VLT-spectroscopy of extraplanar HII-regions in NGC55

Details VLT-spectroscopy of extraplanar HII-regions in NGC55 VLT-spectroscopy of extraplanar HII-regions in NGC55

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003aas...203.7602d&link_type=abstract

American Astronomical Society Meeting 203, #76.02; Bulletin of the American Astronomical Society, Vol. 35, p.1322

Astronomy and Astrophysics

Astronomy

Scientific paper

We present first VLT-multiobject spectroscopy of candidate extraplanar HII-regions in the disk-halo interface of the edge-on galaxy NGC55. In earlier Hα-images these objects appear isolated from the surrounding ISM and fairly compact with central dense cores. The spectra clearly reveal continuum emission from stars and typical emission-lines as observed in ordinary disk HII-regions.
The emission-line fluxes are very similar to those obtained for the diffuse ionized gas (DIG), except for [OIII]5007 which is significantly decreased by more than a factor of 3. Similar to the DIG the prominent ionization stage of oxygen is O+, whereas the corresponding one for low metallicity HII-regions is O++. A comparison with CLOUDY suggests that the ionization mechanism of these compact objects is photoionization by 1-2 late OB stars (O9.5 to B0).
This raises the question whether these extraplanar HII-regions (EHRs) originated within the halo or have just been expelled from the disk. Since the [O/H] abundances of the central disk HII-region are with 45 about 5 times higher than the measured abundance of the extraplanar HII-regions (10 objects must have formed within the halo.
Our data also 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. As both EHRs are located above the main star forming region of NGC55, we suggest, that their formation was triggered by star formation activity in the disk below. If this is true, the extraplanar molecular gas clouds out of which EHRs have formed can survive and collapse only in the period between two successive bursts of star formation.
Research in this field at the Ruhr-University is supported by DLR grant 50OR0102 and DFG SFB 591.

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