Talk given at the August, 2002 URSI meeting

ABSTRACT:

Low Surface Brightness galaxies (LSBs) have remarkable properties, which distinguish them from the more familiar `classical' high surface brightness (HSB) galaxies, like: (1) they seem to constitute at least 50% of the total galaxy population in number, which has strong implications for the galaxy luminosity function, baryonic matter density and galaxy formation scenarios; (2) their disks are among the less evolved objects in the local universe; (3) they are embedded in dark matter halos which are of lower density and more extended than the haloes around HSBs.

Basically all previous studies of LSBs were made in the optical and most objects studied are blue. However, a class of rare, red (B-I~1) LSBs has been identified with hitherto unknown properties: record high HI contents, MHI/LB~30-45 M¤/L¤, i.e. about 100 times the values found for an average HSB spiral galaxy, and objects which do not follow the `standard' Tully-Fisher relation at all.

In order to further investigate the often baffling properties of the LSB class of galaxies, we are undertaking the first large-scale study of LSBs selected in the near-infrared (J, H and K bands), using a homogeneous sample of about 4000 potential LSBs from the Two Micron All Sky Survey (2MASS). The selection criterion we used is a central K-band surface brightness of 18 mag arcsec-2, corresponding to about 22 mag arcsec^-2 in the B-band, a generally used selection criterion to separate LSB from HSB galaxies inthe optical.

About 1000 of these infrared-selected LSBs are being observed in the 21 cm HI line. For this, we have defined two sub-samples, optimised for the respective sensitivity and sky coverage of the Nançay and Arecibo radio telescopes: about 400 objects of both small and larger sizes for the declination range of 0° to 38° observable with the 300m diameter Arecibo dish, and about 600 of the larger sources for observation in the -38° to 0° and 38° to 60° declination range with the Nançay telescope. This strategy has resulted in a similar detection rate of about one third for both sub-samples, generally in the radial velocity range of 0-10,000 km/s. A first, partial analysis of the data shows HI masses ranging from ~8x10⁸ to 4x10¹⁰ M¤ (for H0=75 km s^-1 Mpc^-1), and HI contents ranging from M_HI/L_B 0.07-2.5 M¤/L¤, i.e. from gas-poor to gas rich.

Optical BVRI CCD photometry was obtained of about 50 selected objects. These multi-wavelength data will provide us with important information on a large, homogenous sample of LSB galaxies (e.g., colors, gas content). They will be used to construct models of the star formation histories and evolution of LSBs, and for a study of their Tully-Fisher (TF) relation, particularly to search for deviations from the 'classical' TF relation; our first results did not yet shown any galaxies that are severely underluminous for their total mass.