The Dwarf Galaxy Population in Nearby Groups. the Data

We used V and I CCD photometry to search for low-surface brightness dwarf galaxies in the central (< 0.5 h Mpc) region of the loose groups NGC 6868 (Telescopium) and NGC 5846, the compact group HCG 42 and the poor cluster IC 4765. We used the parameters given by the exponential profile fit (central surface brightness, scale length and limiting diameter) to identify 80 low surfacebrightness dwarf candidates with magnitudes 17 < V < 22 mag (−16.7 > MV > −11.4 at the assumed distances of the groups) and with colors V − I < 1.5 mag at the limiting isophote of 25.8 V mag/arcsec. The selected galaxies have central surface brightnesses μ0 > 22.5 V mag/arcsec , scale lengths h > 1.5, and diameters larger than 1.2 h kpc. Twenty of the eighty galaxies are extended low surface-brightness galaxies that were detected only on smoothed images, after masking all high surface brightness objects. The completeness fraction in the detection of the low surface-brightness dwarf galaxies is ∼ 80% for V . 20 and 22.5 < μ0 < 24.5 V mag/arcsec, and below 50% at fainter magnitudes and central surface brightnesses (μ0 > 24.5 V mag/arcsec 2 and V > 20 mag). In this last bin, the completeness increases to ∼ 80% when we search for galaxies in smoothed images instead. The detected low surface-brightness dwarf galaxies are highly concentrated towards the center of the four groups in the inner 250 h kpc. At larger radii, the projected number density is similar to the value found in the control fields. The best fit power-law slope of the surface density distribution is, on average, β ∼ −1.5 (R < 250 h kpc), in agreement with the values found for satellites dwarfs around isolated E/S0 galaxies and in X-ray groups. The distribution of the low surface-brightness dwarf galaxies in the MV −μ0 plane does not show a clear correlation, suggesting that the correlation noted by other studies could be produced by selection effects. The low surface-brightness 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. The spectroscopic follow-up shows that only 78 of the 412 galaxies with measured velocities in the fields of these groups are group members. We were able to obtain the radial velocity for 11 of the 80 low-surface brightness dwarf galaxies. Five of these galaxies are group members while six are in the background of the groups. We infer, then, that more than 90% of the 80 low surface-brightness dwarf galaxies in our sample must be bonafide group members. In addition, new structures along the group’s lines-of-sights were discovered. These new structures are groups and poor clusters that extend to ∼ 0.3 in redshift space. Subject headings: galaxies: clusters: general – galaxies: photometry – galaxies: fundamental parameters (classification, colors, surface brightness) – galaxies: dwarfs: – galaxies: cluster: individual (HCG 42,IC 4765,NGC 5846, NGC 6868)