Star formation in gaseous galaxy halos: VLT-spectroscopy of extraplanar HII-regions in NGC55

Abstract

We present first deep VLT-spectra of a new class of compact extraplanar objects located in the disk-halo interface of the edge-on spiral galaxy NGC55. Their spectra reveal continuum emission from stars and show typical emission-lines as observed in ordinary disk HII-regions. Similar to the Diffuse Ionized Gas (DIG) the prominent ionization stage of oxygen is O+, whereas the corresponding one for low metallicity HII-regions is O++. A comparison with CLOUDY model simulations reveals that the ionization mechanism of these compact objects is most likely photoionization by late OB stars (O9.5 to B0). Further analysis of diagnostic diagrams unambiguously confirms the HII-region character. This raises the question whether these extraplanar HII-regions (EHRs) originated from the prominent extraplanar gas of this galaxy or have just been expelled from the disk into the halo. From hydrodynamical considerations ejection from the disk can be ruled out. Therefore, these objects must have formed within the halo. Compared to the average abundance of the central disk HII-region (45% Z_solar) both EHRs reveal substantially lower [O/H] abundances of about 10% Z_solar. We could establish for the first time strong differences in the metal content along the minor axis of this galaxy. Oxygen appears to be less abundant in the halo by about a factor of 4. Since both EHRs are located above the central part of NGC55, it appears likely, that their formation was triggered by star formation activity in the disk below. In this environment the molecular gas clouds out of which EHRs have formed can survive and collapse only in the period between two successive bursts of star formation.