Low-surface-brightness galaxies: Hidden galaxies revealed

In 20 years, low-surface-brightness (LSB) galaxies have evolved from being an idiosyncratic notion to being one of the major baryonic repositories in the Universe. The story of their discovery and the characterization of their properties is told here. Their recovery from the noise of the night-sky background is a strong testament to the severity of surface-brightness selection effects. LSB galaxies have a number of remarkable properties which distinguish them from the more familiar Hubble sequence of spirals. The two most important are (1) they evolve at a significantly slower rate and may well experience star formation outside of the molecular-cloud environment, (2) they are embedded in dark-matter halos which are of lower density and more extended than the halos around high-surface-brightness (HSB) disk galaxies. Compared to HSB disks, LSB disks are strongly dark-matter dominated at all radii and show a systematic increase in MIL with decreasing central surface brightness. In addition, the recognition that large numbers of LSB galaxies actually exist has changed the form of the galaxy luminosity function and has clearly increased the space density of galaxies at z = 0. Recent CCD surveys have uncovered a population of red LSB disks that may be related to the excess of faint blue galaxies detected at moderate redshifts. LSB galaxies offer us a new window into galaxy evolution and formation which is every bit as important as those processes which have produced easy-to-detect galaxies, Indeed, the apparent youth of some LB galaxies suggest that galaxy formation is a greatly extended process. While the discovery of LSB galaxies has led to new insights, it remains unwise to presume that we now have a representative sample which encompasses all galaxy types and forms.