Extremely low surface brightness galaxies in the Fornax Cluster: Properties, stability, and luminosity fluctuations

Using the method of photographic amplification, we have discovered additional examples of very low surface brightness (LSB) galaxies in the Fornax Cluster. In general, the properties of these galaxies are very similar to those discovered in the Virgo Cluster and discussed by Impey, Bothun, & Malin. To avoid redundancy, we focus attention on other aspects of this population, including (1) their morphological appearance, (2) their dynamical stability, and (3) their usefulness as distance indicators via the technique of luminosity fluctuations. In addition, we combine this sample with the Virgo sample to reinvestigate the faint-end slope of the galaxy luminosity function (LF). We find that the failure to account properly for surface brightness selection biases leads to artificially flatter slopes in the case where the light profile of dwarf galaxies is exponential in nature. Under this assumption, we derive a faint-end slope of ≈ -1.55. Most of these LSB objects exhibit a ragged morphological appearance, consisting of numerous surface brightness enhancements (i.e., blobs) embedded in an overall LSB envelope. The colors of these blobs are the same as the envelope and thus they are probably not stellar clusters but instead are random piles of stars which reflect a stellar system with chaotic orbits. Crude dynamical arguments involving questions of disk stability and resistance to disruption by the mean tidal field of the cluster provide circumstantial evidence for an M/L ratio which is significantly higher than that implied by their observed blue colors (mean B-V ≈ 0.6, mean V-I ≈1.0). When coupled with the increase in the faint-end slope of the LF, this implies a nonnegligible mass contribution from these objects. The origin of this population, however, continues to be a mystery, since there is no relation between surface brightness and color which would be expected from any fading scenario. Finally, a subset of these LSB galaxies have regions of constant surface brightness that encompass several hundred pixels. As such, they become amenable to the measurement of luminosity fluctuations for purposes of deriving distances. From a sample of five galaxies which meet this criteria, we derive a distance modulus to Fornax of (m - M) = 31.3 ± 0.3. The typical fluctuation in these LSB galaxies is ≈ 10% (e.g., 100 red giants per pixel). Our derived distance modulus should not be taken seriously, since we do not yet understand the dominant stellar spectral type which is responsible for the fluctuations in these systems. However, the fact that we have derived a reasonable distance suggests that this method may ultimately work for these galaxies. More precise distance measurements would require darker backgrounds (especially at I) than can be obtained from the ground.