LOW-SURFACE-BRIGHTNESS GALAXIES - HIDDEN GALAXIES REVEALED

In 20 years, low-surface-brightness (LSB) galaxies have evolved from b eing an idiosyncratic notion to being one of the major baryonic reposi tories in the Universe. The story of their discovery and the character ization of their properties is told here. Their recovery from the nois e of the night-sky background is a strong testament to the severity of surface-brightness selection effects. LSB galaxies have a number of r emarkable properties which distinguish them from the more familiar Hub ble sequence of spirals. The two most important are (1) they evolve at a significantly slower rate and may well experience star formation ou tside of the molecular-cloud environment, (2) they are embedded in dar k-matter halos which are of lower density and more extended than the h alos around high-surface-brightness (HSB) disk galaxies. Compared to H SB disks, LSB disks are strongly dark-matter dominated at all radii an d show a systematic increase in MIL with decreasing central surface br ightness. In addition, the recognition that large numbers of LSB galax ies actually exist has changed the form of the galaxy luminosity funct ion and has clearly increased the space density of galaxies at z = 0. Recent CCD surveys have uncovered a population of red LSB disks that m ay be related to the excess of faint blue galaxies detected at moderat e 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 p rocess. While the discovery of LSB galaxies has led to new insights, i t remains unwise to presume that we now have a representative sample w hich encompasses all galaxy types and forms.