The van den Bergh luminosity classes highlight how luminosity and surface brightness generally go together. Low surface brightness usually means low luminosity and small size, hence a dwarf classification. However, the discovery of rare giant low surface brighness (GLSB) galaxies by Bothun et al. (1987) shows that morphology can sometimes be misleading for judging absolute luminosity. The hallmarks of these objects are a relatively normal bulge and an extremely low surface brightness, very large disk. Disk radial scalelengths and luminosities are unusually large, and extrapolated disk central surface brightnesses are unusually faint, compared to more normal spirals. The disks tend to be relatively smooth with a few large, isolated HII regions. Bothun et al. (1987) point to a model whereby the disks of these galaxies have such a low gas surface density that they are largely unevolved, due to the inefficiency of star formation.
GLSB galaxies can be classified within the Hubble-Sandage and de Vaucouleurs classification systems although, as noted by McGaugh, Schombert, and Bothun (1995), the majority are classified later than stage Sc. Bulges, bars, rings, and spiral patterns are evident in some examples, in spite of the low disk surface brightness. Figure 47 shows three of the originally recognized GLSB examples (McGaugh, Schombert, & Bothun 1995): Malin 2 (also known as F568-6), UGC 1230, and UGC 6614. In the images, the length of a side is 131, 38, and 77 kpc, respectively. These can compared to the giant normal spiral NGC 7531 in the far right frame, where the length of a side is also 38 kpc. Malin 2 and UGC 6614 are especially enormous physical objects. UGC 1230 is also very large for such a late-type morphology. van den Bergh (1998) likens the size of Malin 2 to the core of a cluster of galaxies. He considers "monsters" like Malin 1 and Malin 2 to be only one of three types of LSB galaxies. Some LSB galaxies are as big as normal galaxies, like UGC 1230. Most LSB galaxies, however, are dwarfs: ellipticals, irregulars and, less frequently, spirals. These are described further in section 15.2.
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Figure 47. Examples of giant or large low surface brightness galaxies. In the far right panel, SGC 2311.8-4353 is the diffuse object to the right of high surface brightness spiral NGC 7531. All of these images are B-band. |
An example of another object that could be considered a large LSB galaxy, but which lacks a bulge or any evident recent star formation, is SGC 2311.8-4353, the mysterious ghost-like companion close to the right of NGC 7531 in Figure 47. This peculiar object is 2/3 the size of NGC 7531 (Buta 1987) but has unknown redshift. If it is associated with NGC 7531, it would be as much as 30 kpc in diameter at the faintest detectable isophote level and would clearly not be a dwarf.
A recent study of three GLSBs (Malin 1, UGC 6614, and UGC 9024) by Rahman et al. (2007) showed that IR emission from such objects is consistent with their optically-determined low star formation rates, with the diffuse optical disks being undetected from two of the three. A dynamical study of two GLSBs (Malin 1 and NGC 7589) by Lelli, Fraternali, & Sancisi (2010) led the authors to conclude that at least in these cases, the GLSB galaxy can be thought of as an inner high surface brightness galaxy having a very extended LSB disk. This is based on the steeply rising rotation curves found for these galaxies, which is very much like what is seen in early-type high surface brightness galaxies.
Impey & Bothun (1997) argue that LSB galaxies brighter than MB = -14 contribute significantly to the luminosity density of the local universe, are dark-matter-dominated at almost all radii, and have an evolutionary history involving late collapse of a low amplitude perturbation, a low star formation rate, and very slow changes. Large LSBs are greatly underrepresented in galaxy catalogues but are clearly an important class of objects.