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Astron. Astrophys. 322, 391-399 (1997)

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On the ionization of the diffuse ionized gas: spectroscopy of NGC 2188 *

H. Domgörgen 1 and R.-J. Dettmar 2

1 Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2 Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany

Received 2 September 1996 / Accepted 2 December 1996


NGC 2188 is a highly inclined irregular galaxy which shows spectacular filaments of ionized gas emerging from an HII region complex into the halo of the galaxy. Here we present H [FORMULA] imaging and optical spectroscopy of NGC 2188. Since the galaxy is low in metallicity and dust poor, the spectroscopic observations open up a parameter range for the extraplanar DIG which is not yet explored. Using empirical diagnostic diagrams we find that the ionized gas above the plane is most likely photoionized. Differences in line ratios between the DIG in spiral galaxies and in NGC 2188 can fully be explained by differences in metallicities. We argue that dust is not of high importance in altering the line ratios measured for the DIG either by scattering or photoelectric heating.

Key words: galaxies: halos – galaxies: individual: NGC 2188 – galaxies: ISM – galaxies: irregular

* Based on observations obtained at ESO/La Silla (Chile)

Send offprint requests to: H. Domgörgen

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998