Low surface brightness galaxies around the HDF-S

Abstract

Aims. 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. Methods. To derive redshifts for the LSB candidates, long-slit spectra were obtained covering a spectral range from 3400 Å to 7500 Å. The observations have been obtained using the ESO 3.6 m 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"). Results. 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 scalelength between 2.5 kpc and 7.3 kpc. 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 .Key words. surveys -galaxies: distances and redshifts -galaxies: fundamental parameters IntroductionThe known properties of LSB galaxies are still a challenge for existing theories of galaxy formation and evolution. Although they may not represent a significant amount of the luminosity density in the universe, their number density is very high. Searches for LSB galaxies showed that they account for up to 60% of the local galaxy population (McGaugh 1996;O'Neil & Bothun 2000). Minchin et al. (2004) showed in their study of the bivariate surface brightness distribution, that up to 20% of the dynamical mass is represented by the LSB galaxies and that they account for up to 60% of the number density in the Universe. Thus it seems, that LSB galaxies are a common product of galaxy formation and evolutionary processes. Therefore, it is essential to understand their role in formation and evolution scenarios of galaxies in general. Studies of the properties of local LSB galaxy samples have shown that they populate nearly the whole parameter space derived also for HSB galaxies. The only difference is their low central surface brightness which is below µ 0 = 22.5 mag arcsec −2 in the B-filter. Field LSB galaxies are generally gas rich, although the gas surface densities are very low, too. The typical gas surface densities for LSB galaxies are below the Kennicutt criterion for ongoing star formation (Kennicutt 1989;van der Hulst et al. 1993;Picker...

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