The Dwarf Galaxy Population in Nearby Groups. The data

We used V and I CCD photometry to search for low-surface brightness dwarf galaxies (LSBD) in the central (<0.5 h^-1 Mpc) region of the groups NGC 6868, NGC 5846, HCG 42 and the poor cluster IC 4765. Using the parameters given by the exponential profile fit, we identified 80 LSBD candidates with magnitudes 17 < V < 22 mag and with colors V-I < 1.5 mag at the limiting isophote of 25.8 V mag/arcsec^2. The galaxies have mu0 > 22.5 V mag/arcsec^2, h > 1.5 arcsec, and diameters larger than 1.2 h^-1 kpc. Twenty of the eighty galaxies are extended LSB galaxies that were detected only on smoothed images, after masking all high surface brightness objects. The completeness in the detection is ~80% for galaxies with V<=20 and 22.5<mu0<24.5 V mag/arcsec^2, and below 50% at fainter magnitudes and central surface brightnesses. In this last bin, the completeness increases to ~80% when we search for galaxies in smoothed images instead. The detected LSBD galaxies are highly concentrated towards the center of the four groups in the inner 250 h^-1 kpc. The best fit power-law slope of the surface density distribution is, on average, beta ~ -1.5 (R < 250 h^-1 kpc), in agreement with the values found for satellites dwarfs around isolated E/S0 galaxies and in X-ray groups. The LSBD galaxies in the Mv-mu0 plane does not show a clear correlation, suggesting that the correlation noted by other studies could be produced by selection effects. The LSBD dwarf galaxies follow a well defined color-magnitude relation, extending for more than ten magnitudes (from bright ellipticals to faint dwarfs). A similar well defined color-magnitude relation from giants to dwarfs is known to be valid for galaxy clusters but it is the first time that it is demonstrated in the sparse environments of groups. (Abridge)