2.5.1. Two physical regimes along the tuning fork?

In probing the physical meaning of the Hubble sequence it is perhaps useful to mentally subdivide the sequence into two regimes: E-Sbc and Sc-Irr [85] since in the latter objects the dominant physics seems to be an almost perfect sequence of decreasing mass and luminosity with Hubble type. For very late-type galaxies, correlations between morphology and colour/star-formation history exist but are not strong - a typical Im is not significantly bluer than a typical Scd [69]. It is perhaps worth noting that while the general trends in very late-type systems are clear, there appears to be no really objective way of distinguishing between very late-type spirals (ie. types Sd and Sm) and irregular galaxies [85].

Along the earlier portion of the sequence the Hubble system remains quite successful at ranking galaxies along physical lines, although the story is more complex. In addition to ranking galaxies by bulge-to-disk ratio (partly by definition), the early part of the spiral tines of the Hubble sequence order galaxies by surface HI and total mass density, total HI mass, colour, and, to some extent, mass-to-light ratio. Of these the strongest correlations appear to be with colour [69], and hence with star-formation history. To what extent these correlations would remain if one ranked galaxies simply by bulge-to-disk ratio, as proposed by Morgan [63, 64], is unknown. However, because bulge-to-disk ratio is the dominant morphological characteristic linked to star-formation history, and because this seems the strongest correlation, I would not be surprised if the correlations along the early part of the spiral sequence actually improved if the ranking was purely by bulge-to-disk ratio.