The story of the discovery and characterization of LSB galaxies as important members of the general galaxy population began in 1963 with the publication of the David Dunlap Observatory (DDO) Catalog of Galaxies by Sydney van den Bergh (van den Bergh 1959). This catalog consists of galaxies which exhibit a diffuse appearance and that have angular sizes larger than three arc minutes. While the DDO objects are the first bona fide collection of a sample of LSB galaxies, they are not at all representative of the phenomenon. The galaxies contained in the DDO catalog are exclusively of low mass (some are members of the Local Group). This has fostered an erroneous perception that all LSB galaxies are dwarf galaxies. Today, we know that all masses of galaxies have representation in the LSB class.
The first substantial contribution to our understanding of LSB disk galaxies was made by William Romanishin and his collaborators Steve and Karen Strom in 1983 (see Romanishin et al. 1983). They derived their sample from the Uppsala General Catalog of Galaxies (UGC - Nilsson 1973). The UGC is a diameter selected catalog and is therefore less sensitive to surface brightness selection effects compared to galaxy selection based on apparent flux (see also McGaugh et al. 1995). As a result, the UGC does contain some LSB disk galaxies but most have µ 0 in the range 22-23.0 mag arcsec-2. In early 1984, Allan Sandage and his collaborators published some of the first results of the Las Campanas Photographic Survey of the Virgo Cluster. Contained in those papers were some dramatic examples of dwarf galaxies in the Virgo cluster which were quite diffuse. The existence of such diffuse objects in a cluster was very interesting as this environment should prove hostile to their formation and survival. In addition, Ellis, Grayson and Bond (1984) discovered additional examples of extremely diffuse galaxies in the field. This immediately raised the possibility that perhaps galaxies like this were common but, for the most part, not yet detected and cataloged. If so, perhaps these faint diffuse galaxies were the source of the enigmatic QSO absorption line systems. These considerations forged the Impey/Bothun collaboration as we collectively wondered if the Sandage survey had missed galaxies of even lower surface brightness. To answer this question we enlisted the help of David Malin in Australia. Ultimately, we were trying to improve the determination of the galaxy luminosity function by concentrating on those galaxies which would be the most difficult to detect, due to extreme diffuseness. We had no idea if such extreme LSB galaxies really existed; to say we knew what we were doing would really exploit the advantage of hindsight!
Malin's method of photographic amplification had been used to find low surface brightness shells and other tidal debris around normal galaxies, and it could be extended to find entire very LSB galaxies. Indeed, Malin already had anecdotal evidence that whenever he Malinized a plate, he would find these ``faint little buggers'' everywhere. So David agreed to Malinize selected one square degree areas of the Virgo cluster from which several small diffuse objects emerged. Skeptical colleagues insisted that the peculiar collection of faint smudges that could be seen were all artifacts of the processing, water spots, or specks of dust. Even the detection of 21-cm emission from one of these ``plate flaws'' did not assuage one particularly recalcitrant referee. With youth, low pay and foolishness on our side, we persisted in our efforts to verify the reality of the smudge galaxies.
In late 1985 and early 1986 we used the Las Campanas 100-inch telescope for CCD imaging the very diffuse galaxies found in the Malinization process (all of which turned out to be real). Most of these galaxies were devoid of structure. However, one had what appeared to be very faint spiral structure which was connected to a point-like nuclear region. On the Palomar Sky Survey, this nuclear region is unresolved with no associated nebulosity apparent. This was one of the few Malin objects bright enough for optical spectroscopy, and on May 1986 at the Palomar 200-inch telescope, Jeremy Mould and Bothun took a spectrum of its nucleus. Astonishingly, the spectrum exhibited emission lines at a redshift of z = 0.083, or a recessional velocity of about 25,000 kms-1. Now, we had pursued the Malinization process on UK Schmidt plates of the Virgo cluster in order to find extremely LSB galaxies in the cluster. Virgo has a mean recessional velocity of 1150 kms-1, so this nucleated object clearly was far beyond Virgo. Since the total angular size of the object on our CCD frame was approximately 2.5 arcminutes, quick scaling then indicated that if a galaxy like this was indeed in Virgo then its angular size would be a degree. If it were as close as the Andromeda Galaxy its angular size would be about 20 degrees and of course we would look right through it without noticing it. This seemed absurd, and there was a good chance that this strange galaxy was a composite system, consisting of a background emission line galaxy shining through a foreground dwarf.
In October 1986, 21-cm observations at Arecibo revealed the characteristic signature of a rotating disk galaxy whose systemic velocity was equal to that of the emission line object. The accidental discovery of Malin 1 (Bothun et al. 1987) strongly confirmed Disney's original speculation of the existence of ``crouching giants.'' The existence of Malin 1 (type D in the previous example) certainly implied a non negligible space density of these kinds of objects. The properties of Malin 1 are described in detail by Impey and Bothun (1989). Recent H I observations of Malin 1 using the VLA by Pickering et al. (1997) confirm the presence of a greatly extended gaseous disk around the normal bulge component of the galaxy.
The remaining ``smudges'' in the Virgo cluster area did not turn out to be as spectacular as Malin 1. These smudges were most likely LSB dwarf galaxy members of the Virgo cluster. Their discovery and characterization by Impey, Bothun and Malin (1988) readily showed that the faint end slope of the LF in clusters of galaxies was significantly steeper than previous measured. In the case of Virgo, this meant there were galaxies in the range MB = -12 to -16 which were below the isophotal limits of the plate material used by Sandage. Most of these have µ 0 fainter than 24.5 mag arcsec-2 and l larger than 1 kpc. The presence of these diffuse galaxies at modest luminosities increased the faint end power law slope of the LF to a value of -1.55; significantly steeper than the value of -1.1 which was thought to hold for clusters and perhaps the field (see Efthasthiou et al. 1988; Loveday et al. 1992; Marzke et al. 1994).
The success of the Malin hunt for very diffuse galaxies prompted three new surveys. The first relied on the goodwill of Jim Schombert, who was a Caltech Postdoc associated with the second Palomar Sky Survey. Jim was the quality control person and thus had direct access to the plates themselves for a limited period of time before they were secured in the vault. This allowed an opportunity for hit-and-run visual inspection in pre-defined declination strips to search for diffuse galaxies with sizes larger than one arcminute. This produced the catalogs of Schombert and Bothun (1988) and Schombert et al. (1992). A second survey was initiated in the Fornax cluster using the Malinization technique in order to compare the results to Virgo. These objects are cataloged and described in Bothun et al. (1991) which built on the earlier work of Caldwell and Bothun (1987). The third survey was initiated with Mike Irwin at Cambridge and this involved using the Automatic Plate Machine (APM) to scan UK Schmidt plates using an algorithm optimized to find galaxies of low contrast. This forms the most extensive catalog of LSB galaxies to date (Impey et al. 1996).
The goals of these new surveys was to discover, using various techniques, the extent and nature of this new population of galaxies which had low contrast with respect to the sky background and hence have remained undetected and uncataloged. The importance of discovering this new population cannot be overstated. The existence of LSB galaxies is a clear signal that the samples from which we select galaxies for detailed follow-up studies are incomplete, inadequate and biased. These surveys are now complete and most of the results have been published. They have opened up a new field of inquiry in extragalactic astronomy. Detailed studies of the properties of individual LSB galaxies, and the class as a whole, has resulted in a number of recent Ph.D. theses: Knezek (1993), McGaugh (1992), Sprayberry (1994), Dalcanton (1995), de Jong (1995), Driver (1995), de Blok (1997), O'Neil (1997), and Pickering (1997).
In just over a decade, a whole new population of galaxies has been discovered.
Figure 1 provides only a hint on
their overall space density.
The number of objects with µ 0 24.0 mag arcsec-2 is
unknown, and can only be guessed by extrapolation of the trend in
Figure 1. Significant numbers
of galaxies with µ 0 24.0 mag arcsec-2 have
been detected in CCD surveys, suggesting that the trend remains fairly
constant (Dalcanton 1995,
O'Neil et al. 1997a) or
even rises towards fainter µ 0
(Schwartzenberg et al.
1995). Figure 2 shows an example of one of the
more extreme LSB galaxies turned up in the O'Neil et al.
survey. Reproducing nearly invisible galaxies
on paper is difficult. Interested readers should inspect the digital
gallery of LSB galaxies available at .
These LSB galaxies are of cosmological significance and have properties
which are quite different from those of their HSB
counterparts which dominate existing galaxy catalogs.
Figure 2. An example from the Texas survey (e.g., O'Neil et al. 1997a) of an extremely diffuse object. The image size is 1.2 x 1.5 arc minutes and the galaxy looks to be at least one arc minute in ``diameter'' at a probable redshift of 4-6,000 km/s. A better presentation of these diffuse galaxies can be found at