The dwarf low surface brightness galaxy population of the Virgo cluster - III. Comparisons with different environments

We have previously described the results of a search for low surface brightness (LSB) objects in data drawn from an east–west strip of the Virgo cluster and have compared this to a large-area strip outside the cluster. In this paper, we compare the cluster east–west data with new data along a cluster north–south strip and with data obtained for the Ursa Major (UMa) cluster and fields around the spiral galaxy M101. Our intention is to look for differences in the nature of the dwarf galaxy population with location in the cluster and within other environments. These uniform data sets reach central surface brightness values of ∼26 Bμ, and an apparent B magnitude of 21 (M(B)=−10 for a Virgo cluster distance of 16 Mpc). Over a north–south strip area of ∼15 deg(2), we find ∼22 LSB objects per square degree. This compares with ∼4 deg(−2) in both the UMa and the M101 fields. These results are very similar to what we have previously found for the east–west cluster strip and for other fields outside the cluster. There are far more small LSB features detected in Virgo cluster fields and we associate them with an extensive cluster dwarf galaxy population. In the cluster north–south strip there is an average dwarf-to-giant number ratio of 31 ± 9, which is much higher than would be expected from observations of galaxies in the Local Group, but significantly less than that required by Λ cold dark matter galaxy formation models. The dwarf-to-giant ratios in UMa and M101 are consistent with those in the Local Group. Comparing the cluster east–west and north–south data, we find clear differences in the way the number density of dwarfs decreases with cluster radius and the distributions of galaxy magnitudes and sizes. The north–south strip covers a region of the cluster that contains not only the main cluster region associated with M87, but also two more distant infalling clouds. Additionally, the north–south strip lies approximately along the extended filamentary structure associated with Virgo while the east–west strip is perpendicular to it. We interpret the differences between the two strips as being due to these different environments. We suggest that the Virgo cluster is assembling itself out of subclusters and clouds that are already rich in dwarf galaxies compared to the environment of the general field. There is no evidence for any systematic difference in dwarf galaxy (B−I) colour between dwarf galaxies of different magnitudes or in different parts of the Virgo cluster. A typical value of (B−I)= 1.8 is consistent with the colours of a wide range of stellar systems including the globular clusters of the central elliptical galaxy M87, but is bluer than is typical for giant ellipticals. We discuss and interpret our results in the context of various physical processes that are thought to act on galaxies as they form and evolve. Additionally, we give images and positions of possible new dwarf galaxy companions of M101