Luminosity Classification

In a series of papers van den Bergh [191, 192, 194] suggested that the coherence of the spiral structure could be used to infer the intrinsic luminosity of spiral galaxies. Systems with well-developed global spiral structure are of Type I (supergiants) while those with ragged ill-defined spiral arms are of Type V (dwarfs), as in the stellar luminosity-class nomenclature. The basic criteria of this DDO-classification (DDO = David Dunlap Observatory) are outlined in the table below.

Calibrations of the absolute magnitudes of Sc galaxies as a function of luminosity class are controversial; see discussions in [193] and [160]. de Vaucouleurs [30] has also discussed this problem, but in terms of his Luminosity Index which is a simple average of his stage and the DDO luminosity class. The absolute magnitudes of Sb galaxies as a function of luminosity class have been recently discussed in [176]. Irregular galaxies may be roughly typed according to intrinsic luminosity [190, 194] through Holmberg's [51] relation between absolute magnitude and apparent surface brightness. Similarly, giant ellipticals have a higher mean surface brightness than dwarf ellipticals [179]. In addition, though, ellipticals show a correlation of their luminosity with colour [11, 28] in the sense that giant ellipticals are redder than their dwarf counterparts.

DDO Luminosity Classification for Sc Galaxies [191, 192, 194]
Sc IThese supergiant galaxies are characterized by long well-developed arms of relatively high-surface-brightness.M 101
Sc IIThe spiral structure of these bright giant galaxies is less well developed as compared to Sc I galaxies.NGC 3184
NGC 3319
Sc IIIShort patchy arms extend from a fairly high-surface-brightness main body.NGC 2403
NGC 672
S IVOuter regions give only a hint of spiral structure emanating from a relatively low-surface-brightness disk.NGC 247
NGC 2500
S VDwarf spirals; only a hint of spiral structure is seen in these low-surface-brightness objects.DDO 122

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