for the BCDs. We also compare the results
of NGC604 with those obtained for the 4 BCDs already studied. In NGC604,
we find a global underabundance of N, O, Ar, and Fe in the neutral gas
compared to the ionized gas suggesting the presence of primorial gas along
the line of sight. Furthermore, NI and ArI appear to be good tracers of
nitrogen and argon, respectively, in the neutral phase.