|© CAMBRIDGE UNIVERSITY PRESS 1998|
1.4 Some historical notes
An excellent review of the early history of the classification of spiral galaxies is given in Sandage (1975). In its most primitive form the presently used classification system goes back to Hubble (1926), who placed all spirals within the sequence Sa-Sb-Sc. Subsequently Hubble (1936) sub-divided the ``early'' end of his classification sequence by splitting class Sa into S0 and Sa. Later Shapley & Paraskevopoulos (1940) recognized the need to sub-divide the Sc galaxies at the ``late'' end of the classification sequence into types Sc and Sd. As a result the complete sequence became S0-Sa-Sb-Sc-Sd-Ir. A somewhat different way to classify spirals was introduced by Holmberg (1958) who sub-divided the spiral sequence into stages Sa-Sb--Sb+-Sc--Sc+. Holmberg was able to show that the classification types of galaxies correlate closely with their mean color indices C = mpg - mpv. The mean colors of galaxies (corrected for internal absorption, Galactic foreground absorption and redshift) as a function of Holmberg type are given in Table 1. Note that the mean intrinsic colors of the stellar populations change smoothly from red to blue along this classification sequence. This suggests that the mean age of the stellar populations also changes monotonically along the Hubble classification sequence. Red early-type galaxies are dominated by an old stellar population, whereas the light of blue late-type galaxies is dominated by young stars.
A somewhat different way of sub-dividing the classification of disk galaxies was proposed by de Vaucouleurs (1959a) who used the sequence Sa-Sb-Sc-Sd-Sm-Im to provide even higher resolution at the ``late'' end of the Hubble classification sequence. In this notation the ``m'' denotes magellanic, i.e. like the Magellanic Clouds.
Inspection of the data in Table 2 shows a smooth progression between the B-V and U-B colors of galaxies (corrected for Galactic absorption, to pole-on orientation, and to a redshift of 1000 km s-1) and classification types according to de Vaucouleurs (1961).
|de Vaucouleurs type|
Table 3 and Figure 1 show the distribution of MB versus Hubble type (Sandage & Tammann 1981) for the galaxies in A Revised Shapley-Ames Catalog of Bright Galaxies. Figure 1 clearly shows that the galaxies of latest classification types in Sandage's modification of the Hubble classification system have systematically lower luminosities than do those of earlier types. In other words this classification system does not clearly distinguish between effects on galaxy morphology that are due to Hubble type, and those that result from luminosity effects. The latest type in the sequence of giant spirals occurs for objects such as M101 (= NGC 5457). Supergiant Ir galaxies have not been found in the nearby regions of the Universe.
The frequency distribution of galaxy types in the Shapley-Ames Catalog (Sandage & Tammann 1981, p. 91) is a function of both the intrinsic frequency of each type and the distance to which objects of that particular type are bright enough to be included in this catalog. Very late-type objects are intrinsically fainter than galaxies assigned to earlier Hubble stages. Such late-type galaxies will therefore be heavily under-represented in the Shapley-Ames Catalog. On the other hand galaxies of Hubble stages Sa, Sb and Sc are seen to have very similar luminosity distributions. The relative numbers of these objects in the Shapley-Ames Catalog should therefore closely match their intrinsic values. Combining normal and barred varieties one finds that the ratio Sa + Sab:Sb + Sbc:Sc is 1.0:1.7:2.2. The relative numbers of spirals of different Hubble stages depends critically on luminosity, with relatively early types predominating at high luminosities. Combining normal and barred spirals one finds the following distribution among galaxies in the Shapley-Ames catalog: For 163 bright galaxies (1) with -23.0 < MB -22.0 mag the frequency of Hubble stages is Sa-Sab (17%), Sb-Sbc (44%), Sc (17%), Scd-Im (0%), compared to Sa-Sab (0%), Sb-Sbc (4%), Sc (25%), Scd-Sm (29%) for 55 galaxies with - 19.0 < MB < - 18.0.
|MB||Sa||Sab||Sb||Sbc||Sc||Scd||Sdm||Sm + Im|
|Note: a Due to rounding errors not all columns add up to 1.00|
Fig. 1 Luminosity distributions for Shapley-Ames galaxies (Sandage & Tammann 1981) as a function of Hubble type. Objects of types Sd-Sm-Im are seen to be much less luminous than those with types Sa-Sb-Sc.