Low Surface Brightness Galaxies around the HDF-S: II. Distances and volume densities

Abstract

With this study we aim at the spectroscopic verification of a photometrically selected sample of Low Surface Brightness (LSB) galaxy candidates in a field around the Hubble Deep Field-South (HDF-S). The sample helps to extend the parameter space for LSB galaxies to lower central surface brightnesses and to provide better estimates on the volume densities of these objects. To derive redshifts for the LSB candidates, long-slit spectra were obtained covering a spectral range from 3400{\AA} to 7500{\AA}. The observations have been obtained using the ESO 3.6m telescope, equipped with the EFOSC2 spectrograph. From the measured radial velocities, distances could be estimated. With this distance information, it is possible to differentiate between true LSB galaxies and higher redshift High Surface Brightness (HSB) galaxies which may contaminate the sample. A correction for the surface brightnesses can then be applied, accounting for the cosmological dimming effect (``Tolman Dimming''). We show that ~70% of the LSB candidates, selected based on their location in the color-color space, are real LSB galaxies. Their position in the color-color diagrams, therefore, indicate that the LSB galaxies have a different stellar population mix resulting from a different star formation history compared to HSBs. Our LSB galaxy sample consists only of large disk galaxies with scale-length between 2.5kpc and 7.3kpc. We confirm the flat central surface brightness distribution of previous surveys and extend this distribution down to central surface brightnesses of 27 B mag arcsec^-2.