Читать книгу Galaxies - Группа авторов - Страница 22

The CVRHS sequence of galaxy types from ellipticals to irregulars has astrophysical significance. This is because some measured properties of galaxies correlate with position along the sequence. For example, the integrated color index, , ranges from ≈0.9 for E galaxies to ≈0.4 for magellanic irregular galaxies. Over the same range, the color index varies from ≈0.5 to −0.2. Color images of galaxies of different types available through the Sloan Digital Sky Survey show these differences directly. Types earlier than Sa tend to be yellowish, indicating the predominance of an old stellar population, while types Sc and later tend to be mostly bluish, indicating the predominance of a younger stellar population. Intermediate types (Sa to Sbc) show the presence of both populations, with bars and bulges usually yellowish and spiral arms generally bluish. As shown by Buta et al. (1994), the color indices of galaxies have a dependence on type that is well approximated by an integral Gaussian, showing little variation from type E to type S0⁺, followed by a more rapid variation from type S0/a to types Sd and Sdm. This strong dependence on type, which carries into other colors such as , and (Buta and Williams 1995; Buta 1995a), has been interpreted in terms of a star formation history where the star formation rate declines exponentially with different decay rates (e.g. Kennicutt et al. 1994; Gusev et al. 2015).

Other galactic properties that vary along the VRHS sequence include the average surface brightness and HI mass-to-blue luminosity ratio. The average surface brightness ranges from ≈12.4 mag arcmin⁻² for stages E to S0⁺ to 14.9 mag arcmin⁻² for stage Im, a factor of 10 drop (Buta et al. 1994). This correlation is directly related to one of Hubble’s classification criteria for spirals: that Sa galaxies have more prominent central concentrations than do Sc galaxies. The latest stages (Sd-Im) have virtually no central concentration and the lowest average surface brightnesses. Similarly, there is a relatively smooth variation in HI mass-to-blue light ratio across the spiral sequence, ranging from 0.08 at stage S0/a to 0.5 at stage Im, a factor of 6.5 change (Buta et al. 1994).