The Low Surface Brightness Extent of the Fornax Cluster

We have used a large format CCD camera to survey the nearby Fornax cluster and its immediate environment for low luminosity low surface brightness galaxies. Recent observations indicate that these are the most dark matter dominated galaxies known and so they are likely to be a good tracer of the dark matter in clusters. We have identified large numbers of these galaxies consistent with a steep faint end slope of the luminosity function (alpha~ -2) down to MB ~ -12. These galaxies contribute almost the same amount to the total cluster light as the brighter galaxies and they have a spatial extent that is some four times larger. They satisfy two of the important predictions of N-body hierarchical simulations of structure formation using dark halos. The luminosity (mass ?) function is steep and the mass distribution is more extended than that defined by the brighter galaxies. We also find a large concentration of low surface brightness galaxies around the nearby galaxy NGC1291.Comment: 16 pages, 6 figure

An automated search for nearby low-surface-brightness galaxies - I. The catalogue

We have developed an automatic technique to search for low-surface-brightness (LSB) galaxies in the local Universe (v ≤ 5000 km s(-1)) using the automated plate measuring (APM) scan data of UK Schmidt photographic plates. We optimized our technique and selection criteria by surveying the known LSB galaxies in the Fornax cluster. Plate-to-plate magnitude calibrations were carried out using independent CCD sequences. The galaxies we detected are brighter than 20 B mag, have scalesizes greater than 3 arcsec and a central surface brightness fainter than 22.5 mag arcsec(-2). In total 2435 LSB galaxies were detected over a total area of 2187 deg(2). The survey covers the Fornax cluster, NGC 1400, Sculptor and Dorado groups and the field between. We detect on average 32 LSB galaxies per 5.8 °× 5.8 ° field. We have estimated the background (v>5000 km s(-1)) contamination in three ways: by numerical modelling, using a limited redshift sample and comparing our Fornax data with those of Ferguson. The results indicate a contamination of about 19 galaxies per field

An automated search for nearby low-surface-brightness galaxies - II. The discussion

An automated search for low-surface-brightness (LSB) galaxies over 2187 deg(2) of sky produced 2435 galaxies. The technique, calibration, background contamination, models of the galaxy populations, survey and catalogue were discussed in Paper I. In this paper we present an analysis of our results. The number density of Fornax LSB galaxies drops exponentially with radius from the cluster centre with a scalelength of 1.25° while the bright galaxies have a scalelength of 0.48°. Spectroscopic observations in the Fornax region reveal that two LSB galaxies are at approximately the same redshift as the Fornax cluster, yet they are six bright galaxy scalelengths from the cluster centre. A correlation analysis of the sample indicates that our galaxies are much more strongly clustered (A(ω) =0.82) than the general faint population (at the same magnitude limit) but less so than the bright nearby RC3 galaxies (A(ω) = 2.23) sampled within the same volume. This implies that LSB galaxies are associated with bright galaxies, but distributed over a larger scale. We have compared our observations with a fading model of the faint galaxy number counts. This model predicts ≈60 galaxies per field while we detect on average 13. Either the fading models are incorrect or there is strong differential fading between clusters and field. The luminosity function of the Fornax cluster has a slope of α =-1.58 ± 0.6. The total luminosity of the Fornax cluster is dominated by bright galaxies with a LSB-to-bright luminosity ratio of 0.02 while the field has a ratio of 0.03