A search for low surface brightness dwarf galaxies in different environments

Abstract

Current theories of large scale structure and galaxy formation predict the existence of numerous low mass dark matter haloes in the Universe today. If these haloes contain sufficient stars they should be detectable as low luminosity stellar systems or dwarf galaxies. We have searched for these objects in four regions of increasing density - the general field, the area around a giant spiral galaxy, the low density Ursa Major cluster, and the high density Virgo cluster. Using identical deep optical data covering a total of 602 and probing fainter magnitudes than has been done previously, we used identical selection and detection methods to compare the dwarf galaxy populations in these different environments. We found substantially more dwarfs per giant galaxy in the Virgo cluster (20:1) compared to the field environment (6:1 max). A comparison of the HI properties and (B-I) colours for the objects for which we had additional data also showed that in general, the cluster objects are redder and gas poor compared to the objects in the field. We discuss the possible mechanisms which may have resulted in creating a population of cluster dwarf galaxies, which would explain the high number density which we found in our data. It is likely that a combination of tidal interactions and transformation of infalling dlrrs into dEs will result in the large population of cluster dwarfs. Conclusive evidence regarding their formation must now be obtained by a more detailed investigation of their stellar populations. The lack of dwarf galaxies in the field region is likely to be due to the effect of inefficient star formation in the field environment compared to the cluster. Thus the low mass dark matter haloes predicted by CDM models must still be 'dark' and can only be identified by further deep HI studies of the field environment, and future gravitational lensing studies of substructure.