Interstellar abundances in the neutral and ionized gas of NGC 604

Abstract

Aims. We present FUSE spectra of the giant H ii region NGC 604 in the spiral galaxy M 33. Chemical abundances are tentatively derived from far-UV absorption lines and compared to those derived from optical emission lines. Methods. Absorption lines from neutral hydrogen and heavy elements were observed against the continuum provided by the young massive stars embedded in the H ii region. We derived the column densities of H i, N i, O i, Si ii, P ii, Ar i, and Fe ii, fitting the line profiles with either a single component or several components. We used CLOUDY to correct for contamination from the ionized gas. Archival HST/ STIS spectra across NGC 604 allowed us to investigate how inhomogeneities affect the final H i column density. Results. Kinematics show that the neutral gas is physically related to the H ii region. The STIS spectra reveal H i column density fluctuations up to 1 dex across NCG 604. Nevertheless, we find that the H i column density determined from the global STIS spectrum does not differ significantly from the average over the individual sightlines. Our net results using the column densities derived with FUSE , assuming a single component, show that N, O, Si, and Ar are apparently underabundant in the neutral phase by a factor of ${\sim} $10 or more with respect to the ionized phase, while Fe is the same. However, we discuss the possibility that the absorption lines are made of individual unresolved components, and find that only P ii, Ar i, and Fe ii lines should not be affected by the presence of hidden saturated components, while N i, O i, and Si ii might be much more affected. Conclusions. If N, O, and Si are actually underabundant in the neutral gas of NGC 604 with respect to the ionized gas, this would confirm earlier results obtained for the blue compact dwarfs, and their interpretations. However, a deeper analysis focused on P, Ar, and Fe mitigates the above conclusion and indicates that the neutral gas and ionized gas could have similar abundances.