2. Sample selection, observations and reduction
LSB galaxies from the samples of van der Hulst et al. (1993) and de Blok et al. (1995) were examined for the presence of HII regions using H images. We selected as possible targets only those galaxies with two or more distinct HII regions. This introduces a bias in our sample, and it therefore cannot be considered complete in any sense. Nevertheless it makes it possible to study the ISM in a significant subset of the LSB galaxy population.
Optical spectra of 64 HII regions in 12 of the selected LSB galaxies were obtained from 6-10 February 1992 with the 4.3 meter William Herschel Telescope (WHT) at La Palma. The ISIS spectrograph was used. The gratings were adjusted such that the blue arm covered a wavelength range from Angstrom to Angstrom, while the red arm covered a range from Angstrom to Angstrom. An amount of overlap therefore existed, which was used to tie the spectra together. The resolution for both arms was 1.4 Angstrom per pixel in the dispersion direction.
Conditions during the observing run were variable: usually photometric but with seeing varying between and . Total exposure times per long-slit spectrum were 4000 seconds. A slit width of was used during the first two nights. This was later adjusted to for the last two nights.
The data were reduced and analyzed using standard long-slit reduction tasks from the IRAF package. Two independent reductions of the data resulted in a mean difference between integrated fluxes of the same HII regions of . These systematic differences are not taken into account in further analyses, but should be kept in mind.
© European Southern Observatory (ESO) 1998
Online publication: June 18, 1998