A search for warm neutral gas associated with the giant HII region NGC 604

The characteristics of the halo gas and its interaction with the galactic d isk in spiral galaxies are poorly known; this is particularly true for the warm neutral gas associated with HVCs and galactic chimneys. The detection of absorption features such as the NaI D or the CaII (H,K) lines is instrum ental to study its detailed physical properties but requires very long inte gration times. In this work very deep optical spectra of NGC 604, the brigh test giant HII region among the nearby spirals, are presented. The detectio n of two absorption components at LSR velocities -255 km/s and -20 km/s res pectively, is reported; the first component is associated with the HII regi on. The inferred line width after deconvolution is 155 km/s; this large wid th is produced by the blending of the multiple absorption components produc ed by the diverse sources of internal motion (expanding shells and general turbulence). The radial velocity of the CaII absorption is slightly larger than the measured in the HII emission lines suggesting a possible flow of g as into the halo above the young star cluster. The large ratio x = W-lambda (NaI D-2)/W-lambda(CaIIK) = 0.7 indicates the probable presence of shocks w hich release Ca from the dust grains into the gas phase. The lower velocity component most likely trace galactic gas.