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Extraplanar Star Formation in NGC 55
R. Tüllmann [1],
M. R. Rosa [2],
T. Elwert [1],
D. J. Bomans [1],
A. M. N. Ferguson [3], R.-J. Dettmar [1]
[1] Astronomisches Institut, Ruhr-Universität Bochum, Germany;
[2] Space Telescope-European Coordinating Facility, Garching, Germany;
[3] Max-Planck-Institut für Astrophysik, Garching, Germany
Abstract
We present the first VLT spectra of two compact Ha-emitting objects, located in the halo of the edge-on spiral galaxy NGC 55. The detection of stellar continuum and the observed emission-line characteristics indicate that these objects are extraplanar HII-regions. CLOUDY model simulations establish photoionisation by single OB-type stars as ionisation mechanism and finally confirm the HII-region character. Hydrodynamical considerations unambiguously restrict the origin of these regions to the halo. Their creation was most likely triggered by star formation activity in the disc below. In this picture the gas clouds, out of which the OB stars formed, could cool and collapse only between two successive bursts of star formation.
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